Internet DRAFT - draft-ietf-rtgwg-bgp-pic
draft-ietf-rtgwg-bgp-pic
Network Working Group A. Bashandy, Ed.
Internet Draft C. Filsfils
Intended status: Informational Cisco Systems
Expires: April 2024 P. Mohapatra
Sproute Networks
October 1, 2023
BGP Prefix Independent Convergence
draft-ietf-rtgwg-bgp-pic-20.txt
Abstract
In a network comprising thousands of BGP peers exchanging millions of
routes, many routes are reachable via more than one next-hop. Given
the large scaling targets, it is desirable to restore traffic after
failure in a time period that does not depend on the number of BGP
prefixes.
This document describes an architecture by which traffic can be re-
routed to equal cost multi-path (ECMP) or pre-calculated backup paths
in a timeframe that does not depend on the number of BGP prefixes.
The objective is achieved through organizing the forwarding data
structures in a hierarchical manner and sharing forwarding elements
among the maximum possible number of routes. The described technique
yields prefix independent convergence while ensuring incremental
deployment, complete automation, and zero management and provisioning
effort. It is noteworthy to mention that the benefits of BGP Prefix
Independent Convergence (BGP-PIC) are hinged on the existence of more
than one path whether as ECMP or primary-backup.
Status of this Memo
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Table of Contents
1. Introduction...................................................3
1.1. Terminology...............................................3
2. Overview.......................................................6
2.1. Dependency................................................6
2.1.1. Hierarchical Hardware FIB (Forwarding Information Base)
............................................................6
2.1.2. Availability of more than one BGP next-hops..........7
2.2. BGP-PIC Illustration......................................7
3. Constructing the Shared Hierarchical Forwarding Chain.........10
3.1. Constructing the BGP-PIC Forwarding Chain................10
3.2. Example: Primary-Backup Pic-path Scenario................11
4. Forwarding Behavior...........................................12
5. Handling Platforms with Limited Levels of Hierarchy...........13
6. Forwarding Chain Adjustment at a Failure......................13
6.1. BGP-PIC core.............................................14
6.2. BGP-PIC edge.............................................15
6.2.1. Adjusting Forwarding Chain in egress node failure...15
6.2.2. Adjusting Forwarding Chain on PE-CE link Failure....15
6.3. Handling Failures for Flattened Forwarding Chains........17
7. Properties....................................................18
7.1. Coverage.................................................18
7.1.1. A remote failure on the pic-path to a BGP next-hop..18
7.1.2. A local failure on the pic-path to a BGP next-hop...18
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7.1.3. A remote IBGP next-hop fails........................18
7.1.4. A local EBGP next-hop fails.........................18
7.2. Performance..............................................19
7.3. Automated................................................19
7.4. Incremental Deployment...................................19
8. Security Considerations.......................................20
9. IANA Considerations...........................................20
10. References...................................................20
10.1. Normative References....................................20
10.2. Informative References..................................20
11. Acknowledgments..............................................21
Appendix A. Handling Platforms with Limited Levels of Hierarchy..23
Appendix B. Example: Flattening a forwarding chain...............25
Appendix C. Perspective..........................................32
1. Introduction
BGP speakers exchange reachability information about prefixes
[RFC4271] and, for labeled address families an edge router assigns
local labels to prefixes and associates the local label with each
advertised prefix using technologies such as L3VPN [RFC4364], 6PE
[RFC4798], and Softwire [RFC5565] using BGP label unicast (BGP-LU)
technique [RFC8277]. A BGP speaker then applies the path selection
steps to choose the best route. In modern networks, it is not
uncommon to have a prefix reachable via multiple edge routers.
Multiple techniques have been described to allow for BGP to
advertise more than one path for a given prefix [I.D.ietf-idr-best-
external][RFC7911][RFC6774], whether in the form of equal cost
multipath or primary-backup. Another common and widely deployed
scenario is L3VPN with multi-homed VPN sites with unique Route
Distinguisher.
This document describes a hierarchical and shared forwarding chain
organization that allows traffic to be restored to a pre-
calculated alternative equal cost primary path or backup path in a
time period that does not depend on the number of BGP prefixes.
The technique relies on internal router behavior that is
completely transparent to the operator and can be incrementally
deployed and enabled with zero operator intervention. In other
words, once it is implemented and deployed on a router, nothing is
required from the operator to make it work. It is noteworthy to
mention that this document describes a Forwarding Information Base
(FIB) architecture that can be implemented in both hardware and/or
software, although we refer to hardware implementation in most of
the cases because of the additional complexity and performance
requirements associated with hardware implementations.
1.1. Terminology
This section defines the terms used in this document.
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o BGP-LU: BGP Label Unicast. Refers to carrying label unicast
address family (SAFI-4) in BGP4 as in [RFC8277].
o BGP prefix: A IP address prefix as described in [RFC4271].
o IGP prefix: A prefix that is learnt via an Interior Gateway
Protocol (IGP), such as OSPF and ISIS. The prefix may be learnt
directly through the IGP or statically configured.
o Customer Edge (CE) [RFC4364]: An external router through which
an egress PE can reach a prefix P/m.
o Egress PE [RFC4364], "ePE": A BGP speaker that learns about a
prefix through an external BGP (EBGP) peer and chooses that EBGP
peer as the next-hop for that prefix.
o Ingress PE, "iPE": A BGP speaker that learns about a prefix
through a Internal BGP (IBGP) peer and chooses an egress PE as
the next-hop for the prefix.
o Pic-path: The next-hop in a sequence of nodes starting from the
current node and ending with the destination node or network
identified by the prefix. The nodes may not be directly
connected.
o Recursive pic-path: A pic-path consisting only of the IP
address of the next-hop without the outgoing interface.
Subsequent lookups are necessary to determine the outgoing
interface and a directly connected next-hop.
o Non-recursive pic-path: A pic-path consisting of the IP address
of a directly connected next-hop and outgoing interface.
o Adjacency: The layer 2 encapsulation leading to the layer 3
directly connected next-hop. An adjacency is identified by a
next-hop and an outgoing interface
o Primary pic-path: A recursive or non-recursive pic-path that
can be used for forwarding as long as forwarding engine can
walk (See section 2.2 for explanation of forwarding chain and
Section 4 forwarding engine behavior) starting from this pic-
path can end to an adjacency. A prefix can have more than one
primary pic-path.
o Backup pic-path: A recursive or non-recursive pic-path that can
be used only after some or all primary pic-paths become
unreachable.
o Primary Next-hop. The next-hop in a primary pic-path
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o Secondary next-hop: The next-hop in the backup pic-path
o Leaf: A container data structure for a prefix or local label.
Alternatively, it is the data structure that contains prefix
specific information.
o IP leaf: The leaf corresponding to an IPv4 or IPv6 prefix.
o Label leaf. The leaf corresponding to a locally allocated label
such as the VPN label on an egress PE [RFC4364].
o Pathlist: An array of pic-paths used by one or more prefixes to
forward traffic to destination(s) covered by an IP prefix. Each
pic-path in the pathlist carries its "path-index" that identifies
its position in the array of paths. In general, the value of the
path-index in a pic-path is the same as its position in the
pathlist, except in the case outlined in Section 5. For example
the 3rd pic-path may carry a path-index value of 1. A pathlist
may contain a mix of primary and backup pic-paths.
o OutLabel-List: Each labeled prefix is associated with an
OutLabel-List. The OutLabel-List is an array of one or more
outgoing labels and/or label actions where each label or label
action has 1-to-1 correspondence to a pic-path in the pathlist.
Label actions are: push (add) the label as specified in
[RFC3031], pop (remove) the label as specified in [RFC3031],
swap (replace) the incoming label with the label in the
OutLabel-List entry, or don't push anything at all in case of
"unlabeled". The prefix may be an IGP or BGP prefix.
o Forwarding chain: It is a compound data structure consisting of
multiple connected blocks that a forwarding engine walks one
block at a time to forward the packet out of an interface.
Section 2.2 explains an example of a forwarding chain.
Subsequent sections provide additional examples
o Dependency: An object X is said to be a dependent or child of
object Y if there is at least one forwarding chain where the
forwarding engine must visit the object X before visiting the
object Y in order to forward a packet. Note that if object X is
a child of object Y, then Y cannot be deleted unless object X
is no longer a dependent/child of object Y.
o Pic-route: A prefix with one or more pic-paths associated with
it. The minimum set of objects needed to construct a pic-route
is a leaf and a pathlist.
o IGP pic-route: a pic-route whose prefix is learned from an IGP
o BGP pic-route: a pic-route whose prefix is learned from BGP
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o Routing-table: A table where each entry is a pic-route as
defined in this section.
o ASN: Autonomous System Number
2. Overview
The idea of BGP-PIC is based on two pillars
o A shared hierarchical forwarding chain: It is not uncommon to see
multiple destinations reachable via the same list of next-hops.
Instead of having a separate list of next-hops for each
destination, all destinations sharing the same list of next-hops
can point to a single copy of this list thereby allowing fast
convergence by making changes to a single shared list of next-
hops rather than possibly a large number of destinations. Because
pic-paths in a pathlist may be recursive, a hierarchy is formed
between pathlist and the resolving prefix whereby the pathlist
depends on the resolving prefix.
o A forwarding plane that supports multiple levels of indirection:
A forwarding chain that starts with a destination and ends with
an outgoing interface is not a simple flat structure. Instead, a
forwarding entry is constructed via multiple levels of
indirections. A BGP prefix uses a recursive next-hop, which in
turn resolves via an IGP next-hop, which in turn resolves via an
adjacency consisting of one or more outgoing interface(s) and
next-hop(s).
Designing a forwarding plane that constructs multi-level forwarding
chains with maximal sharing of forwarding objects allows rerouting a
large number of destinations by modifying a small number of objects
thereby achieving convergence in a time frame that does not depend
on the number of destinations. For example, if the IGP prefix that
resolves a recursive next-hop is updated there is no need to update
the possibly large number of BGP NLRIs that use this recursive next-
hop.
2.1. Dependency
This section describes the required functionalities in the
forwarding and control planes to support BGP-PIC as described in
this document.
2.1.1. Hierarchical Hardware FIB (Forwarding Information Base)
BGP-PIC requires a hierarchical hardware FIB support: if the
destination address of a forwarded packet matches a BGP prefix, a
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BGP leaf is looked up, then a BGP pathlist is consulted, then an IGP
pathlist, then an adjacency. Section 4 has more details about how
a packet is forwarded
An alternative method consists in "flattening" the dependencies when
programming the BGP destinations into HW FIB resulting in
potentially eliminating both the BGP pathlist and IGP pathlist
consultation. Such an approach decreases the number of memory
lookups per forwarding operation at the expense of HW FIB memory
increase (flattening means less sharing thereby less duplication),
loss of equal cost multi-path (ECMP) properties (flattening means
less pathlist entropy) and loss of BGP-PIC properties. Section 5
explains the concept of flattening for hardware with limited number
of levels of indirections.
2.1.2. Availability of more than one BGP next-hops
When the BGP next-hop in the primary pic-path becomes unresolved,
BGP-PIC depends on the availability of one or more pre-computed and
pre-programmed backup pic-paths(s) in the BGP pathlist in the
forwarding engine.
The existence of a backup pic-path is clearly required for the
following reason: a network connectivity service caring for network
availability will require two disjoint network connections resulting
in two BGP next-hops.
The BGP distribution of secondary next-hops is available thanks to
the following BGP mechanisms: Add-Path [RFC7911], BGP Best-External
[I.D.ietf-idr-best-external], diverse path [RFC6774], and the
frequent use in VPN deployments of different VPN RD's per PE.
Another option to learn multiple BGP next-hops/paths is to receive
IBGP paths from multiple BGP RRs [RFC9107] selecting a different
path as best. It is noteworthy to mention that the availability of
another BGP path does not mean that all failure scenarios can be
covered by simply forwarding traffic to the available secondary
path. The discussion of how to cover various failure scenarios is
beyond the scope of this document.
2.2. BGP-PIC Illustration
To illustrate the two pillars above as well as the platform
dependency, this document will use an example of a multihomed L3VPN
prefix in a BGP-free core running LDP [RFC5036] or segment routing
over MPLS forwarding plane [RFC8660].
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+--------------------------------+
| |
| ePE2 (IGP-IP1 192.0.2.1, Loopback)
| | \
| | \
| | \
iPE | CE....VRF "Blue", ASN 65000
| | / (VPN-IP1 198.51.100.0/24)
| | / (VPN-IP2 203.0.113.0/24)
| LDP/Segment-Routing Core | /
| ePE1 (IGP-IP2 192.0.2.2, Loopback)
| |
+--------------------------------+
Figure 1: VPN prefix reachable via multiple PEs
Referring to Figure 1, suppose the iPE (the ingress PE) receives
NLRIs for the VPN prefixes VPN-IP1 and VPN-IP2 from two egress PEs,
ePE1 and ePE2 with next-hop BGP-NH1 (192.0.2.1) and BGP-NH2
(192.0.2.2), respectively. Assume that ePE1 advertise the VPN labels
VPN-L11 and VPN-L12 while ePE2 advertise the VPN labels VPN-L21 and
VPN-L22 for VPN-IP1 and VPN-IP2, respectively. Suppose that BGP-NH1
and BGP-NH2 are resolved via the IGP prefixes IGP-IP1 and IGP-IP2,
where each happen to have 2 equal cost paths with IGP-NH1 and IGP-
NH2 reachable via the interfaces I1 and I2 on iPE, respectively.
Suppose that local labels (whether LDP [RFC5036] or segment routing
[RFC8660]) on the downstream LSRs for IGP-IP1 are IGP-L11 and IGP-
L12 while for IGP-IP2 are IGP-L21 and IGP-L22. As such, the pic-
routing table at iPE is as follows:
65000: 198.51.100.0/24
via ePE1 (192.0.2.1), VPN Label: VPN-L11
via ePE2 (192.0.2.2), VPN Label: VPN-L21
65000: 203.0.113.0/24
via ePE1 (192.0.2.1), VPN Label: VPN-L12
via ePE2 (192.0.2.2), VPN Label: VPN-L22
192.0.2.1/32 (ePE2)
via I1, Label: IGP-L11
via I2, Label: IGP-L12
192.0.2.2/32 (ePE1)
via I1, Label: IGP-L21
via I2, Label: IGP-L22
Based on the above pic-routing-table, a hierarchical forwarding
chain can be constructed as shown in Figure 2.
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IP Leaf: pathlist: IP Leaf: pathlist:
-------- +-----------+ --------
| | +-------------+
|BGP-NH1------->IGP-IP1 ----->| |
VPN-IP1-->| | | | IGP-NH1,I1----->adjacency1
| |BGP-NH2------->... | | |
| | | | | IGP-NH2,I2----->adjacency2
| +-----------+ | | |
| | +-------------+
| |
v v
OutLabel-List: OutLabel-List:
+--------+ +--------+
|VPN-L11 | |IGP-L11 |
|VPN-L21 | |IGP-L12 |
+--------+ +--------+
Figure 2: Shared Hierarchical Forwarding Chain at iPE
The forwarding chain depicted in Figure 2 illustrates the first
pillar, which is sharing and hierarchy. It can be seen that the BGP
pathlist consisting of BGP-NH1 and BGP-NH2 is shared by all NLRIs
reachable via ePE1 and ePE2. As such, it is possible to make changes
to the pathlist without having to make changes to the NLRIs. For
example, if BGP-NH2 becomes unreachable, there is no need to modify
any of the possibly large number of NLRIs. Instead only the shared
pathlist needs to be modified. Likewise, due to the hierarchical
structure of the forwarding chain, it is possible to make
modifications to the IGP pic-routes without having to make any
changes to the BGP NLRIs. For example, if the interface "I2" goes
down, only the shared IGP pathlist needs to be updated, but none of
the IGP prefixes sharing the IGP pathlist nor the BGP NLRIs using
the IGP prefixes for resolution need to be modified.
Figure 2 can also be used to illustrate the second BGP-PIC pillar.
Having a deep forwarding chain such as the one illustrated in Figure
2 requires a forwarding plane that is capable of accessing multiple
levels of indirection in order to calculate the outgoing
interface(s) and next-hops(s). While a deeper forwarding chain
minimizes the re-convergence time on topology change, there will
always exist platforms with limited capabilities and hence imposing
a limit on the depth of the forwarding chain. Section 5 describes
how to gracefully trade off convergence speed with the number of
hierarchical levels to support platforms with different
capabilities.
Another example using IPv6 addresses can be something like the
following
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65000: 2001:DB8:1::/48
via ePE1 (65000: 2001:DB8:192::1), VPN Label: VPN6-L11
via ePE2 (65000: 2001:DB8:192::2), VPN Label: VPN6-L21
65000: 2001:DB8:2:/48
via ePE1 (65000: 2001:DB8:192::1), VPN Label: VPN6-L12
via ePE2 (65000: 2001:DB8:192::2), VPN Label: VPN6-L22
65000: 2001:DB8:192::1/128
via Core, Label: IGP6-L11
via Core, Label: IGP6-L12
65000: 2001:DB8:192::2/128
via Core, Label: IGP6-L21
via Core, Label: IGP6-L22
The same hierarchical forwarding chain described can be constructed
for IPv6 addresses/prefixes.
3. Constructing the Shared Hierarchical Forwarding Chain
Constructing the forwarding chain is an application of the two
pillars described in Section 2. This section describes how to
construct the forwarding chain in a hierarchical shared manner.
3.1. Constructing the BGP-PIC Forwarding Chain
The whole process starts when a BGP prefix is downloaded to FIB. The
prefix contains one or more outgoing pic-paths. For certain labeled
prefixes, such as L3VPN [RFC4364] prefixes, each pic-path may be
associated with an outgoing label and the prefix itself may be
assigned a local label. The list of outgoing pic-paths defines a
pathlist. If such pathlist does not already, then the FIB manager
(software or hardware entity responsible for managing the FIB)
creates a new pathlist, otherwise the existing pathlist with the
same list of pic-paths exist (the pathlist may already exist because
there is another pic-route that is already using the same list of
pic-paths) is used. The BGP prefix is added as a dependent of the
pathlist.
The previous step constructs the upper part of the hierarchical
forwarding chain. The forwarding chain is completed by resolving the
pic-paths of the pathlist. A BGP pic-path usually consists of a
next-hop. The next-hop is resolved by finding a matching IGP prefix.
The end result is a hierarchical shared forwarding chain where the
BGP pathlist is shared by all BGP prefixes that use the same list of
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pic-paths and the IGP prefix is shared by all pathlists that have a
pic-path resolving via that IGP prefix.
The remainder of this section goes over an example to illustrate the
applicability of BGP-PIC in a primary-backup pic-path scenario.
3.2. Example: Primary-Backup Pic-path Scenario
Consider the egress PE ePE1 in the case of the multi-homed VPN
prefixes shown in Figure 1. Suppose ePE1 determines that the primary
pic-path is the external pic-path, while the backup pic-path is the
IBGP pic-path to the other PE ePE2 with next-hop BGP-NH2. ePE1
constructs the forwarding chain depicted in Figure 3. The figure
shows only a single VPN prefix for simplicity. But all prefixes that
are multihomed to ePE1 and ePE2 share the BGP pathlist.
BGP OutLabel-List
+---------+
VPN-L11 |Unlabeled|
(Label-leaf)---+---->+---------+
| | VPN-L21 |
v | (swap) |
| +---------+
|
|
|
|
| BGP pathlist
| +--------------+
| | |
| | CE-NH ------->(to the CE)
| | path-index=0 |
VPN-IP1 -----+------------------>+--------------+
(IP leaf) | VPN-NH2 |
| | (backup) ------->IGP Leaf
| | path-index=1 | (Towards ePE2)
| +--------------+
|
| BGP OutLabel-List
| +---------+
| |Unlabeled|
+------------->+---------+
| VPN-L21 |
| (push) |
+---------+
Figure 3: VPN Prefix Forwarding Chain with eiBGP pic-paths on egress
PE
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The example depicted in Figure 3 differs from the example in Figure
2 in two main aspects. First, as long as the primary pic-path
towards the CE (external pic-path) can be used for forwarding, it
will be the only pic-path used for forwarding while the OutLabel-
List contains both the unlabeled (primary pic-path) and the VPN
label (backup pic-path) advertised by the backup pic-path ePE2. The
second aspect is presence of the label leaf corresponding to the VPN
prefix. This label leaf is used to match VPN traffic arriving from
the core. Note that the label leaf shares the pathlist with the IP
prefix.
4. Forwarding Behavior
This section explains how the forwarding plane uses the hierarchical
shared forwarding chain to forward a packet.
When a packet arrives at a router, assume it matches a leaf. If not,
the packet is handled according to the local policy (such as
silently dropping the packet), which is beyond the scope of this
document. A labeled packet matches a label leaf while an IP packet
matches an IP leaf. The forwarding engines walks the forwarding
chain starting from the leaf until the walk terminates on an
adjacency. Thus when a packet arrives, the chain is walked as
follows:
1. Lookup the leaf based on the destination address or the label at
the top of the packet.
2. Retrieve the parent pathlist of the leaf.
3. Pick an outgoing pic-path "Pi" from the list of resolved pic-
paths in the pathlist. The method by which the outgoing pic-path
is picked is beyond the scope of this document (e.g. flow-
preserving hash exploiting entropy within the MPLS stack and IP
header). Let the "path-index" of the outgoing pic-path "Pi" be
"j". Remember that, as described in the definition of the term
pathlist in Section 1.1, the path-index of a pic-path may not
always be identical the position of the pic-path in the pathlist.
4. If the prefix is labeled, use the "path-index" "j" to retrieve
the label "Lj" stored position j in the OutLabel-List and apply
the label action of the label on the packet (e.g. for VPN label
on the ingress PE, the label action is "push"). As mentioned in
Section 1.1 the value of the "path-index" stored in the pic-
path may not necessarily be the same value of the location of the
pic-path in the pathlist.
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5. If the chosen pic-path "Pi" is recursive, move to its parent
prefix and go to step 2.
6. If the chosen pic-path is non-recursive move to its parent
adjacency.
7. Encapsulate the packet in the layer string specified by the
adjacency and send the packet out.
Let's apply the above forwarding steps to the forwarding chain
depicted in Figure 2 in Section 2. Suppose a packet arrives at
ingress PE iPE from an external neighbor. Assume the packet matches
the VPN prefix VPN-IP1. While walking the forwarding chain, the
forwarding engine applies a hashing algorithm to choose the pic-path
and the hashing at the BGP level chooses the first pic-path in the
BGP pathlist while the hashing at the IGP level yields the second
pic-path in the IGP pathlist. In that case, the packet will be sent
out of interface I2 with the label stack "IGP-L12,VPN-L11".
5. Handling Platforms with Limited Levels of Hierarchy
This section describes the construction of the forwarding chain if a
platform does not support the number of recursion levels required to
resolve the NLRIs. There are two main design objectives.
o Being able to reduce the number of hierarchical levels from any
arbitrary value to a smaller arbitrary value that can be
supported by the forwarding engine.
o Minimal modifications to the forwarding algorithm due to such
reduction.
Appendix A provides details on how to handle limited hardware
capabilities.
6. Forwarding Chain Adjustment at a Failure
The hierarchical and shared structure of the forwarding chain
explained in the previous section allows modifying a small number of
forwarding chain objects to re-route traffic to a pre-calculated
equal-cost or backup pic-path without the need to modify the
possibly very large number of BGP prefixes. This section goes over
various core and edge failure scenarios to illustrate how the FIB
manager can utilize the forwarding chain structure to achieve BGP
prefix independent convergence.
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6.1. BGP-PIC core
This section describes the adjustments to the forwarding chain when
a core link or node fails but the BGP next-hop remains reachable.
There are two case: remote link failure and attached link failure.
Node failures are treated as link failures.
When a remote link or node fails, the IGP on the ingress PE receives
an advertisement indicating a topology change so IGP re-converges to
either find a new next-hop and/or outgoing interface or remove the
pic-path completely from the IGP prefix used to resolve BGP next-
hops. IGP and/or LDP download the modified IGP leaves with modified
outgoing labels for the labeled core.
When a local link fails, FIB manager detects the failure almost
immediately. The FIB manager marks the impacted pic-path(s) as
unusable so that only useable pic-paths are used to forward packets.
Hence only IGP pathlists with pic-paths using the failed local link
need to be modified. All other pathlists are not impacted. Note that
in this particular case there is no need to backwalk (walk back the
forwarding chain) to IGP leaves to adjust the OutLabel-Lists because
FIB can rely on the path-index stored in the useable pic-paths in
the pathlist to pick the right label.
It is noteworthy to mention that because FIB manager modifies the
forwarding chain starting from the IGP leaves only. BGP pathlists
and leaves are not modified. Hence traffic restoration occurs within
the time frame of IGP convergence, and, for local link failure,
assuming a backup pic-path has been precomputed, within the
timeframe of local detection (e.g. 50ms). Examples of solutions that
can pre-compute backup pic-paths are IP FRR [RFC5714] remote LFA
[RFC7490], TI-LFA [I-D.ietf-rtgwg-segment-routing-ti-lfa] and MRT
[RFC7812] or EBGP pic-path having a backup pic-path [bonaventure].
Let's apply the procedure mentioned in this subsection to the
forwarding chain depicted in Figure 2. Suppose a remote link failure
occurs and impacts the first ECMP IGP pic-path to the remote BGP
next-hop. Upon IGP convergence, the IGP pathlist used by the BGP
next-hop is updated to reflect the new topology (one pic-path
instead of two) and the new forwarding state is immediately
available to all dependent BGP prefixes. The same behavior would
occur if the failure was local such as an interface going down. As
soon as the IGP convergence is complete for the BGP next-hop IGP
pic-route, all its BGP depending routes benefit from the new pic-
path. In fact, upon local failure, if LFA protection is enabled for
the IGP pic-route to the BGP next-hop and a backup pic-path was pre-
computed and installed in the pathlist, upon the local interface
failure, the LFA backup pic-path is immediately activated (e.g. sub-
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50msec) and thus protection benefits all the depending BGP traffic
through the hierarchical forwarding dependency between the routes.
6.2. BGP-PIC edge
This section describes the adjustments to the forwarding chains as a
result of edge node or edge link failure.
6.2.1. Adjusting Forwarding Chain in egress node failure
When a node fails, IGP on neighboring core nodes send updates
indicating that the edge node is no longer a direct neighbor. If the
node that failed is an egress node, such as ePE1 and ePE2 in Figure
1, IGP running on an ingress node, such as iPE in Figure 1,
converges and the realizes that the egress node is no longer
reachable. As such IGP on the ingress node instructs FIB to remove
the IP and label leaves corresponding to the failed edge node from
FIB. So FIB manager on the ingress node performs the following
steps:
o FIB manager deletes the IGP leaf corresponding to the failed edge
node
o FIB manager backwalks to all dependent BGP pathlists and marks
that pic-path using the deleted IGP leaf as unresolved
o Note that there is no need to modify the possibly large number of
BGP leaves because each pic-path in the pathlist carries its pic-
path index and hence the correct outgoing label will be picked.
Consider for example the forwarding chain depicted in Figure 2.
If the 1st BGP pic-path becomes unresolved, then the forwarding
engine will only use the second pic-path for forwarding. Yet the
path-index of that single resolved pic-path will still be 1 and
hence the label VPN-L21 will be pushed.
6.2.2. Adjusting Forwarding Chain on PE-CE link Failure
Suppose the link between an edge router and its external peer fails.
There are two scenarios (1) the edge node attached to the failed
link performs next-hop self (where BGP advertises the IP address of
its own loopback as next-hop) and (2) the edge node attached to the
failure advertises the IP address of the failed link as the next-hop
attribute to its IBGP peers.
In the first case, the rest of IBGP peers will remain unaware of the
link failure and will continue to forward traffic to the edge node
until the edge node attached to the failed link withdraws the BGP
prefixes. If the destination prefixes are multi-homed to another
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IBGP peer, say ePE2, then FIB manager on the edge router detecting
the link failure applies the following steps to the forwarding chain
(see Figure 3):
o FIB manager backwalks to the BGP pathlists marks the pic-path
through the failed link to the external peer as unresolved.
o Hence traffic will be forwarded using the backup pic-path towards
ePE2.
o Labeled traffic arriving at the egress PE ePE1 matches the BGP
label leaf.
o The OutLabel-List attached to the BGP label leaf already
contains an entry corresponding to the backup pic-path.
o The label entry in OutLabel-List corresponding to the
internal pic-path to backup egress PE has a swap action to
the label advertised by the backup egress PE.
o For an arriving label packet (e.g. VPN), the top label is
swapped with the label advertised by backup egress PE and the
packet is sent towards that the backup egress PE.
o Unlabeled traffic arriving at the egress PE ePE1 matches the BGP
IP leaf
o The OutLabel-List attached to the BGP label leaf already
contains an entry corresponding to the backup pic-path.
o The label entry in OutLabel-List corresponding to the
internal pic-path to backup egress PE has a push (instead of
the swap action in for the labeled traffic case) action to
the label advertised by the backup egress PE.
o For an arriving IP packet, the label advertised by backup
egress PE is pushed and the packet is sent towards that the
backup egress PE.
In the second case where the edge router uses the IP address of the
failed link as the BGP next-hop, the edge router will still perform
the previous steps. But, unlike the case of next-hop self, the IGP
on the failed edge node informs the rest of the IBGP peers that the
IP address of the failed link is no longer reachable. Hence the FIB
manager on IBGP peers will delete the IGP leaf corresponding to the
IP prefix of the failed link. The behavior of the IBGP peers will be
identical to the case of edge node failure outlined in Section
6.2.1.
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It is noteworthy to mention that because the edge link failure is
local to the edge router, sub-50 msec convergence can be achieved as
described in [bonaventure].
Let's try to apply the case of next-hop self to the forwarding chain
depicted in Figure 3. After failure of the link between ePE1 and CE,
the forwarding engine will route traffic arriving from the core
towards VPN-NH2 with path-index=1. A packet arriving from the core
will contain the label VPN-L11 at top. The label VPN-L11 is swapped
with the label VPN-L21 and the packet is forwarded towards ePE2.
6.3. Handling Failures for Flattened Forwarding Chains
As explained in the in Section 5 if the number of hierarchy levels
of a platform cannot support the native number of hierarchy levels
of a recursive forwarding chain, the instantiated forwarding chain
is constructed by flattening two or more levels. Hence a 3-levels
chain in Figure 5 is flattened into the 2-levels chain in Figure 6.
While reducing the benefits of BGP-PIC, flattening one hierarchy
into a shallower hierarchy does not always result in a complete loss
of the benefits of the BGP-PIC. To illustrate this fact suppose
ASBR12 is no longer reachable in domain 1. If the platform supports
the full hierarchy depth, the forwarding chain is the one depicted
in Figure 5 and hence the FIB manager needs to backwalk one level to
the pathlist shared by "ePE1" and "ePE2" and adjust it. If the
platform supports 2 levels of hierarchy, then a useable forwarding
chain is the one depicted in Figure 6. In that case, if ASBR12 is no
longer reachable, the FIB manager has to backwalk to the two
flattened pathlists and updates both of them.
The main observation is that the loss of convergence speed due to
the loss of hierarchy depth depends on the structure of the
forwarding chain itself. To illustrate this fact, let's take two
extremes. Suppose the forwarding objects in level i+1 depend on the
forwarding objects in level i. If every object on level i+1 depends
on a separate object in level i, then flattening level i into level
i+1 will not result in loss of convergence speed. Now let's take the
other extreme. Suppose "n" objects in level i+1 depend on 1 object
in level i. Now suppose FIB flattens level i into level i+1. If a
topology change results in modifying the single object in level i,
then FIB has to backwalk and modify "n" objects in the flattened
level, thereby losing all the benefit of BGP-PIC. Experience shows
that flattening forwarding chains usually results in moderate loss
of BGP-PIC benefits. Further analysis is needed to corroborate and
quantify this statement.
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7. Properties
7.1. Coverage
All the possible failures, except CE node failure, are covered,
whether they impact a local or remote IGP pic-path or a local or
remote BGP next-hop as described in Section 6. This section provides
details for each failure and how the hierarchical and shared FIB
structure described in this document allows recovery that does not
depend on number of BGP prefixes.
7.1.1. A remote failure on the pic-path to a BGP next-hop
Upon IGP convergence, the IGP leaf for the BGP next-hop is updated
and all the BGP depending routes leverage the new IGP forwarding
state immediately. Details of this behavior can be found in Section
6.1.
This results in BGP traffic recovery that only depends on IGP
convergence and is independent of the number of BGP prefixes
impacted.
7.1.2. A local failure on the pic-path to a BGP next-hop
Upon LFA protection, the IGP leaf for the BGP next-hop is updated to
use the precomputed backup pic-path and all the BGP depending routes
leverage this protection. Details of this behavior can be found in
Section 6.1.
This BGP resiliency property only depends on LFA protection and is
independent of the number of BGP prefixes impacted.
7.1.3. A remote IBGP next-hop fails
Upon IGP convergence, the IGP leaf for the BGP next-hop is deleted
and all the depending BGP Path-Lists are updated to either use the
remaining ECMP BGP best-paths or if none remains available to
activate precomputed backups. Details about this behavior can be
found in Section 6.2.1.
This BGP resiliency property only depends on IGP convergence and is
independent of the number of BGP prefixes impacted.
7.1.4. A local EBGP next-hop fails
Upon local link failure detection, the adjacency to the BGP next-hop
is deleted and all the depending BGP pathlists are updated to either
use the remaining ECMP BGP best-paths or if none remains available
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to activate precomputed backups. Details about this behavior can be
found in Section 6.2.2.
This BGP resiliency property only depends on local link failure
detection and is independent of the number of BGP prefixes impacted.
7.2. Performance
When the failure is local (a local IGP next-hop failure or a local
EBGP next-hop failure), a pre-computed and pre-installed backup is
activated by a local-protection mechanism that does not depend on
the number of BGP destinations impacted by the failure. Sub-50msec
is thus possible even if millions of BGP prefixes are impacted.
When the failure is remote (a remote IGP failure not impacting the
BGP next-hop or a remote BGP next-hop failure), an alternate pic-
path is activated upon IGP convergence. All the impacted BGP
destinations benefit from a working alternate pic-path as soon as
the IGP convergence occurs for their impacted BGP next-hop even if
millions of BGP pic-routes are impacted.
Appendix A puts the BGP-PIC benefits in perspective by providing
some results using actual numbers.
7.3. Automated
The BGP-PIC solution does not require any operator involvement. The
process is entirely automated as part of the FIB implementation.
The salient points enabling this automation are:
o Extension of the BGP Best path to compute more than one primary
([RFC7911] and [RFC6774]) or backup BGP next-hop ([I.D.ietf-idr-
best-external] and [I-D.pmohapat-idr-fast-conn-restore]).
o Sharing of BGP Pathlist across BGP destinations with the same
primary and backup BGP next-hop.
o Hierarchical indirection and dependency between BGP pathlist and
IGP pathlist.
7.4. Incremental Deployment
As soon as one router supports BGP-PIC solution, it is possible to
benefit from all its benefits (most notably convergence that does
not depend in the number of prefixes) without any requirement for
other routers to support BGP-PIC.
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8. Security Considerations
The behavior described in this document is internal functionality
to a router that result in significant improvement to convergence
time as well as reduction in CPU and memory used by FIB while not
showing change in basic routing and forwarding functionality. As
such no additional security risk is introduced by using the
mechanisms described in this document.
9. IANA Considerations
This document has no IANA actions.
10. References
10.1. Normative References
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4), RFC 4271, January 2006.
[RFC3031] E. Rosen, A. Viswanathan, R. Callon, "Multiprotocol Label
Switching Architecture", RFC 3031, January 2001
10.2. Informative References
[I-D.ietf-idr-best-external] Marques,P., Fernando, R., Chen, E,
Mohapatra, P., Gredler, H., "Advertisement of the best
external route in BGP", draft-ietf-idr-best-external-
05.txt, January 2012.
[RFC5565] Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
Framework", RFC 5565, June 2009.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC4798] De Clercq, J. , Ooms, D., Prevost, S., Le Faucheur, F.,
"Connecting IPv6 Islands over IPv4 MPLS Using IPv6
Provider Edge Routers (6PE)", RFC 4798, February 2007.
[bonaventure] O. Bonaventure, C. Filsfils, and P. Francois.
"Achieving sub-50 milliseconds recovery upon bgp peering
link failures, " IEEE/ACM Transactions on Networking,
15(5):1123-1135, 2007
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[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC, October 2007
[RFC7911] D. Walton, A. Retana, E. Chen, J. Scudder, "Advertisement
of Multiple Paths in BGP", RFC 7911, July 2016
[RFC6774] R. Raszuk, R. Fernando, K. Patel, D. McPherson, K. Kumaki,
"Distribution of diverse BGP paths", RFC 6774, November
2012
[I-D.pmohapat-idr-fast-conn-restore] P. Mohapatra, R. Fernando, C.
Filsfils, and R. Raszuk, "Fast Connectivity Restoration
Using BGP Add-path", draft-pmohapat-idr-fast-conn-restore-
03, Jan 2013
[I-D.ietf-rtgwg-segment-routing-ti-lfa] S. Litkowski, A. Bashandy,
C. Filsfils, P. Francois, B. Decraene, D. Voyer, "Topology
Independent Fast Reroute using Segment Routing", draft-
ietf-rtgwg-segment-routing-ti-lfa-09 (work in progress),
December 2022
[RFC5714] M. Shand and S. Bryant, "IP Fast Reroute Framework", RFC
5714, January 2010
[RFC7490] S. Bryant, C. Filsfils, S. Previdi, M. Shand, N So, "
Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)", RFC
7490 April 2015
[RFC7812] A. Atlas, C. Bowers, G. Enyedi, " An Architecture for
IP/LDP Fast-Reroute Using Maximally Redundant Trees", RFC
7812, June 2016
[RFC8277] E. Rosen, " Carrying Label Information in BGP-4", RFC
8277, October 2017
[RFC8660] A. Bashandy, C. Filsfils, S. Previdi, B. Decraene, S.
Litkowski, M. Horneffer, R. Shakir, "Segment Routing with
MPLS data plane", RFC 8660, December 2019
[RFC9107] R. Raszuk, B. Decraene, C. Cassar, E. Aman, K Wang, " BGP
Optimal Route Reflection (BGP ORR)", RFC9107, August 2021
11. Acknowledgments
Special thanks to Neeraj Malhotra and Yuri Tsier for the valuable
help
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Special thanks to Bruno Decraene, Theresa Enghardt, Ines Robles,
Luc Andre Burdet, and Alvaro Retana for the valuable comments
This document was prepared using 2-Word-v2.0.template.dot.
Authors' Addresses
Ahmed Bashandy
Cisco Systems
Email: abashandy.ietf@gmail.com
Clarence Filsfils
Cisco Systems
Brussels, Belgium
Email: cfilsfil@cisco.com
Prodosh Mohapatra
Sproute Networks
Email: mpradosh@yahoo.com
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Appendix A. Handling Platforms with Limited Levels of Hierarchy
This section provides additional details on how to handle platforms
with limited number of hierarchical levels.
Let's consider a pathlist associated with the leaf "R1" consisting
of the list of pic-paths <P1, P2,..., Pn>. Assume that the leaf "R1"
has an OutLabel-list <L1, L2,..., Ln>. Suppose the pic-path Pi is a
recursive pic-path that resolves via a prefix represented by the
leaf "R2". The leaf "R2" itself is pointing to a pathlist consisting
of the pic-paths <Q1, Q2,..., Qm>.
If the platform supports the number of hierarchy levels of the
forwarding chain, then a packet that uses the pic-path "Pi" will be
forwarded according to the steps in Section 4.
Suppose the platform cannot support the number of hierarchy levels
in the forwarding chain. FIB manager needs to reduce the number of
hierarchy levels when programming the forwarding chain in the FIB.
The idea of reducing the number of hierarchy levels is to "flatten"
two chain levels into a single level. The "flattening" steps are as
follows
1. FIB manager walks to the parent of "Pi", which is the leaf "R2".
2. FIB manager extracts the parent pathlist of the leaf "R2", which
is <Q1, Q2,..., Qm>.
3. FIB manager also extracts the OutLabel-list of R2 associated with
the leaf "R2". Remember that the OutLabel-list of R2 is <L1,
L2,..., Lm>.
4. FIB manager replaces the pic-path "Pi", with the list of pic-
paths <Q1, Q2,..., Qm>.
5. Hence the pic-path list <P1, P2,..., Pn> now becomes "<P1,
P2,...,Pi-1, Q1, Q2,..., Qm, Pi+1, Pn>.
6. The path-index stored inside the locations "Q1", "Q2", ..., "Qm"
must all be "i" because the index "i" refers to the label "Li"
associated with leaf "R1".
7. FIB manager attaches an OutLabel-list with the new pathlist as
follows: <Unlabeled,..., Unlabeled, L1, L2,..., Lm, Unlabeled,
..., Unlabeled>. The size of the label list associated with the
flattened pathlist equals the size of the pathlist. Thus there is
a 1-1 mapping between every pic-path in the "flattened" pathlist
and the OutLabel-list associated with it.
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It is noteworthy to mention that the labels in the OutLabel-list
associated with the "flattened" pathlist may be stored in the same
memory location as the pic-path itself to avoid additional memory
access.
The same steps can be applied to all pic-paths in the pathlist <P1,
P2,..., Pn> so that all pic-paths are "flattened" thereby reducing
the number of hierarchical levels by one. Note that that
"flattening" a pathlist pulls in all pic-paths of the parent pic-
paths, a desired feature to utilize all pic-paths at all levels. A
platform that has a limit on the number of pic-paths in a pathlist
for any given leaf may choose to reduce the number pic-paths using
methods that are beyond the scope of this document.
The steps can be recursively applied to other pic-paths at the same
levels or other levels to recursively reduce the number of
hierarchical levels to an arbitrary value so as to accommodate the
capability of the forwarding engine.
Because a flattened pathlist may have an associated OutLabel-list
the forwarding behavior has to be slightly modified. The
modification is done by adding the following step right after step 4
in Section 4.
5. If there is an OutLabel-list associated with the pathlist, then
if the pic-path "Pi" is chosen by the hashing algorithm, retrieve
the label at location "i" in that OutLabel-list and apply the
label action of that label on the packet.
The steps in this Section to are applied to an example in the next
Section.
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Appendix B. Example: Flattening a forwarding chain.
This example uses a case of inter-AS option C [RFC4364] where there
are 3 levels of hierarchy. Figure 4 illustrates the sample topology.
The Autonomous System Border Routers (ASBRs) on the ingress domain
(Domain 1) use BGP to advertise the core routers (ASBRs and ePEs) of
the egress domain (Domain 2) to the iPE. The end result is that the
ingress PE (iPE) has 2 levels of recursion for the VPN prefixes VPN-
IP1 and VPN-IP2.
Domain 1 Domain 2
+-------------+ +-------------+
| | | |
| LDP/SR Core | | LDP/SR core |
| | | |
| (192.0.2.4) | |
| ASBR11-------ASBR21........ePE1(192.0.2.1)
| | \ / | . . |\
| | \ / | . . | \
| | \ / | . . | \
| | \/ | .. | \VPN-IP1(198.51.100.0/24)
| | /\ | . . | /VRF "Blue" ASN: 65000
| | / \ | . . | /
| | / \ | . . | /
| | / \ | . . |/
iPE ASBR12-------ASBR22........ePE2 (192.0.2.2)
| (192.0.2.5) | |\
| | | | \
| | | | \
| | | | \VRF "Blue" ASN: 65000
| | | | /VPN-IP2(203.0.113.0/24)
| | | | /
| | | | /
| | | |/
| ASBR13-------ASBR23........ePE3(192.0.2.3)
| (192.0.2.6) | |
| | | |
| | | |
+-------------+ +-------------+
<=========== <========= <============
Advertise ePEx Advertise Redistribute
Using IBGP-LU ePEx Using ePEx routes
EBGP-LU into BGP
Figure 4: Sample 3-level hierarchy topology
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The following assumptions about connectivity are made:
o In "Domain 2", both ASBR21 and ASBR22 can reach both ePE1 and
ePE2 using the same metric.
o In "Domain 2", only ASBR23 can reach ePE3.
o In "Domain 1", iPE (the ingress PE) can reach ASBR11, ASBR12, and
ASBR13 via IGP using the same metric.
The following assumptions are made about the labels:
o The VPN labels advertised by ePE1 and ePE2 for prefix VPN-IP1 are
VPN-L11 and VPN-L21, respectively.
o The VPN labels advertised by ePE2 and ePE3 for prefix VPN-IP2 are
VPN-L22 and VPN-L32, respectively.
o The labels advertised by ASBR11 to iPE using BGP-LU for the
egress PEs ePE1 and ePE2 are LASBR111(ePE1) and LASBR112(ePE2),
respectively.
o The labels advertised by ASBR12 to iPE using BGP-LU for the
egress PEs ePE1 and ePE2 are LASBR121(ePE1) and LASBR122(ePE2),
respectively.
o The label advertised by ASBR13 to iPE using BGP-LU for the egress
PE ePE3 is LASBR13(ePE3).
o The IGP labels advertised by the next hops directly connected to
iPE towards ASBR11, ASBR12, and ASBR13 in the core of domain 1
are IGP-L11, IGP-L12, and IGP-L13, respectively.
o Both the routers ASBR21 and ASBR22 of Domain 2 advertise the same
label LASBR21 and LASBR22 for the egress PEs ePE1 and ePE2,
respectively, to the routers ASBR11 and ASBR22 of Domain 1.
o The router ASBR23 of Domain 2 advertises the label LASBR23 for
the egress PE ePE3 to the router ASBR13 of Domain 1.
Based on these connectivity assumptions and the topology in Figure
4, the routing table on iPE is
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65000: 198.51.100.0/24
via ePE1 (192.0.2.1), VPN Label: VPN-L11
via ePE2 (192.0.2.2), VPN Label: VPN-L21
65000: 203.0.113.0/24
via ePE2 (192.0.2.2), VPN Label: VPN-L22
via ePE3 (192.0.2.3), VPN Label: VPN-L32
192.0.2.1/32 (ePE1)
via ASBR11, BGP-LU Label: LASBR111(ePE1)
via ASBR12, BGP-LU Label: LASBR121(ePE1)
192.0.2.2/32 (ePE2)
via ASBR11, BGP-LU Label: LASBR112(ePE2)
via ASBR12, BGP-LU Label: LASBR122(ePE2)
192.0.2.3/32 (ePE3)
Via ASBR13, BGP-LU Label: LASBR13(ePE3)
192.0.2.4/32 (ASBR11)
via Core, Label: IGP-L11
192.0.2.5/32 (ASBR12)
via Core, Label: IGP-L12
192.0.2.6/32 (ASBR13)
via Core, Label: IGP-L13
The diagram in Figure 5 illustrates the forwarding chain in iPE
assuming that the forwarding hardware in iPE supports 3 levels of
hierarchy. The leaves corresponding to the ASBRs on domain 1
(ASBR11, ASBR12, and ASBR13) are at the bottom of the hierarchy.
There are few important points:
o Because the hardware supports the required depth of hierarchy,
the sizes of a pathlist equal the size of the label list
associated with the leaves using this pathlist.
o The path-index inside the pathlist entry indicates the label that
will be picked from the OutLabel-List associated with the child
leaf if that pic-path is chosen by the forwarding engine hashing
function.
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OutLabel-List OutLabel-List
For VPN-IP1 For VPN-IP2
+------------+ +--------+ +-------+ +------------+
| VPN-L11 |<---| VPN-IP1| |VPN-IP2|-->| VPN-L22 |
+------------+ +---+----+ +---+---+ +------------+
| VPN-L21 | | | | VPN-L32 |
+------------+ | | +------------+
| |
V V
+---+---+ +---+---+
| 0 | 1 | | 0 | 1 |
+-|-+-\-+ +-/-+-\-+
| \ / \
| \ / \
| \ / \
| \ / \
v \ / \
+-----+ +-----+ +-----+
+----+ ePE1| |ePE2 +-----+ | ePE3+-----+
| +--+--+ +-----+ | +--+--+ |
v | / v | v
+--------------+ | / +--------------+ | +-------------+
|LASBR111(ePE1)| | / |LASBR112(ePE2)| | |LASBR13(ePE3)|
+--------------+ | / +--------------+ | +-------------+
|LASBR121(ePE1)| | / |LASBR122(ePE2)| | OutLabel-List
+--------------+ | / +--------------+ | For ePE3
OutLabel-List | / OutLabel-List |
For ePE1 | / For ePE2 |
| / |
| / |
| / |
v v v
+---+---+ Shared pathlist +---+ pathlist
| 0 | 1 | For ePE1 and ePE2 | 0 | For ePE3
+-|-+-\-+ +-|-+
| \ |
| \ |
| \ |
| \ |
v v v
+------+ +------+ +------+
+---+ASBR11| |ASBR12+--+ |ASBR13+---+
| +------+ +------+ | +------+ |
v v v
+-------+ +-------+ +-------+
|IGP-L11| |IGP-L12| |IGP-L13|
+-------+ +-------+ +-------+
Figure 5: Forwarding Chain for hardware supporting 3 Levels
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Now suppose the hardware on iPE (the ingress PE) supports 2 levels
of hierarchy only. In that case, the 3-levels forwarding chain in
Figure 5 needs to be "flattened" into 2 levels only.
OutLabel-List OutLabel-List
For VPN-IP1 For VPN-IP2
+------------+ +-------+ +-------+ +------------+
| VPN-L11 |<---|VPN-IP1| | VPN-IP2|--->| VPN-L22 |
+------------+ +---+---+ +---+---+ +------------+
| VPN-L21 | | | | VPN-L32 |
+------------+ | | +------------+
| |
| |
| |
Flattened | | Flattened
pathlist V V pathlist
+===+===+ +===+===+===+ +==============+
+--------+ 0 | 1 | | 0 | 0 | 1 +---->|LASBR112(ePE2)|
| +=|=+=\=+ +=/=+=/=+=\=+ +==============+
v | \ / / \ |LASBR122(ePE2)|
+==============+ | \ +-----+ / \ +==============+
|LASBR111(ePE1)| | \/ / \ |LASBR13(ePE3) |
+==============+ | /\ / \ +==============+
|LASBR121(ePE1)| | / \ / \
+==============+ | / \ / \
| / \ / \
| / + + \
| + | | \
| | | | \
v v v v v
+------+ +------+ +------+
+----|ASBR11| |ASBR12+---+ |ASBR13+---+
| +------+ +------+ | +------+ |
v v v
+-------+ +-------+ +-------+
|IGP-L11| |IGP-L12| |IGP-L13|
+-------+ +-------+ +-------+
Figure 6: Flattening 3 levels to 2 levels of Hierarchy on iPE
Figure 6 represents one way to "flatten" a 3 levels hierarchy into
two levels. There are a few important points:
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o As mentioned in Section Appendix A, a flattened pathlist may have
label lists associated with them. The size of the label list
associated with a flattened pathlist equals the size of the
pathlist. Hence it is possible that an implementation includes
these label lists in the flattened pathlist itself.
o Again as mentioned in Section Appendix A, the size of a flattened
pathlist may not be equal to the size of the OutLabel-lists of
leaves using the flattened pathlist. So the indices inside a
flattened pathlist still indicate the label index in the
OutLabel-Lists of the leaves using that pathlist. Because the
size of the flattened pathlist may be different from the size of
the OutLabel-lists of the leaves, the indices may be repeated.
o Let's take a look at the flattened pathlist used by the prefix
"VPN-IP2". The pathlist associated with the prefix "VPN-IP2" has
three entries.
o The first and second entry have index "0". This is because
both entries correspond to ePE2. Thus when hashing performed
by the forwarding engine results in using the first or the
second entry in the pathlist, the forwarding engine will pick
the correct VPN label "VPN-L22", which is the label advertised
by ePE2 for the prefix "VPN-IP2".
o The third entry has the index "1". This is because the third
entry corresponds to ePE3. Thus when the hashing is performed
by the forwarding engine results in using the third entry in
the flattened pathlist, the forwarding engine will pick the
correct VPN label "VPN-L32", which is the label advertised by
"ePE3" for the prefix "VPN-IP2".
Now let's try and apply the forwarding steps in Section 4 together
with the additional step in Section Appendix A to the flattened
forwarding chain illustrated in Figure 6.
o Suppose a packet arrives at "iPE" and matches the VPN prefix
"VPN-IP2".
o The forwarding engine walks to the parent of the "VPN-IP2", which
is the flattened pathlist and applies a hashing algorithm to pick
a pic-path.
o Suppose the hashing by the forwarding engine picks the second
pic-path in the flattened pathlist associated with the leaf "VPN-
IP2".
o Because the second pic-path has the index "0", the label "VPN-
L22" is pushed on the packet.
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o Next the forwarding engine picks the second label from the
OutLabel-List associated with the flattened pathlist resulting in
"LASBR122(ePE2)" being the next pushed label.
o The forwarding engine now moves to the parent of the flattened
pathlist corresponding to the second pic-path. The parent is the
IGP label leaf corresponding to "ASBR12".
o So the packet is forwarded towards the ASBR "ASBR12" and the IGP
label at the top will be "IGP-L12".
Based on the above steps, a packet arriving at iPE and destined to
the prefix VPN-L22 reaches its destination as follows:
o iPE sends the packet along the shortest pic-path towards ASBR12
with the following label stack starting from the top: {L12,
LASBR122(ePE2), VPN-L22}.
o The penultimate hop of ASBR12 pops the top label "L12". Hence the
packet arrives at ASBR12 with the remaining label stack
{LASBR122(ePE2), VPN-L22} where "LASBR12(ePE2)" is the top label.
o ASBR12 swaps "LASBR122(ePE2)" with the label "LASBR22(ePE2)",
which is the label advertised by ASBR22 for the ePE2 (the egress
PE).
o ASBR22 receives the packet with "LASBR22(ePE2)" at the top.
o Hence ASBR22 swaps "LASBR22(ePE2)" with the IGP label for ePE2
advertised by the next-hop towards ePE2 in domain 2, and sends
the packet along the shortest pic-path towards ePE2.
o The penultimate hop of ePE2 pops the top label. Hence ePE2
receives the packet with the top label VPN-L22 at the top
o ePE2 pops "VPN-L22" and sends the packet as a pure IP packet
towards the destination VPN-IP2.
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Appendix C. Perspective
The following table puts the BGP-PIC benefits in perspective
assuming
o 1M impacted BGP prefixes
o IGP convergence ~ 500 msec
o local protection ~ 50msec
o FIB Update per BGP destination ~ 100usec conservative,
~ 10usec optimistic
o BGP best route recalculation per BGP destination
~ 10usec optimistic,
~ 100usec optimistic
Without PIC With PIC
Local IGP Failure 10 to 100sec 50msec
Local BGP Failure 100 to 200sec 50msec
Remote IGP Failure 10 to 100sec 500msec
Local BGP Failure 100 to 200sec 500msec
Upon local IGP next-hop failure or remote IGP next-hop failure, the
existing primary BGP next-hop is intact and usable hence the
resiliency only depends on the ability of the FIB mechanism to
reflect the new pic-path to the BGP next-hop to the depending BGP
destinations. Without BGP-PIC, a conservative back-of-the-envelope
estimation for this FIB update is 100usec per BGP destination. An
optimistic estimation is 10usec per entry.
Upon local BGP next-hop failure or remote BGP next-hop failure,
without the BGP-PIC mechanism, a new BGP Best-Path needs to be
recomputed and new updates need to be sent to peers. This depends on
BGP processing time that will be shared between best-path
computation, RIB update and peer update. A conservative back-of-the-
envelope estimation for this is 200usec per BGP destination. An
optimistic estimation is 100usec per entry.
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