Internet DRAFT - draft-bashandy-bgp-edge-node-frr
draft-bashandy-bgp-edge-node-frr
Network Working Group A. Bashandy
Internet Draft B. Pithawala
Intended status: Standards Track K. Patel
Expires: January 2013 Cisco Systems
July 16, 2012
Scalable BGP FRR Protection against Edge Node Failure
draft-bashandy-bgp-edge-node-frr-03.txt
Abstract
Consider a BGP free core scenario. Suppose the edge BGP speakers PE1,
PE2,..., PEn know about a prefix P/m via the external routers CE1,
CE2,..., CEm. If the edge router PEi crashes or becomes totally
disconnected from the core, it is desirable for a core router "P"
carrying traffic to the failed edge router PEi to immediately restore
traffic by re-tunneling packets originally tunneled to PEi and
destined to the prefix P/m to one of the other edge routers that
advertised P/m, say PEj, until BGP re-converges. In doing so, it is
highly desirable to keep the core BGP-free while not imposing
restrictions on external connectivity. Thus (1) a core router should
not be required to learn any BGP prefix, (2) the size of the
forwarding and routing tables in the core routers should be
independent of the number of BGP prefixes,(3) provisioning overhead
should be kept at minimum, (4) re-routing traffic without waiting for
re-convergence must not cause loops, and (4) there should be no
restrictions on what edge routers advertise what prefixes. For
labeled prefixes, (6) the label stack on the packet must allow the
repair PEj to correctly forward the packet and (7) there must not be
any need to perform more than one label lookup on any edge or core
router during steady state
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Table of Contents
1. Introduction...................................................3
1.1. Conventions used in this document.........................4
1.2. Terminology...............................................5
1.3. Problem definition........................................6
2. Overview of the solution in an MPLS Core.......................7
2.1. Control Plane operation for Automated pNH Assignment......7
2.2. Control Plane operation for Configured pNH...............10
2.3. Forwarding behavior at Steady State (When pPE is reachable)11
2.4. Forwarding behavior when pPE Fails.......................12
3. Overview of the solution in a Pure IP Core....................13
3.1. Control Plane operation..................................13
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3.2. Forwarding Behavior at Steady State (while pPE is reachable)
..............................................................13
3.3. Forwarding Behavior at Failure (when pPE is not reachable)14
4. Example.......................................................15
4.1. Control Plane............................................16
4.2. Forwarding Plane at Steady State (When PE0 is reachable).16
4.3. Forwarding Plane at Failure (When PE0 is not reachable)..17
5. Inter-operability with Existing IP FRR Mechanisms.............19
6. Security Considerations.......................................19
7. IANA Considerations...........................................19
8. Conclusions...................................................19
9. References....................................................20
9.1. Normative References.....................................20
9.2. Informative References...................................21
10. Acknowledgments..............................................21
Appendix A. How to protect Against Misconfigured pNH.............22
Appendix B. Alternative Approach for advertising (pNH,rNH) to iPE23
Appendix C. Modification History.................................24
A.1.1. Changes from Version 02.............................24
A.1.2. Changes from Version 01.............................24
1. Introduction
In a BGP free core, where traffic is tunneled between edge routers,
BGP speakers advertise reachability information about prefixes to
other edge routers not to core routers. For labeled address
families, namely AFI/SAFI 1/4, 2/4, 1/128, and 2/128, an edge
router assigns local labels to prefixes and associates the local
label with each advertised prefix such as L3VPN [10], 6PE [11], and
Softwire [9]. Suppose that a given edge router is chosen as the
best next-hop for a prefix P/m. An ingress router that receives a
packet from an external router and destined to the prefix P/m
"tunnels" the packet across the core to that egress router. If the
prefix P/m is a labeled prefix, the ingress router pushes the label
advertised by the egress router before tunneling the packet to the
egress router. Upon receiving the packet from the core, the egress
router takes the appropriate forwarding decision based on the
content of the packet or the label pushed on the packet.
In modern networks, it is not uncommon to have a prefix reachable
via multiple edge routers. One example is the best external path
[8]. Another more common and widely deployed scenario is L3VPN [10]
with multi-homed VPN sites. As an example, consider the L3VPN
topology depicted in Figure 1.
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PE1 .............+
|
+--------+---------------+
| |
| VPN 1 Network |
| |
| VPN prefix |
| (10.0.0.0/8) |
| |
+---+--------------------+
|
/------CE1
/
/
BGP-free core P--------PE0
\
\
\------CE2
|
+---+--------------------+
| |
| VPN 2 Network |
| |
| VPN prefix |
| (20.0.0.0/8) |
| |
+--------+---------------+
|
PE2 .............+
Figure 1 VPN prefix reachable via multiple PEs
As illustrated in Figure 1, the edge router PE0 is the primary NH
for both 10.0.0.0/8 and 20.0.0.0/8. At the same time, both
10.0.0.0/8 and 20.0.0.0/8 are reachable through the other edge
routers PE1 and PE2, respectively.
1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC-2119 [1].
In this document, these words will appear with that interpretation
only when in ALL CAPS. Lower case uses of these words are not to be
interpreted as carrying RFC-2119 significance.
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1.2. Terminology
This section defines the terms used in this document. For ease of
use, we will use terms similar to those used by L3VPN [10]
o BGP-Free core: A network where BGP prefixes are only known to
the edge routers and traffic is tunneled between edge routers
o External prefix: It is a prefix P/m (of any AFI/SAFI) that a BGP
speaker has an external path for. The BGP speaker may learn
about the prefix from an external peer through BGP, some other
protocol, or manual configuration. The protected prefix is
advertised to some or all of the internal peers.
o Protectable prefix: It is an external prefix P/m of any
AFI/SAFI) that a BGP speaker has an external path to and is
eligible to have a repair path.
o Primary Egress PE, "ePE": It is an IBGP peer that can reach the
prefix P/m through an external path and advertised the prefix to
the other IBGP peers. The primary egress PE was chosen as the
best path by one or more internal peers. In other words, the
primary egress PE is an egress PE that will normally be used by
some ingress PEs when there is no failure. Referring to Figure
1, PE0 is an egress PE.
o Protected egress PE, "pPE" (Protected PE for simplicity): It is
an egress PE that has or eligible to have a repair path for some
or all of the prefixes to which it has an external path
Referring to Figure 1, PE0 is a protected egress PE.
o Protected edge router: Any protected egress PE.
o Protected next-hop (pNH): It is an IPv4 or IPv6 host address
belonging to the protected egress PE. Traffic tunneled to this
IP address will be protected via the mechanism proposed in this
document. Note that the protected next-hop MUST be different
from the next-hop attribute in the BGP update message [2][3].
o CE: It is an external router through which an egress PE can
reach a prefix P/m. The routers "CE1" and "CE2" in Figure 1 are
examples of such CEs.
o Ingress PE, "iPE": It is a BGP speaker that learns about a
prefix through another IBGP peer and chooses that IBGP peer as
the next-hop for the prefix.
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o Repairing P router "rP" (Also "Repairing core router" and
"repairing router"): A core router that attempts to restore
traffic when the primary egress PE is no longer reachable
without waiting for IGP or BGP to re-converge. The repairing P
router restores the traffic by rerouting the traffic (through a
tunnel) towards the pre-calculated repair PE when it detects
that the primary egress PE is no longer reachable. Referring to
Figure 1, the router "P" is the repairing P router.
o Repair egress PE "rPE" (Repair PE for simplicity): It is an
egress PE other than the primary egress PE that can reach the
protected prefix P/m through an external neighbor. The repair PE
is pre-calculated prior to any failure. Referring to Figure 1,
PE1 is the repair PE for 10.0.0.0/8 while PE2 is the repair PE
for 20.0.0.0/8.
o Underlying Repair label (rL): The underlying repair label is the
label that will be pushed so that the repair PE can forward
repaied traffic correctly. A repair label is defined for labeled
protected prefixes only.
o Repair next-hop (rNH): It is an IPv4 or IPv6 host address
belonging to the repair egress PE. If the protected prefix is
advertised via BGP, then the repair next-hop SHOULD be the next-
hop attribute in the BGP update message [2][3].
o Repair path (Also Repair Egress Path): It is the repair next-
hop. If an underlying repair label exists, the repair path is
the repair next-hop together with the underlying repair label.
o Primary tunnel: It is the tunnel from the ingress PE to the
primary egress PE
o Repair tunnel: It is the tunnel from the repairing P router to
the repair egress PE
1.3. Problem definition
The problem that we are trying to solve is as follows
o Even though multiple prefixes may share the same egress router,
they have different repair edge router. In Figure 1 above, both
10.0.0.0/8 and 20.0.0.0/8 share the same primary next hop PE0,
the routing protocol(s) must identify that the node protecting
repair node for 10.0.0.0/8 is PE1 while the node protecting
repair node for 11.0.0.0/8 is PE2
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o On loosing connection to the edge router, the core router "P"
MUST reroute traffic towards the *correct* repair edge router
without waiting for IGP or BGP to re-converge and update the
routing tables. On the failure of PE0 illustrated in Figure 1,
the core router P needs to reroute traffic for 10.0.0.0/8
towards PE1 and traffic for 11.0.0.0/8 towards PE2
o The repairing core router P MUST NOT be forced to learn about
the BGP prefixes on any of the edge router. The same applies for
all core routers.
o The size of the routing table on any core router MUST be
independent of the number of BGP prefixes in the network.
o Rerouting traffic without waiting for IGP and BGP to re-converge
after a failure MUST NOT cause loops.
o For labeled prefixes, when a packet gets re-routed to the repair
PE, the label stack on the packet MUST ensure correct
forwarding.
o Provisioning overhead must be kept at minimum. In addition,
misconfiguration should be detectable.
o At steady state, when pPE is reachable, a path taken by traffic
flow must not be impacted by enabling the solution proposed in
this document on some or all routers
2. Overview of the solution in an MPLS Core
The solution proposed in this document relies on the collaboration of
egress PE, ingress PE, penultimate hop routers, and repairing router.
This section gives an overview of how to the solution works for
labeled and unlabeled protected prefixes in an MPLS core.
2.1. Control Plane operation for Automated pNH Assignment
This section outlines the solution for the case where the protected
next hop "pNH" is automatically calculated instead of being assigned
by an operator.
1. Each egress router that is capable of handling repaired traffic
assigns each protectable labeled prefix a repair label: "rL". "rL"
is advertised as optional path attribute. "rL" MUST be per-CE or
per-VRF for good BGP attribute packing and forwarding simplicity.
For unlabeled prefix, no repair label is needed. A router that is
capable of handling repaired traffic is called a repair PE "rPE".
The semantics of the repair label "rL" is:
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a. pop *two* labels
b. If "rL" is per-CE, then and send the packet to the appropriate
CE
c. If "rL" is per-VRF, forward the packet based on the contents
under the two popped labels
2. If an Egress PE knows that a P/m to which it has an external path
is also reachable via another PE and that other PE advertises a
repair label "rL" for P/m,
a. It chooses the other PE as a repair PE. Let's call the chosen
repair PE "rPE". The ePE chooses an IP address "rNH" local to
or advertised by rPE.
i. "rNH" SHOULD be the next-hop attribute advertised by rPE
when it announces reachability to the protected prefix
P/m to minimize the number of prefixes advertised into
IGP.
ii. if rPE also advertised a protected next-hop (pNH) for any
BGP prefix that rPE can protect, then rNH MUST NOT be any
protected next-hop (pNH) advertised by rPE.
b. Allocates a local IP address corresponding to the chosen rPE,
say "pNH". "pNH" represents the protected NH. I.e. Traffic
tunneled to "pNH" will be protected against edge node failure
via the BGP FRR mechanism proposed in this document
c. A separate pNH is needed for every rPE (for a given protected
PE). Each pNH must be unique within a single BGP-free core.
d. Now that "ePE" has a repair path for P/m, it becomes a
protected PE "pPE".
e. Advertise pNH as a prefix into IGP
f. Re-advertise the protected prefix P/m to other iBGP peers with
"pNH" as optional non-transitive attribute
g. pPE advertises the mapping (pNH,rNH) separately to all ingress
PEs. A method analogous to how tunnel information is
advertised [4] can be used to advertise this mapping (pNH,rNH)
to ingress PE's.
h. Once iPE receives the pNH for each prefix and the mapping
(pNH,rNH), the iPE can retrieve "rL" for P/m from the
advertisement of rPE for P/m.
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i. "pPE" advertises the pair (pNH,rNH) to candidate repairing
core routers.
j. "pPE" advertises the protected next-hop "pNH" to the
penultimate hops to indicate that traffic flowing through the
tunnel to the tail end "pNH" is protected against the failure
of the node "pPE" and requires special processing by the
penultimate hop as will be described in the next few steps
k. pPE advertises an explicit label for pNH instead of the usual
implicit NULL. This way pPE can carry out the special label
popping behavior (described in the next section if the
penultimate hop cannot perform this task
3. Ingress PE "iPE"
a. iPE receives the protected prefix P/m with "pNH" as an
optional attribute
b. iPE also receives the mapping (pNH,rNH) from pPE
c. When iPE receives "rL" with P/m from rPE, then iPE can
associate "rL" with P/m as described in Section 2.1.
As a result of the above steps, the following nodes store the
following information
o Ingress PE (iPE)
o Receives from pPE NLRI advertisement for the protected labeled
prefix P/m containing the usual next-hop attribute and the
optional information "pNH". iPE also receives that mapping
(pNH, rNH).
o iPE retrieves "rL" from the advertisement of rPE for the
protected prefix P/m.
o Assume that iPE chooses pPE as the primary NH. Then the iPE
will use pNH as the tunnel tail end to pPE instead of the
usual BGP next-hop
o Penultimate Hop
o Receives the "pNH" from pPE
o As such, it knows that traffic destined to pNH needs certain
special forwarding treatment as described in the next few
steps
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o Penultimate hop advertises "pNH" as its own prefix but with
one of the following conditions
. For link-state IGPs, "pNH" MAY be advertised with
*maximum metric* so as not to affect the path taken by
the traffic flowing from iPE's to pPE's
. For distance vector IGPs, the penultimate hop MAY
advertise the metric of "pNH" as follows
PHP-metric(pNH) =
pPE-metric(pNH) + metric-From-PHP-to-pPE
That is the metric advertised by the penultimate hop for
pNH equals the metric advertised by pPE for pNH plus the
metric from the penultimate hop to pPE
. This way the advertisement of pNH by the penultimate hop
does not impact the path taken by the traffic from iPE's
to pPE's
o Repairing core router "rP" (which may also be the penultimate hop)
o Receives the pair (pNH,rNH) from pPE
o Installs the following forwarding entry for pNH
. If pNH is not reachable, re-tunnel traffic to rNH
2.2. Control Plane operation for Configured pNH
In Section 2.1, the pPE assigned pNH to a protected prefix P/m
based on the chosen rPE. The result of this behavior is the need to
re-advertise the protected prefix P/m with the associated "pNH". In
this section, we outline the procedure by which the operator can pre-
assign pNH to protected prefixes and hence avoid the need to re-
advertise protected prefixes.
1. Protected PE "pPE"
a. The operator groups prefixes such that two prefixes belong to
the same group if the operator knows that the two prefixes are
protected by the same rPE
b. The operator assigns a distinct protected next-hop "pNH" for
every group of prefixes. The assignment occurs even a repair
path for P/m is not yet known.
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c. pPE advertises "pNH" as an optional non-transitive attribute
with the protected prefix P/m *all the time* even of no other
PE advertises P/m
d. When pPE receives an advertisement for P/m from another PE
i. pPE chooses the other PE as rPE
ii. pPE advertises the mapping (pNH,rNH) separately to all
ingress PEs. rNH SHOULD be the next-hop attribute
advertised by rPE. A method analogous to how tunnel
information is advertised [4] can be used to advertise
this mapping (pNH,rNH) to ingress PE's.
e. The rest of the behavior is identical to what specified in
Section 2.1.
2. How to Protect the network against misconfigured pNH?
See Appendix A.
What is left it to outline the forwarding behavior before and after
"pPE" failure.
2.3. Forwarding behavior at Steady State (When pPE is reachable)
This section outlines the packet forwarding procedure when pPE is
still reachable
1. Ingress PE (iPE) receives a packet matching P/m and reachable via
pPE
2. The iPE pushes three labels:
o Bottom label: VPN label advertised by pPE
o Second label: rL
o Top label: IGP label towards pNH (not the BGP next-hop
attribute)
3. Penultimate Hop
a. Receives a packet with top label bound to pNH
b. Pops *two* labels *all the time*.
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c. Sends packet to pNH
4. Protected PE (pPE)
a. Receives a packet with top label as VPN label
b. Forwards the packet as usual
c. For unlabeled packets, the iPE only pushes the rL and the IGP
label of pNH and the pPE uses the IP header for forwarding.
Thus the packet can be delivered correctly to its destination.
2.4. Forwarding behavior when pPE Fails
The repairing core router directly connected to a failure detects
that pNH is no longer reachable. The following steps are applied.
1. Repairing core router "rP"
a. Receives packet with top label bound to pNH
b. pNH is not reachable
c. Swap the top label with the label of rNH
d. Send packet towards rPE
In effect, the repairing router re-tunnels the packet towards
the repair PE
2. Penultimate hop of rPE
a. rNH is not a protected NH for rPE
b. Thus the penultimate hop employs the usual penultimate hop
popping and then forwards the packet to rPE
3. Repair PE (rPE)
a. Receives packet with top label rL (which rPE advertised) and
underneath it the regular VPN label advertised by the
protected PE "pPE"
b. Make a lookup on "rL"
c. rL per CE
i. Pop *two* labels.
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ii. Send to correct CE
d. rL per VRF
i. Pop *two* labels.
ii. Make IP lookup in appropriate VRF
iii. Send to the CE
e. rL is assigned to unlabeled prefix
i. Pop "rL"
ii. Send the packet to the correct CE
3. Overview of the solution in a Pure IP Core
This section provides an overview of the solution when operating in a
pure IP core where core routers only understand IPv4 or IPv6
protocols. Thus traffic between PEs is transported using IP tunnels
such as [4][6][7].
3.1. Control Plane operation
The control plane behavior in an IP core is identical to its behavior
in an MPLS core.
3.2. Forwarding Behavior at Steady State (while pPE is reachable)
1. Ingress PE (iPE) receives a packet matching P/m and reachable via
pPE
2. Ingress PE:
o For labeled traffic, Pushes two labels
. Bottom label: VPN label advertised by pPE
. Second label: rL
o For unlabeled traffic, just push "rL"
o Encapsulates the packet into the IP tunnel header towards the
pNH
3. Penultimate Hop
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o No special behavior is needed from the penultimate hop while
pPE is reachable
4. Protected PE
a. Receives an IP packet encapsulated in an IP tunnel header with
destination address pNH
b. Decapsulate the IP tunnel header and the label right under it
(which will be the repair label "rL")
c. For labeled traffic, the VPN label is exposed. So pPE makes a
lookup using the VPN label. Otherwise the usual IP forwarding
is applied
d. Forwards the packet as usual
3.3. Forwarding Behavior at Failure (when pPE is not reachable)
The repairing router directly connected to a failure detects that pNH
is no longer reachable. The following steps are applied.
1. Repairing router "rP"
a. Receives IP packet with a tunnel header destined to pNH
b. pNH is not reachable
c. Replace the tunnel header with a tunnel header with
destination address rNH
d. Forward the packet to rNH
2. Repair PE (rPE)
a. Receives IP packet with a tunnel header destined to rNH
b. Decapsulate the tunnel header to expose the repair label "rL"
c. The rest of the behavior is identical to the behavior in an
MPLS Core.
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4. Example
We will use an LDP core as an example. Consider the diagram
depicted in Figure 2 below.
+-----------------------------------+
| |
| LDP Core |
| |
| PE1 Lo = 9.9.9.1
| |\
| | \
| | \
| | \
| | CE1.......VRF "Blue"
| | / (10.0.0.0/8)
| | / (11.0.0.0/8)
| | /
| |/
PE11 P--------PE0 Lo1 = 1.1.1.1/32
| |\ pNH Range = 2.1.1.0/24
| | \
| | \
| | \
| | CE2.......VRF "Red"
| | / (20.0.0.0/8)
| | / (21.0.0.0/8)
| | /
| |/
| PE2 Lo = 9.9.9.2
| |
| |
+-----------------------------------+
Figure 2 : Edge node BGP FRR in LDP core
o In Figure 2, PE0 is the pPE for VRFs "Blue" and "Red" while PE1
and PE2 are the rPEs for VRFs "Blue" and "Red", respectively. VRF
Blue has 10.0.0.0/8 and 11.0.0.0/8 and VRF Red has 20.0.0.0/8 and
21.0.0.0/8
o Assuming PE0 uses per prefix label allocation, PE0 assigns the VPN
labels 4100, 4200, 4300, and 4400 to 10.0.0.0/8, 11.0.0.0/8,
20.0.0.0/8, and 21.0.0.0/8 respectively. PE0 advertises the
prefixes 10.0.0.0/8, 11.0.0.0/8, 20.0.0.0/8, and 21.0.0.0/8 using
MP/BGP as usual
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4.1. Control Plane
1. rPEs Allocate and advertise Repair labels
a. Acting as a rPE, PE1 allocates (on per-CE basis) and
advertises a repair label rL1=3100 with the prefixes
10.0.0.0/8 and 11.0.0.0/8 to all iBGP peers
b. Similarly, PE2 allocates and advertises the repair label
rL2=3200 with the prefixes 20.0.0.0/8 and 21.0.0.0/8
2. pPE calculates and advertises the pNHs
a. For prefixes belonging to VRF "blue", PE0 allocates
rNH1=2.1.1.1 because all of them are protected by PE1
b. Similarly, for prefixes belonging to VRF "red", PE0
allocates rNH2=2.1.1.2 because VRF "red" is protected by PE2
c. PE0 advertises (pNH1,rNH1)=(2.1.1.1, 9.9.9.1) and
(pNH2,rNH2)=(2.1.1.2, 9.9.9.2) to the ingress PE PE11 and
the repairing core router "P".
d. PE0 re-advertises 10.0.0.0/8 & 11.0.0.0/8 with the optional
attribute pNH1=2.1.1.1, and 20.0.0.0/8 & 21.0.0.0/8 with
pNH=2.1.1.2 to the ingress PE PE11
3. The ingress PE "PE11" creates the following forwarding state
a. For prefixes 10.0.0.0/8 & 11.0.0.0/8: Push the VPN labels
4100 and 4200, respectively, followed by rL=3100 then tunnel
the packet to 2.1.1.1
b. For prefixes 20.0.0.0/8 & 21.0.0.0/8: Push the VPN labels
4300 and 4400, respectively, followed by rL=3200; then
tunnel the packet to 2.1.1.2
4.2. Forwarding Plane at Steady State (When PE0 is reachable)
1. Ingress PE PE11
a. Traffic for VRF "Blue"
i. PE11 receives a packet for VRF Blue with destination
address 10.1.1.1 from an external router.
ii. PE11 pushes the following labels
1. The VPN label 4100
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2. The Repair label 3100
3. The LDP label for the pNH 2.1.1.1
b. Traffic for VRF "Red"
i. PE11 receives a packet for VRF Red with destination
address 20.1.1.1 from an external router
ii. PE11 pushes the following labels
1. The VPN label 4300
2. The Repair label 3200
3. The LDP label for the pNH 2.1.1.2
2. Penultimate Hop of PE0 (Which is also the rP "P")
a. Receives a packet with top label for the protected next-hop
2.1.1.1 or 2.1.1.2
b. Pops *2* labels
c. Forwards the packet to pPE which is 1.1.1.1
3. Protected PE PE0
a. Traffic for VRF "Blue"
i. PE0 receives traffic with the top label 4100.
ii. 4100 is the VPN label 10.1.1.1 belonging to VRF "Blue"
iii. PE0 pops the label 4100 and forwards the packet to CE1
b. Traffic for VRF "Red"
i. PE0 receives traffic with the top label 4300.
ii. 4300 is the VPN label for 20.1.1.1 belonging to VRF "Red"
iii. PE0 pops the label 4300 and forwards the packet to CE2
4.3. Forwarding Plane at Failure (When PE0 is not reachable)
1. The ingress PE PE11
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Does not know about the failure yet and hence it does not
change its behavior.
2. Repair PE rP
a. Traffic for VRF "Blue"
i. Receives a packet with the top label being the LDP label
for 2.1.1.1
ii. 2.1.1.1 is not reachable
iii. Swap the LDP label for 2.1.1.1 with the LDP label of
9.9.9.1
iv. Forward the packet towards 9.9.9.1
b. Traffic for VRF "Blue"
i. Receives a packet with the top label being the LDP label
for 2.1.1.2
ii. 2.1.1.2 is not reachable
iii. Swap the LDP label for 2.1.1.1 with the LDP label of
9.9.9.2
iv. Forward the packet towards 9.9.9.2
3. The repair Router "PE1"
a. The penultimate hop of PE1 performs the usual penultimate hop
popping
b. PE1 receives a packet with the top label equals the repair
label 3100, which was allocated on per-CE basis and points to
CE1
c. PE1 pops *2* labels and forwards the packet to CE1
4. The repair Router "PE2"
a. The penultimate hop of PE2 performs the usual penultimate hop
popping
b. PE1 receives a packet with the top label equals the repair
label 3200, which was allocated on per-CE basis and points to
CE2
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c. PE2 pops *2* labels and forwards the packet to CE2
5. Inter-operability with Existing IP FRR Mechanisms
Current existing IP FRR mechanisms can be divided into two
categories: core protection and edge protection. Core protection
techniques, such as [12], [13], and [14], provide protection against
internal node and/or link failure. Thus the technique proposed in
this document is not related to existing IP FRR mechanisms. If the
failure of an internal node or link results in completely
disconnecting a protectable edge node, then an administrator MAY
configure the repairing router to prefer the technique proposed in
this document over existing IP FRR mechanisms.
Edge protection techniques, such as [16] and its practical
implementation [15] provide protection against the failure of the
link between PE and CE routers. Thus existing PE-CE link protection
can co-exist with the techniques proposed in this document because
the two techniques are independent of each other.
6. Security Considerations
No additional security risk is introduced by using the mechanisms
proposed in this document
7. IANA Considerations
No requirements for IANA
8. Conclusions
This document proposes a method that allows fast re-route
protection against edge node failure or complete disconnected from
the core in a BGP-free core. The proposed method has few advantages
o Easy to apply protection policies. pPE is the router that chooses
the rPE. Hence if an operator wants to control what prefixes/VRFs
get to be protected or what router can be chosen as repair PE, the
operator needs to apply the policy on the pPE only.
o Simple forwarding plane. The only change in forwarding plane is
the need to pop/push two labels on the iPE, rP, and rPEs.
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o Single label lookup even during failure. Forwarding decisions are
taken based on a single label lookup on all routers all the time
even during failure
o Immunity to mis-configuration. The only required configuration is
to choose non-overlapping address ranges on different pPEs. If an
operator configures overlapping IP address ranges on two different
pPEs, then one of the pPE will eventually allocate a pNH that is
covered by the IP address range of another pPE and hence the mis-
configuration can be detected
o No Need for IP or TE FRR: Because the exit point of the repair
tunnel from rP to rPE is different from the primary tunnel exit
point
o Works in both MPLS core and IP core
o Works with per-CE, per-VRF, and per-prefix label allocation
o Can be incrementally deployed. There is no flag day. Different
routers can be upgraded at different times
o Zero impact on the paths taken by traffic: Enabling/deploying the
feature described in this document has no effect on the paths
taken by traffic at steady state
9. References
9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4
(BGP-4), RFC 4271, January 2006
[3] Bates, T., Chandra, R., Katz, D., and Rekhter Y.,
"Multiprotocol Extensions for BGP", RFC 4760, January 2007
[4] Malhotra, P. and Rosen, E., " The BGP Encapsulation Subsequent
Address Family Identifier (SAFI) and the BGP Tunnel
Encapsulation Attribute", RFC 5512, April 2009
[5] Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed., "Layer Two
Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.
[6] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina,
"Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.
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[7] Perkins, C., "IP Encapsulation within IP", RFC 2003, October
1996.
9.2. Informative References
[8] Marques,P., Fernando, R., Chen, E, Mohapatra, P., Gredler, H.,
"Advertisement of the best external route in BGP", draft-ietf-
idr-best-external-04.txt, April 2011.
[9] Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
Framework", RFC 5565, June 2009.
[10] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private Networks
(VPNs)", RFC 4364, February 2006.
[11] 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
[12] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
Reroute: Loop-Free Alternates", RFC 5286, September 2008.
[13] Shand, S., and Bryant, S., "IP Fast Reroute", RFC5714, January
2010
[14] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, January 2010.
[15] Bashandy, A., Pithawala, P., and Heitz, J., "Scalable, Loop-
Free BGP FRR using Repair Label", draft-bashandy-idr-bgp-
repair-label-02.txt", July 2011
[16] 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
10. Acknowledgments
Special thanks to Eric Rosen, Clarence Filsfils, Maciek
Konstantynowicz, Stewart Bryant, Pradosh Malhotra, Nagendra Kumar,
George Swallow, Les Ginsberg, and Anton Smirnov for the valuable
comments
This document was prepared using 2-Word-v2.0.template.dot.
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Appendix A. How to protect Against Misconfigured pNH
Section 2.2 outlines a method by which the operator can configure
the protected next-hop "pNH". There is a possibility of a
misconfiguration as follows
o The operator configures the same pNH for two protected prefixes
P1/m1 and P2/m2 but the two prefixes are protected by different
rPEs
o The operator configures two different pNH's for two protected
prefixes P1/m1 and P2/m2 but the two prefixes are protected by
same rPE
The second configuration does not cause a lot of harm. Either way,
routers implementing the BGP FRR scheme proposed in this document can
detect both misconfigurations.
Suppose the operator configures the same "pNH" for P1/m1 and P2/m2
but P1/m1 is protected by rPE1 and P2/m2 is protected by rPE2. In
that case, the iPE and misconfigured pPE will detect this
inconsistency because both will see that P1/m1 and P2/m2 are assigned
the same pNH but are protected by two different rPEs. The reaction to
the misconfiguration is beyond the scope of this document.
Similarly, iPE and pPE can detect that the operator configured
different pNH's for P1/m1 and P2/m2 even though they are protected by
the same rPE because both iPE and pPE will receive an advertisement
for P1/m1 and P2/m2 from the same rPE. Reactions and remedy to the
misconfiguration is beyond the scope of this document.
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Appendix B. Alternative Approach for advertising (pNH,rNH) to iPE
In Section 2.1, pPE re-advertises the protected prefixes with (pNH)
as optional non-transitive attribute and advertises mapping (pNH,rNH)
separately. Alternatively, iPE can re-advertise the protected prefix
P/m to other iBGP peers with the mapping (pNH,rNH) as optional non-
transitive attributes. Advertising (pNH) only with the prefixes has
some advantages
o Advertising pNH only with the prefixes can easily be used for
configured pNH as described in Section 2.2.
o If the repair PE changes from one PE to another, there is no need
to re-advertise all the prefixes. Only the mapping (pNH,rNH) needs
to be re-advertised plus possibly some of the protected prefixes
o Advertising pNH only with the prefix slightly reduces the BGP
message size
Irrespective of whether (pNH,rNH) is advertised with the prefix or
separately, (pNH,rNH) is better than advertising (pNH,rL) because
there are many rL's for the same rNH. Hence advertising (pNH,rNH)
yields better attribute packing
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Appendix C. Modification History
C.1.1. Changes from Version 02
The whole scheme has been changed to a single next-hop per pPE-rPE.
As a result, unlike version 00 and 01, there will be a need for
behavioral changes in pPE, rP, iPE. The behavior for rPE remains
almost unchanged
The second important change is requiring rP to advertise the pNH with
maximum metric so that traffic does not get disrupted when the pPE
disappears
C.1.2. Changes from Version 01
1. Use the term "underlying repair label" instead of just "repair
label" to avoid confusion with the term "repair label" used in
[15].
2. In version 01, it was assumed in many places that the repairing
router is the penultimate hop P router. Although this would
probably be the most common case, it is not always true. Hence in
this version the repairing router may be any core router
3. Merged handling labeled and unlabeled prefixes into a single
algorithm.
4. Allowed sending a repair label for unlabeled prefixes and added
the "Push" flag. This ensures loop-free repair even for unlabeled
prefixes in case that the repair PE has eiBGP paths as mentioned
in Section Error! Reference source not found.
5. In Section Error! Reference source not found. discussing the rules
governing the choice of the underlying repair label for labeled
prefix, we changed the wording so that the primary egress PE
"SHOULD" instead of "MAY" use the repair label advertised
according to [15] as an underlying repair label.
6. All occurrences of the term "backup" were replaced by "repair" as
the term "repair" is the commonly used term in the IP FRR context
such as [14][13][12]
7. Added the definition of primary and repair tunnels in Section 1.2.
8. Added a definition of the term "Repair Next-hop" in Section 1.2.
9. Modified the definition of "repair path" in Section 1.2. to being
the repair next-hop plus the underlying repair label instead of
being the repair PE plus the underlying repair label.
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10.Outlined inter-operability with existing IP FRR techniques in
Section 5.
11.There were few editorial corrections.
Authors' Addresses
Ahmed Bashandy
Cisco Systems
170 West Tasman Dr, San Jose, CA 95134
Email: bashandy@cisco.com
Burjiz Pithawala
Cisco Systems
170 West Tasman Dr, San Jose, CA 95134
Email: bpithaw@cisco.com
Keyur Patel
Cisco Systems
170 West Tasman Dr, San Jose, CA 95134
Email: keyupate@cisco.com
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