Internet DRAFT - draft-ketant-idr-bgp-ls-bgp-only-fabric
draft-ketant-idr-bgp-ls-bgp-only-fabric
Inter-Domain Routing K. Talaulikar
Internet-Draft C. Filsfils
Intended status: Standards Track K. Swamy
Expires: March 12, 2021 Cisco Systems
S. Zandi
G. Dawra
LinkedIn
M. Durrani
Equinix
September 8, 2020
BGP Link-State Extensions for BGP-only Fabric
draft-ketant-idr-bgp-ls-bgp-only-fabric-05
Abstract
BGP is used as the only routing protocol in some networks today. In
such networks, it is useful to get a detailed view of the nodes and
underlying links in the topology along with their attributes similar
to one available when using link state routing protocols. Such a
view of a BGP-only fabric enables use cases like traffic engineering
and forwarding of services along paths other than the BGP best path
selection.
This document defines extensions to the BGP Link-state address-family
(BGP-LS) and the procedures for advertisement of the topology in a
BGP-only network. It also describes a specific use-case for traffic
engineering based on Segment Routing.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 12, 2021.
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Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. BGP Routing in the Fabric . . . . . . . . . . . . . . . . . . 3
3. Topology Collection Mechanism . . . . . . . . . . . . . . . . 4
3.1. Peering Models . . . . . . . . . . . . . . . . . . . . . 5
4. Advertising BGP-only Network Topology . . . . . . . . . . . . 6
4.1. Node Advertisements . . . . . . . . . . . . . . . . . . . 6
4.2. Link Advertisements . . . . . . . . . . . . . . . . . . . 7
4.3. Prefix Advertisements . . . . . . . . . . . . . . . . . . 10
4.4. TE Policy Advertisements . . . . . . . . . . . . . . . . 12
5. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Advertisement of Router's Node Attributes . . . . . . . . 13
5.2. Advertisement of Router's Local Links Attributes . . . . 15
5.3. Advertisement of Router's Prefix Attributes . . . . . . . 17
5.4. Advertisement of Router's TE Policy Attributes . . . . . 17
6. Usage of BGP Topology . . . . . . . . . . . . . . . . . . . . 18
6.1. Topology View for Monitoring . . . . . . . . . . . . . . 18
6.2. SR-TE in BGP Networks . . . . . . . . . . . . . . . . . . 18
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
8. Manageability Considerations . . . . . . . . . . . . . . . . 21
8.1. Operational Considerations . . . . . . . . . . . . . . . 21
8.1.1. Operations . . . . . . . . . . . . . . . . . . . . . 21
9. Security Considerations . . . . . . . . . . . . . . . . . . . 21
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
11.1. Normative References . . . . . . . . . . . . . . . . . . 21
11.2. Informative References . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
Network operators are going for a BGP-only routing protocol for
certain networks like Massively Scaled Data Centers (MSDCs).
[RFC7938] describes the requirement, design and operational aspects
for use of BGP as the only routing protocol in MSDCs. The underlying
link and topology information between BGP routers is hidden or
abstracted in this design from the underlay routing for improving
scalability and stability in a large scale network. On the flip
side, there is no detailed topology view similar to one available in
form of the Traffic Engineering (TE) Database (TED) when running link
state routing protocols like OSPF [RFC2328] with extensions specified
in [RFC3630].
BGP Link-State (BGP-LS)[RFC7752] enables advertisement of a link
state topology via BGP that can be consumed by a controller or in
general any software component to get a complete topology view of the
network. BGP-LS extensions for advertisement of a BGP topology for
the Egress Peer Engineering (EPE) use-case
[I-D.ietf-spring-segment-routing-central-epe] are specified in
[I-D.ietf-idr-bgpls-segment-routing-epe]. This document leverages
the BGP-LS TLVs defined for BGP-LS EPE and other BGP-LS documents and
specifies the procedures for advertising the underlying topology in a
more generic BGP-only fabric use-case.
This document specifies the operations and procedures when using the
design involving BGP use for hop-by-hop routing between directly
connected network nodes (refer [RFC7938] for details).
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. BGP Routing in the Fabric
This document does not change base BGP routing protocol operations in
the fabric that provides routing using the BGP best path selection
process [RFC4271] .
The applicability of this specification is limited to those
deployments where BGP is used as hop-by-hop routing protocol between
directly connected nodes in the fabric. While a data-center design
[RFC7938] is used as a reference, the topology advertisement and its
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use for computation may also apply to other networks with BGP-only
fabric or to BGP-only portions of a larger network topology.
BGP hop-by-hop routing can be setup using EBGP single-hop sessions
over individual links between directly connected routers using their
link addresses for peering as described in [RFC7938]. In such a
design, the neighbors' link addresses may be provisioned for peering
and the EBGP session operating directly over the link performs the
monitoring of the neighbor on that link. A variation of this design
would be that the EBGP session is setup between directly connected
routers using their loopback sessions. The mechanisms for discovery
of the neighbor's link addresses and their monitoring on a per link
basis are outside the scope of this document.
[I-D.xu-idr-neighbor-autodiscovery] describes one such mechanism and
the same may be also realized by other means.
Though this document uses the EBGP based design as a reference, it
does not preclude other alternate designs using IBGP.
3. Topology Collection Mechanism
BGP-LS [RFC7752] has been defined to allow BGP to convey topology
information in the form of Link-State objects - node, link and
prefix. The properties of each of these objects are encoded using
BGP-LS Attribute TLVs. Applications need a topological view and
visibility even for networks where BGP is the only routing protocol.
In such networks, each BGP router advertises its local information
which includes its node, links and prefix attributes via BGP-LS.
Figure 1 describes a typical deployment scenario. Every BGP router
in the network is enabled for BGP-LS and forms BGP-LS sessions with
one or more centralized BGP-LS speakers over which it sends its local
topology information. Each BGP router MAY also receive the topology
information from all other BGP routers via these centralized BGP-LS
speakers. This way, any BGP router (as also the centralized BGP-LS
speakers) MAY obtain aggregated Link-State information for the entire
BGP network. An external component (e.g. a controller) can obtain
this information from the centralized BGP-LS speakers or directly by
doing BGP-LS peering to the BGP routers. An internal software
component on any of the BGP routers (e.g. TE module) can also
receive the entire BGP network topology information from its local
BGP process.
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+------------+
| Controller |
+------------+
^
|
v
+-------------------+
| BGP-LS Speaker | +------------+
| (Centralized) | | Controller |
+-------------------+ +------------+
^ ^ ^ ^
| | | |
+-----------+ | +---------------+ |
| | | |
v v v v
+-----------+ +-----------+ +-----------+ +----------+
| BGP | | BGP | | BGP |<-->| Local |
| Router | | Router | . . . | Router | | Consumer |
+-----------+ +-----------+ +-----------+ +----------+
^ ^ ^
| | |
Local Info Local Info Local Info
(node & links) (node & links) (node & links)
Figure 1: Link State info collection
3.1. Peering Models
The peering model described above relies on the base BGP IPv4 or IPv6
routing underlay (e.g. as described in [RFC7938]) or any other
mechanism for reachability for the BGP-LS session establishment with
the centralized BGP speakers. A variation of this model would be to
setup reachability to the centralized BGP speakers (or controller)
over the out of band management network, where available, and for
each BGP router in the fabric use the same for the BGP-LS session
establishment with the centralized BGP speakers. This variation
removes the dependency between the topology learning via BGP-LS from
the base best effort reachability over the BGP routing in the fabric.
Another alternate design would be to enable BGP-LS as well on the hop
by hop EBGP sessions in the underlay as described in [RFC7938]. This
approach results in the topology information being flooded via BGP-LS
hop-by-hop along the BGP routers in the network. Other peering
designs for BGP-LS sessions may also be possible and they are not
precluded by this document.
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4. Advertising BGP-only Network Topology
This section specifies the BGP-LS TLVs and sub-TLVs and their use for
advertising the topology of a BGP-only network in the form of BGP-LS
Node, Link and Prefix NLRIs.
BGP-LS [RFC7752] defines the BGP-LS NLRI types (i.e. Node NLRI, Link
NLRI and Prefix NLRI) along with their corresponding BGP-LS Attribute
(i.e. Node Attribute, Link Attribute or Prefix Attribute) and the
TLVs that map to the respective NLRI and Attribute for each type.
[I-D.ietf-idr-bgpls-segment-routing-epe] specifies the BGP Protocol
ID to be used for signaling BGP EPE information and the same is used
for advertising of BGP topology.
[I-D.ietf-idr-te-lsp-distribution] defines the BGP-LS NLRI that can
be used to advertise the RSVP-TE or Segment Routing (SR) policies
instantiated on a BGP Router head-end along with their corresponding
BGP-LS Attribute TLVs to advertise their properties and state.
The following sub-sections specify the use of these encodings by a
router running BGP protocol.
4.1. Node Advertisements
[RFC7752] defines Node NLRI Type and the Node Descriptor TLVs as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Protocol-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier |
| (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Local Node Descriptors (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
[I-D.ietf-idr-bgpls-segment-routing-epe] introduces additional Node
Descriptor TLVs for BGP protocol that are required to be used.
The following Node Descriptors TLVs MUST appear in the Node NLRI as
Local Node Descriptors:
o BGP Router-ID, which contains the BGP Identifier of the
originating BGP router
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o Autonomous System Number, which contains the advertising router
ASN.
The BGP-LS Attribute associated with the Node NLRI MAY include the
following TLVs that are defined in respective documents to signal the
router properties and capabilities (Section 5.1 defines the
procedures for their advertisements):
+--------+--------------+-------------------------------------------+
| TLV | Description | Reference Document |
| Code | | |
| Point | | |
+--------+--------------+-------------------------------------------+
| 1026 | Node Name | [RFC7752] |
| 1028 | IPv4 TE | [RFC7752] |
| | Router-ID | |
| 1029 | IPv6 TE | [RFC7752] |
| | Router-ID | |
| 1161 | SID/Label | [I-D.ietf-idr-bgp-ls-segment-routing-ext] |
| 1034 | SRGB & | [I-D.ietf-idr-bgp-ls-segment-routing-ext] |
| | Capabilities | |
| 1035 | SR Algorithm | [I-D.ietf-idr-bgp-ls-segment-routing-ext] |
| 1036 | SR Local | [I-D.ietf-idr-bgp-ls-segment-routing-ext] |
| | Block | |
| 266 | Node MSD | [RFC8814] |
| TBD | Flex | [I-D.ietf-idr-bgp-ls-flex-algo] |
| | Algorithm | |
| | Definition | |
+--------+--------------+-------------------------------------------+
Table 1: Node Attribute TLVs
The above list of TLVs is not exhaustive but indicative as of the
time of writing of this document.
4.2. Link Advertisements
[RFC7752] defines Link NLRI Type and its Node and Link Descriptor
TLVs as follows:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Protocol-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier |
| (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Local Node Descriptors (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Remote Node Descriptors (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Link Descriptors (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following Node Descriptors TLVs MUST appear in the Link NLRI as
Local Node Descriptors:
o BGP Router-ID, which contains the BGP Identifier of the
originating BGP router
o Autonomous System Number, which contains the advertising router
ASN.
The following Node Descriptors TLVs MUST appear in the Link NLRI as
Remote Node Descriptors:
o BGP Router-ID, which contains the BGP Identifier of the peer BGP
router
o Autonomous System Number, which contains the peer ASN.
The following Link Descriptors TLVs MUST appear in the Link NLRI as
Link Descriptors:
o Link Local/Remote Identifiers containing the 4-octet Link Local
Identifier followed by the 4-octet Link Remote Identifier. The
value 0 MUST be used for the Link Remote Identifier when the value
is unknown.
In addition, the following Link Descriptors TLVs SHOULD appear in the
Link NLRI as Link Descriptors based on the address family used for
setting up the BGP Peering or the addresses configured on the links:
o IPv4 Interface Address contains the address of the local interface
through which the BGP session is established using IPv4 address.
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o IPv6 Interface Address contains the address of the local interface
through which the BGP session is established using IPv6 address.
o IPv4 Neighbor Address contains the IPv4 address of the peer
interface used by the BGP session establishment using IPv4
address.
o IPv6 Neighbor Address contains the IPv6 address of the peer
interface used by the BGP session establishment using IPv6
address.
The BGP-LS Attribute associated with the Link NLRI MAY include the
following TLVs that are defined in respective documents to signal the
router's local links' properties and capabilities (Section 5.2
defines the procedures for their advertisements) :
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+------+----------------+-------------------------------------------+
| TLV | Description | Reference Document |
| Code | | |
| Poin | | |
| t | | |
+------+----------------+-------------------------------------------+
| 1088 | Administrative | [RFC7752] |
| | group (color) | |
| 1173 | Extended | [I-D.ietf-idr-eag-distribution] |
| | Administrative | |
| | group (color) | |
| 1089 | Maximum link | [RFC7752] |
| | bandwidth | |
| 1092 | TE Default | [RFC7752] |
| | Metric | |
| 1096 | SRLG | [RFC7752] |
| 1098 | Link Name | [RFC7752] |
| 267 | Link MSD | [RFC8814] |
| 1172 | L2 Bundle | [I-D.ietf-idr-bgp-ls-segment-routing-ext] |
| | Member | |
| 1104 | Unidirectional | [RFC8571] |
| | link delay | |
| 1105 | Min/Max | [RFC8571] |
| | Unidirectional | |
| | link delay | |
| 1106 | Min/Max | [RFC8571] |
| | Unidirectional | |
| | link delay | |
| 1107 | Unidirectional | [RFC8571] |
| | packet loss | |
| 1101 | EPE Peer Node | [I-D.ietf-idr-bgpls-segment-routing-epe] |
| | SID | |
| 1102 | EPE Peer Adj | [I-D.ietf-idr-bgpls-segment-routing-epe] |
| | SID | |
| 1103 | EPE Peer Set | [I-D.ietf-idr-bgpls-segment-routing-epe] |
| | SID | |
+------+----------------+-------------------------------------------+
Table 2: Link Attribute TLVs
The above list of TLVs is not exhaustive but indicative as of the
time of writing of this document.
4.3. Prefix Advertisements
[RFC7752] defines Prefix NLRI Type and its Node and Prefix Descriptor
TLVs as follows:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Protocol-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier |
| (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Local Node Descriptors (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Prefix Descriptors (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following Node Descriptors TLVs MUST appear in the Prefix NLRI as
Local Node Descriptors:
o BGP Router-ID, which contains the BGP Identifier of the
originating BGP router
o Autonomous System Number, which contains the advertising router
ASN.
The Prefix Descriptor MUST contain the IP Reachability information
TLV to identify the prefix.
This document defines a new BGP Route Type TLV that MUST be included
in the Prefix Descriptor when the BGP node advertises the Prefix
NLRI. The format of this TLV is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Type |
+-+-+-+-+-+-+-+-+
Where:
Type: 2 octet field with value TBD, see Section 7.
Length: 2 octet field with value set to 1.
Route Type: one octet with the following values defined:
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+-----+---------------+------------------------------------------+
|Value| Type | Description |
+-----+---------------+------------------------------------------+
| 1 | Local | Local interface prefix e.g. Loopback |
| 2 | Attached | Directly attached node's prefix e.g host |
| 3 | External BGP | Prefix learnt via EBGP |
| 4 | Internal BGP | Prefix learnt via IBGP |
| 5 | Redistributed | Prefix redistributed into BGP |
+-------+-------------+------------------------------------------+
Figure 2: BGP Route Types
The BGP-LS Attribute associated with the Prefix NLRI MAY include the
following TLVs that are defined in respective documents to signal the
router's own prefix properties and capabilities (Section 5.3 defines
the procedures for their advertisements):
+---------+-------------+-------------------------------------------+
| TLV | Description | Reference Document |
| Code | | |
| Point | | |
+---------+-------------+-------------------------------------------+
| 1155 | Prefix | [RFC7752] |
| | Metric | |
| 1158 | Prefix SID | [I-D.ietf-idr-bgp-ls-segment-routing-ext] |
+---------+-------------+-------------------------------------------+
Table 3: Prefix Attribute TLVs
The above list of TLVs is not exhaustive but indicative as of the
time of writing of this document.
4.4. TE Policy Advertisements
[I-D.ietf-idr-te-lsp-distribution] defines TE Policy NLRI Type and
its Headend Node and TE Policy Descriptor TLVs as follows:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Protocol-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier |
| (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Headend (Node Descriptors) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// TE Policy Descriptors (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Node Descriptors TLVs are the same as specified in Section 4.1.
The semantics for the TE Policy Descriptor TLVs and the TLVs
associated with the BGP-LS Attribute are used as specified in
[I-D.ietf-idr-te-lsp-distribution].
5. Procedures
In a network where BGP is the only routing protocol, the BGP-LS
session is used to advertise the necessary information about the
local node properties, its local links' properties and where
necessary the prefix's owned by the node. TE Policies, that are
instantiated on the local node (i.e. when it is the head-end for the
policy), along with their properties are also advertised via the BGP-
LS session. This information, once collected across all BGP routers
in the network, provides a complete topology view of the network.
Many of these attributes are not part of the base BGP protocol
operations and are either configured or provided by other components
on the router. BGP-LS performs the role of collecting this
information and propagating it across the BGP network.
The following sections describe the procedures for the propagation of
the BGP-LS NLRIs on a BGP router into the BGP-LS session. These
procedures for propagation of BGP topology information via BGP-LS
SHOULD be applied only in deployments and use-cases where necessary
and SHOULD NOT be applied in every BGP deployment when BGP-LS is
enabled. Implementations MAY provide a configuration option to
enable these procedures in required deployments.
5.1. Advertisement of Router's Node Attributes
Advertisement of the Node NLRI via BGP-LS by each BGP router in a
BGP-only network enables the discovery of all the router nodes in the
topology. The Node NLRI MUST be generated by a BGP router only for
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itself and even when there are no attributes to be advertised along
with it.
The Node attributes defined currently related to Segment Routing (SR)
[RFC8402] have been described in Table 1 and are to be advertised
when SR is enabled. This includes:
o All SR enabled routers support the default SR algorithm 0 and MUST
advertise it in the SR Algorithm TLV. Other algorithms (including
Flexible Algorithm [I-D.ietf-lsr-flex-algo]) SHOULD be advertised
when supported.
o The Segment Routing Global Block (SRGB) provisioned on the router
which is used by BGP Prefix SIDs [RFC8669] and other SR control
plane protocols on the router MUST be advertised. The value for
Flags field in the TLV is not defined for BGP protocol and MUST be
set to 0 by the originator and ignored by receivers.
o The Segment Routing Local Block (SRLB) provisioned on the router
which MAY be used by BGP EPE SIDs
[I-D.ietf-idr-bgpls-segment-routing-epe] SHOULD be advertised.
The value for Flags field in the TLV is not defined for BGP
protocol and MUST be set to 0 by the originator and ignored by
receivers.
o The Node level MSD provides the Node's capabilities for SR SID
operations and SHOULD be advertised.
o When the router supports SR Flexible Algorithms and is provisioned
with the Flexible Algorithm Definition (FAD), then it MUST
advertise the same.
The Node Name Attribute SHOULD be advertised when available.
This document introduces some of the TE concepts into BGP-only
networks. Provisioning of TE Router-ID with a unique address
normally associated with a loopback interface on the router enables
TE use-cases for both IPv4 and IPv6 SHOULD be supported. The BGP
Router-ID along with the ASN also provides the capability for
uniquely identifying a BGP router in the network.
Other Node Attributes applicable to a BGP Router may also be included
and this document does not describe the exhaustive list.
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5.2. Advertisement of Router's Local Links Attributes
Each BGP router in a BGP-only network also advertises its local links
using the Link NLRIs thru BGP-LS. The Link NLRI for a given link
between two BGP routers is advertised as uni-directional logical
"half-link" and its link descriptors allow the correlation between
the two NLRIs "half-links" originated by the peering routers to
describe the bi-directional logical link and its attributes on both
routers.
The discovery of all the links and their local and remote identifiers
in a BGP-only network relies on the design that uses EBGP sessions
over each interconnecting link using the link IP addresses (refer
[RFC7938]). In this case, a Link NLRI MUST be generated by a BGP
router for each of its local link regardless of whether it has any
link attributes to be advertised for it.
When doing EBGP multi-hop sessions between directly connected BGP
routers, the underlying link information would need to learn by some
discovery protocol or provisioning entity. The mechanisms to learn
the underlying link information for BGP-LS advertisements are outside
the scope of this document. However, to provide a true link topology
picture, the advertisement of underlying links is RECOMMENDED for
most use-cases instead of a single EBGP peering representation of a
link between the routers using their loopback addresses.
The Link NLRI represents an adjacency between BGP routers and its
association with the underlying Layer 3 link. When the underlying
Layer 3 link or the BGP session on top of it goes down, the Link NLRI
MUST be withdrawn by the BGP router. The monitoring of links,
detecting of their failures and notification to BGP may be performed
using mechanisms like BFD. This enables faster detection of failures
and verification of the underlying links.
Advertisement of the Link NLRIs via BGP-LS by each BGP router in a
BGP-only network enables the discovery of all the active links in the
topology.
TE attributes for links have been traditionally associated with Link
State Routing protocols. However, with the ability to discover the
link topology via BGP-LS as specified in this document, the TE
attributes and their principles can also be applied to a network
running BGP alone. The TE attributes for a link have been described
in Table 2 and MAY be advertised when TE use-cases are enabled. This
includes:
o The maximum bandwidth of a link is its protocol independent
attribute and SHOULD be advertised.
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o TE concepts of Administrative Groups (also known as affinities)
and Shared Risk Link Groups (SRLGs) MAY be provisioned locally on
links and then MUST be advertised.
o The BGP base protocol does not operate with link metrics, however,
a TE metric concept can be introduced in a BGP only network as
well for TE use-cases. Implementations MAY provide the ability to
provision TE metric value for a link for BGP use including a
different default value for it. The TE metric attribute SHOULD be
advertised for each link when configured and its default value is
taken as 100. When not advertised for a link, implementations who
intend to use the TE metric MUST assume the value to be 100.
o The delay and loss TE metrics for links are measured via MPLS
Performance Monitoring [RFC6374] and their measurement mechanism
over a link are independent of the routing protocol. The same
mechanism MAY be enabled in BGP-only networks and their values
advertised via BGP-LS.
The Link attributes defined currently related to the Segment Routing
feature BGP EPE [I-D.ietf-idr-bgpls-segment-routing-epe] have been
described in Table 2 and are to be advertised when SR use-cases are
enabled. This includes:
o The BGP Peering SIDs provide a functionality similar to Adjacency-
SID (refer [RFC8402]) in BGP-only networks. Implementations
SHOULD allocate the BGP Peer-Adjacency SID for all its links and
the BGP Peer-Node SID for all its peer routers. Implementations
MAY allocate the BGP Peer-Set SID based on local configuration.
o The Link level MSD provides the per link capabilities for SR SID
operations and SHOULD be advertised when the router links have
differing capabilities.
The use of Layer 3 bundle links which comprise of multiple layer 2
member links are often used in BGP networks. When BGP session is
configured over such a layer 3 link, the link attributes of the
underlying layer 2 links MAY be advertised individually using the L2
Bundle Member TLV. The applicable attributes for the L2 links are
described in [I-D.ietf-idr-bgp-ls-segment-routing-ext].
The Link Name Attribute MAY be advertised when available.
Other Link Attributes applicable to a BGP Router may also be included
and this document does not describe the exhaustive list.
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5.3. Advertisement of Router's Prefix Attributes
Advertisement of the Prefix NLRI via BGP-LS may be required only in
specific use-cases. Since the base BGP protocol along with its
extensions already signals Prefix reachability via different NLRIs,
there is no necessity to duplicate the information via BGP-LS
session. However, for specific use-cases related to SR Traffic
Engineering (SR-TE), it is required for each router to advertise it's
Prefix SID(s) (refer [RFC8402]) that can be used to direct traffic
via specific BGP routers. Advertising such BGP Prefix SID for every
BGP router provides this key attribute via BGP-LS and avoids the
requirement for the consumer of the topology information (e.g. a
controller or local TE process) to tap into other BGP NLRI
information.
Advertisement of the Prefix NLRI via BGP-LS MUST be done for its
locally configured prefixes (e.g. its loopback interface address) and
when BGP is advertising the BGP Prefix SID ([RFC8669]) for it. The
advertisement of the Prefix NLRI via BGP-LS for other prefixes learnt
by the router MAY be done based on the specific use-case requirement
and the BGP Route Type as described in Figure 2 indicates the type of
route being advertised.
The Prefix attributes defined currently related to SR [RFC8402] have
been described in Table 3 and MAY be advertised when SR is enabled.
This includes:
o The Prefix SID TLV is included with the SID advertised as the
index to be consistent with the Label-Index TLV of BGP Prefix SID
attribute. The default algorithm is MUST be set to 0 by the
originator except in the case where a local prefix is associated
with a specific SR Algorithm. The flags are defined as the most
significant 8 bits of the 16 bit field defined for Label-Index TLV
in [RFC8669].
o For certain SR-TE uses, the Prefix Metric value MAY be included
and it is set based on the SR-TE computation based on the link-
state topology learnt via BGP-LS.
Other Prefix Attributes applicable may also be included and this
document does not describe the exhaustive list.
5.4. Advertisement of Router's TE Policy Attributes
TE Policies that are setup using RSVP-TE or SR-TE mechanisms MAY be
instantiated on a BGP router. One use-case that results in such SR
Policy instantiation on a BGP router is described later in this
document in Section 6.2. Advertising such TE Policies instantiated
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for every BGP router as head-end via BGP-LS provides the consumer of
the topology information (e.g. a controller or local TE process) a
policy view of the BGP fabric as well.
The procedures for advertisement of the TE Policy NLRI via BGP-LS
MUST be done only for its locally instantiated TE Policies and as
specified in [I-D.ietf-idr-te-lsp-distribution]). Implementation MAY
provide configuration options to control the specific set of TE
Policies that are to be advertised from the local node.
6. Usage of BGP Topology
This section describes some of the use-cases for the building of the
BGP topology information as specified in this document and leveraging
it for enabling new functionality.
6.1. Topology View for Monitoring
The BGP-LS advertisement of the BGP topology as specified in this
document provides a live topology view of the BGP network for an
application or controller that is monitoring the network. The
topology view is from the BGP protocol perspective and includes the
underlying links as well that aids in network monitoring as well as
diagnostics use-cases. BGP-LS is the de-facto protocol for
northbound propagation of network topology related information for
most IGP networks and extending this capability for BGP-only networks
allows existing controllers and applications to consume the
information with some incremental BGP protocol awareness.
6.2. SR-TE in BGP Networks
The SR-TE use-case for BGP builds on top of functionality specified
in [RFC8669] and also described in [RFC8670].The BGP SR Prefix SID
signaled, provides the basic connectivity between all BGP routers
using their loopback addresses. This provides the basic best-effort
paths in the network using the base BGP decision process that is
unchanged. BGP and other overlay routes and services recurse on top
of these loopback addresses of the egress nodes and the forwarding is
done via the BGP SR Prefix SID labels in the underlay. While this
version of the document focuses on the examples with MPLS dataplane
instantiation for SR, the same is applicable for the IPv6 dataplane
instantiation (SRv6) as well.
SR-TE for BGP provides underlay paths through the network for the
overlay routes and services with specific SLA requirements and use-
cases like path disjointness, low latency paths, inclusion or
exclusion and other TE considerations.
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[I-D.ietf-spring-segment-routing-policy] specifies the SR-TE
architecture and the SR Policy construct.
[I-D.ietf-idr-segment-routing-te-policy] describes the extensions to
BGP for signaling of SR Policies from a controller to the SR-TE
headend BGP router. BGP-LS has been extended to allow signaling of
the SR Policies from SR-TE head-end to controllers via
[I-D.ietf-idr-te-lsp-distribution] which allows the controllers to
learn the state of SR Policies instantiated on routers in the
network. This document completes the missing piece that is related
to getting the BGP topology information from all the routers to a
controller as well the local SRTE process on each router for their
path computation requirements.
The signaling of SR Polices from controller to SR-TE headend and
reporting of the state back to the controller can also be done using
PCEP ([RFC8664], [RFC8281], [RFC8231]). However, the BGP topology
learning via BGP-LS which is specified in this document is also
required for the deployments that uses PCEP in the BGP-only network.
The topology collected via BGP-LS in a BGP-only fabric in a Segment
Routing deployment comprise of:
o The properties of every BGP router node and the Prefix SIDs to
reach that node.
o The properties of all the links between the BGP routers and the
Peer-Adjacency-SIDs (and other EPE SIDs) corresponding to them
that allow directing traffic over specific links and/or to
specific neighbors.
o The properties and state of the SR Policies instantiatied on each
of the BGP routers along with their end points, their properties
and most importantly the Binding SID to steer traffic into the SR
Policies.
This topology information allows a computation node to build SR
Policies for services over the BGP fabric for a given traffic
engineering objective at any given node.
The topology of the BGP fabric is advertised to a centralized
controller or application for use-cases that need a centralized
computation of SR Policy which can then be signaled to the SR-TE
head-end node via PCEP or BGP-SRTE. The topology may also be
distributed to any node in the BGP fabric to be used by its local SR-
TE process to perform path computation for its own SR Policies for
use-cases that are addressed by local computation.
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A high level summary of the key topology information advertised via
BGP-LS by BGP routers can be used for TE computations as follows
o The BGP SR Prefix SIDs and the BGP EPE Peering Adjacency SIDs
provide the equivalent of the IGP Prefix and Adjacency SIDs and
can be used to direct traffic to a specific BGP router and over a
specific BGP peer session or link respectively. Traffic for the
BGP SR Prefix SIDs follow the path computed by the BGP decision
process.
o The TE metric can be used to tailor the choice of specific paths
in the network for SR-TE.
o The TE administrative group (also known as affinities) and SRLG
attributes can be configured over links to enable computation of
paths with inclusion and exclusion of specific links or paths that
are mutually disjoint.
o The enabling of link delay and loss measurements and their
advertisements can help monitoring the link quality and carve out
paths based on latency and other SLA requirements.
o The signaling of the Node and Link MSD allows controllers to
instantiate SR Policies based on the capability of the routers.
This section attempts to highlight and describe at a high level some
of the possible SR-TE solutions and use-cases in a BGP-only network.
The actual SR-TE computation and algorithms are outside the scope of
this document.
7. IANA Considerations
IANA maintains a registry called "Border Gateway Protocol - Link
State (BGP-LS) Parameters" with a sub-registry called "Node Anchor,
Link Descriptor and Link Attribute TLVs".
The following TLV codepoints are suggested (to be assigned by IANA):
+----------+----------------------------------------+---------------+
| TLV Code | Description | Value defined |
| Point | | in |
+----------+----------------------------------------+---------------+
| TBD | BGP Route Type TLV | this document |
+----------+----------------------------------------+---------------+
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8. Manageability Considerations
This section is structured as recommended in [RFC5706].
8.1. Operational Considerations
8.1.1. Operations
Existing BGP and BGP-LS operational procedures apply. No additional
operation procedures are defined in this document.
9. Security Considerations
Procedures and protocol extensions defined in this document do not
affect the BGP security model. See the 'Security Considerations'
section of [RFC4271] for a discussion of BGP security. Also refer to
[RFC4272] and [RFC6952] for analysis of security issues for BGP.
10. Acknowledgements
The authors would like to thank Bruno Decraene for his review and
comments on this document.
11. References
11.1. Normative References
[I-D.ietf-idr-bgp-ls-flex-algo]
Talaulikar, K., Psenak, P., Zandi, S., and G. Dawra,
"Flexible Algorithm Definition Advertisement with BGP
Link-State", draft-ietf-idr-bgp-ls-flex-algo-04 (work in
progress), July 2020.
[I-D.ietf-idr-bgp-ls-segment-routing-ext]
Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
and M. Chen, "BGP Link-State extensions for Segment
Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-16
(work in progress), June 2019.
[I-D.ietf-idr-bgpls-segment-routing-epe]
Previdi, S., Talaulikar, K., Filsfils, C., Patel, K., Ray,
S., and J. Dong, "BGP-LS extensions for Segment Routing
BGP Egress Peer Engineering", draft-ietf-idr-bgpls-
segment-routing-epe-19 (work in progress), May 2019.
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[I-D.ietf-idr-eag-distribution]
Wang, Z., WU, Q., Tantsura, J., and K. Talaulikar,
"Distribution of Traffic Engineering Extended Admin Groups
using BGP-LS", draft-ietf-idr-eag-distribution-12 (work in
progress), May 2020.
[I-D.ietf-idr-te-lsp-distribution]
Previdi, S., Talaulikar, K., Dong, J., Chen, M., Gredler,
H., and J. Tantsura, "Distribution of Traffic Engineering
(TE) Policies and State using BGP-LS", draft-ietf-idr-te-
lsp-distribution-13 (work in progress), April 2020.
[I-D.ietf-lsr-flex-algo]
Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
algo-10 (work in progress), August 2020.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8571] Ginsberg, L., Ed., Previdi, S., Wu, Q., Tantsura, J., and
C. Filsfils, "BGP - Link State (BGP-LS) Advertisement of
IGP Traffic Engineering Performance Metric Extensions",
RFC 8571, DOI 10.17487/RFC8571, March 2019,
<https://www.rfc-editor.org/info/rfc8571>.
[RFC8669] Previdi, S., Filsfils, C., Lindem, A., Ed., Sreekantiah,
A., and H. Gredler, "Segment Routing Prefix Segment
Identifier Extensions for BGP", RFC 8669,
DOI 10.17487/RFC8669, December 2019,
<https://www.rfc-editor.org/info/rfc8669>.
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[RFC8814] Tantsura, J., Chunduri, U., Talaulikar, K., Mirsky, G.,
and N. Triantafillis, "Signaling Maximum SID Depth (MSD)
Using the Border Gateway Protocol - Link State", RFC 8814,
DOI 10.17487/RFC8814, August 2020,
<https://www.rfc-editor.org/info/rfc8814>.
11.2. Informative References
[I-D.ietf-idr-segment-routing-te-policy]
Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P.,
Rosen, E., Jain, D., and S. Lin, "Advertising Segment
Routing Policies in BGP", draft-ietf-idr-segment-routing-
te-policy-09 (work in progress), May 2020.
[I-D.ietf-spring-segment-routing-central-epe]
Filsfils, C., Previdi, S., Dawra, G., Aries, E., and D.
Afanasiev, "Segment Routing Centralized BGP Egress Peer
Engineering", draft-ietf-spring-segment-routing-central-
epe-10 (work in progress), December 2017.
[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", draft-
ietf-spring-segment-routing-policy-08 (work in progress),
July 2020.
[I-D.xu-idr-neighbor-autodiscovery]
Xu, X., Talaulikar, K., Bi, K., Tantsura, J., and N.
Triantafillis, "BGP Neighbor Discovery", draft-xu-idr-
neighbor-autodiscovery-12 (work in progress), November
2019.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
DOI 10.17487/RFC3630, September 2003,
<https://www.rfc-editor.org/info/rfc3630>.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.
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[RFC5706] Harrington, D., "Guidelines for Considering Operations and
Management of New Protocols and Protocol Extensions",
RFC 5706, DOI 10.17487/RFC5706, November 2009,
<https://www.rfc-editor.org/info/rfc5706>.
[RFC6374] Frost, D. and S. Bryant, "Packet Loss and Delay
Measurement for MPLS Networks", RFC 6374,
DOI 10.17487/RFC6374, September 2011,
<https://www.rfc-editor.org/info/rfc6374>.
[RFC6952] Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
BGP, LDP, PCEP, and MSDP Issues According to the Keying
and Authentication for Routing Protocols (KARP) Design
Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013,
<https://www.rfc-editor.org/info/rfc6952>.
[RFC7938] Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
BGP for Routing in Large-Scale Data Centers", RFC 7938,
DOI 10.17487/RFC7938, August 2016,
<https://www.rfc-editor.org/info/rfc7938>.
[RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for Stateful PCE", RFC 8231,
DOI 10.17487/RFC8231, September 2017,
<https://www.rfc-editor.org/info/rfc8231>.
[RFC8281] Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for PCE-Initiated LSP Setup in a Stateful PCE
Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
<https://www.rfc-editor.org/info/rfc8281>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "Path Computation Element Communication
Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
DOI 10.17487/RFC8664, December 2019,
<https://www.rfc-editor.org/info/rfc8664>.
[RFC8670] Filsfils, C., Ed., Previdi, S., Dawra, G., Aries, E., and
P. Lapukhov, "BGP Prefix Segment in Large-Scale Data
Centers", RFC 8670, DOI 10.17487/RFC8670, December 2019,
<https://www.rfc-editor.org/info/rfc8670>.
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Authors' Addresses
Ketan Talaulikar
Cisco Systems
Pune 411057
India
Email: ketant@cisco.com
Clarence Filsfils
Cisco Systems
Brussels
Belgium
Email: cfilsfil@cisco.com
Krishna Swamy
Cisco Systems
San Jose
USA
Email: kriswamy@cisco.com
Shawn Zandi
LinkedIn
USA
Email: szandi@linkedin.com
Gaurav Dawra
LinkedIn
USA
Email: gdawra.ietf@gmail.com
Muhammad Durrani
Equinix
USA
Email: mdurrani@equinix.com
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