BGP Link-State (LS) is an Address Family Identifier (AFI) and Sub-address Family Identifier (SAFI) originally defined to carry
interior gateway protocol (IGP) link-state information through BGP. The BGP Network Layer Reachability Information (NLRI)
encoding format for BGP-LS and a new BGP Path Attribute called the BGP-LS attribute are defined in RFC7752. The identifying key of each Link-State object, namely a node, link, or prefix, is encoded in the NLRI and the properties
of the object are encoded in the BGP-LS attribute.
The BGP-LS Extensions for Segment Routing are documented in RFC9085.
BGP-LS applications like an SR Path Computation Engine (SR-PCE) can learn the SR capabilities of the nodes in the topology
and the mapping of SR segments to those nodes. This can enable the SR-PCE to perform path computations based on SR-TE and
to steer traffic on paths different from the underlying IGP-based distributed best-path computation.
The following figure shows a typical deployment scenario. In each IGP area, one or more nodes (BGP speakers) are configured
with BGP-LS. These BGP speakers form an iBGP mesh by connecting to one or more route-reflectors. This way, all BGP speakers
(specifically the route-reflectors) obtain Link-State information from all IGP areas (and from other ASes from eBGP peers).
Usage Guidelines and Limitations
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BGP-LS supports IS-IS and OSPFv2.
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The identifier field of BGP-LS (referred to as the Instance-ID) identifies the IGP routing domain where the NLRI belongs.
The NLRIs representing link-state objects (nodes, links, or prefixes) from the same IGP routing instance must use the same
Instance-ID value.
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When there is only a single protocol instance in the network where BGP-LS is operational, we recommend configuring the Instance-ID
value to 0.
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Assign consistent BGP-LS Instance-ID values on all BGP-LS Producers within a given IGP domain.
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NLRIs with different Instance-ID values are considered to be from different IGP routing instances.
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Unique Instance-ID values must be assigned to routing protocol instances operating in different IGP domains. This allows the
BGP-LS Consumer (for example, SR-PCE) to build an accurate segregated multi-domain topology based on the Instance-ID values,
even when the topology is advertised via BGP-LS by multiple BGP-LS Producers in the network.
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If the BGP-LS Instance-ID configuration guidelines are not followed, a BGP-LS Consumer may see duplicate link-state objects
for the same node, link, or prefix when there are multiple BGP-LS Producers deployed. This may also result in the BGP-LS Consumers
getting an inaccurate network-wide topology.
For segment routing, the following attributes have been added to BGP-LS:
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Node—Segment routing capability (including SRGB range) and algorithm
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Link—Adjacency SID and LAN adjacency SID
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Prefix—Prefix SID and segment routing mapping server (SRMS) prefix range
Exchange Link State Information with BGP Neighbor
The following example shows how to exchange link-state information with a BGP neighbor:
Router# configure
Router(config)# router bgp 1
Router(config-bgp)# neighbor 10.0.0.2
Router(config-bgp-nbr)# remote-as 1
Router(config-bgp-nbr)# address-family link-state link-state
Router(config-bgp-nbr-af)# exit
IGP Link-State Database Distribution
A given BGP node may have connections to multiple, independent routing domains. IGP link-state database distribution into
BGP-LS is supported for both OSPF and IS-IS protocols in order to distribute this information on to controllers or applications
that desire to build paths spanning or including these multiple domains.
To distribute IS-IS link-state data using BGP-LS, use the distribute link-state command in router configuration mode.
Router# configure
Router(config)# router isis isp
Router(config-isis)# distribute link-state instance-id 32
To distribute OSPFv2 link-state data using BGP-LS, use the distribute link-state command in router configuration mode.
Router# configure
Router(config)# router ospf 100
Router(config-ospf)# distribute link-state instance-id 32
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