Label Distribution Protocol
Page 240 7210 SAS M, T, X, R6, Mxp MPLS Configu-
ration Guide
When the peer is several hops away, the BFD session used will be the one associated with the
loopback interface corresponding to LSR-ID of the T-LDP session. The LSR-ID is used to
establish the Hello adjacency with the peer. By default the LSR-ID matches the system interface
address but the user can change it to any other loopback interface address [ldp-instances]. In that
case, the source address and destination address in the BFD packets will match the local end LSR-
ID and the far-end address specified for the peer respectively. The parameters used for the BFD
session are also those configured under the loopback interface corresponding to the LSR-ID using
the bfd command in the config>router>interface context.
Given that the BFD session used to track the same T-LDP peer may move from a link interface to
a loopback interface depending on route reachability, it is important that the user configures the
BFD session parameters consistently on both interfaces.
The link interface BFD session is sourced and maintained on the IOM while the loopback interface
BFD session is sourced and maintained on the CPM. As a result, the system level BFD resource
count reflects the worst case where each T-LDP session is using two BFD sessions.
Configuration
The user enables the stitching of routes between LDP and BGP by configuring separately tunnel
table route export policies in both protocols and enabling the advertising of RFC 3107 formatted
labeled routes for prefixes learned from LDP FECs.
The route export policy in BGP instructs BGP to listen to LDP route entries in the CPM tunnel
table. If a /32 LDP FEC prefix matches an entry in the export policy, BGP originates a BGP
labeled route, stitches it to the LDP FEC, and re-distributes the BGP labeled route to its iBGP
neighbors.
The user adds LDP FEC prefixes with the statement ‘from protocol ldp’ in the configuration of the
existing BGP
• configure>router>bgp>export policy-name
• configure>router>bgp>group>export policy-name
• configure>router>bgp>group>neighbour>export policy-name
To indicate to BGP to evaluate the entries with the ‘from protocol ldp’ statement in the export
policy when applied to a specific BGP neighbor, a new argument is added to the existing advertise
label command:
configure>router>bgp>group>neighbour>advertise-label ipv4 include-ldp-prefix
Without the new include-ldp-prefix argument, only core IPv4 routes learned from RTM are
advertised as BGP labeled routes to this neighbor. And the stitching of LDP FEC to the BGP
labeled route is not performed for this neighbor even if the same prefix was learned from LDP.
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