The segment routing module adds a shortest path SR tunnel entry to TTM for each resolved remote node SID prefix and programs the data path with the corresponding LTN with the push operation pointing to the primary and LFA backup NHLFEs. The LFA backup next hop for a prefix which was advertised with a node SID is only computed if the loopfree-alternates option is enabled in the IS-IS or OSPF instance. The resulting SR tunnel that is populated in TTM is automatically protected with FRR when an LFA backup next hop exists for the prefix of the node SID.
With ECMP, a maximum of 32 primary next-hops (NHLFEs) are programmed for the same tunnel destination for each IGP instance. ECMP and LFA next-hops are mutually exclusive.
The default preference for shortest path SR tunnels in the TTM is set lower than LDP tunnels but higher than BGP tunnels to allow controlled migration of customers without disrupting their current deployment when they enable segment routing. The following is the setting of the default preference of the tunnel types. This includes the preference of both SR tunnels based on shortest path (SR-ISIS and SR-OSPF).
The global default TTM preference for the tunnel types is as follows:
ROUTE_PREF_RSVP 7
ROUTE_PREF_SR_TE 8
ROUTE_PREF_LDP 9
ROUTE_PREF_OSPF_TTM 10
ROUTE_PREF_ISIS_TTM 11
ROUTE_PREF_BGP_TTM 12
ROUTE_PREF_GRE 255
The default value for SR-ISIS or SR-OSPF is the same, even if one or more IS-IS or OSPF instances programmed a tunnel for the same prefix. The selection of an SR tunnel in this case is based on lowest IGP instance ID.
The TTM preference is used in the case of BGP shortcuts, VPRN auto-bind, or BGP transport tunnel when the tunnel binding commands are configured to the any value, which parses the TTM for tunnels in the protocol preference order. The user can use the global TTM preference or explicitly list the tunnel types to be used. When the tunnel types are listed explicitly, the TTM preference is still used to select one type over the other. In both cases, if the selected tunnel type fails, the system falls back to the next preferred tunned type. When a more preferred tunnel type becomes available, the system reverts to that tunnel type. See BGP shortcut using segment routing tunnel, BGP label route resolution using segment routing tunnel, and Service packet forwarding with segment routing for the detailed service and shortcut binding CLI.
For SR-ISIS and SR-OSPF, the user can change the preference of each IGP instance away from the default values.
config>router>isis>segment-routing>tunnel-table-pref preference 1 to 255
config>router>ospf>segment-routing>tunnel-table-pref preference 1 to 255