The segment routing module adds to TTM a shortest path SR tunnel entry 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 given 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 which 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 per IGP instance. ECMP and LFA next-hops are mutually exclusive as per existing implementation.
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 various tunnel types. This includes the preference of both SR tunnels based on shortest path (referred to as 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 regardless 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 choose to either go with the global TTM preference or list explicitly 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, a fallback to the next preferred tunnel type is performed if the selected one fails. Also, a reversion to a more preferred tunnel type is performed as soon as one is available. 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 configure the preference of each specific IGP instance away from the above 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>