Segment Routing Data Path Support

A packet received with a label matching either a node SID or an adjacency SID is forwarded according to the ILM type and operation, as described in Table 1.

Table 1. Data Path Support
Label type	Operation
Top label is a local node SID	Label is popped and the packet is further processed. If the popped node SID label is the bottom of stack label, the IP packet is looked up and forwarded in the appropriate FIB.
Top or next label is a remote node SID	Label is swapped to the calculated label value for the next-hop and forwarded according to the primary or backup NHLFE. With ECMP, a maximum of 32 primary next-hops (NHLFEs) are programmed for the same destination prefix and for each IGP instance. ECMP and LFA next-hops are mutually exclusive as per existing implementation.
Top or next label is an adjacency SID	Label is popped and the packet is forwarded out on the interface to the next-hop associated with this adjacency SID label. In effect, the data path operation is modeled like a swap to an implicit-null label instead of a pop.
Next label is BGP 3107 label	The packet is further processed according to the ILM operation as in current implementation. The BGP label may be popped and the packet looked up in the appropriate FIB. The BGP label may be swapped to another BGP label. The BGP label may be stitched to an LDP label.
Next label is a service label	The packet is looked up and forwarded in the Layer 2 or VPRN FIB as in current implementation.

A router forwarding an IP or a service packet over an SR tunnel pushes a maximum of two transport labels with a remote LFA next-hop. This is illustrated in Figure 1.

Figure 1. Transport Label Stack in Shortest Path Forwarding with Segment Routing

Assume that a VPRN service in node B forwards a packet received on a SAP to a destination VPN-IPv4 prefix X advertised by a remote PE2 via ASBR/ABR node A. Router B is in a segment routing domain while PE2 is in an LDP domain. BGP label routes are used to distribute the PE /32 loopbacks between the two domains.

When node B forwards over the primary next-hop for prefix X, it pushes the node SID of the ASBR followed by the BGP 3107 label for PE2, followed by the service label for prefix X. When the remote LFA next-hop is activated, node B pushes one or more segment routing label: the node SID for the remote LFA backup node (node N).

When node N receives the packet while the remote LFA next-hop is activated, it pops the top segment routing label which corresponds to a local node SID. This results in popping this label and forwarding of the packet to the ASBR node over the shortest path (link N-Z).

When the ABR/ASBR node receives the packet from either node B or node Z, it pops the segment routing label which corresponds to a local node SID, then swaps the BGP label and pushes the LDP label of PE2 which is the next-hop of the BGP label route.