In this mode of operation, the user configures the LSP name, primary path name and optional secondary path name with the path information in the referenced path name, entering a full or partial explicit path with all or some hops to the destination of the LSP. Each hop is specified as an address of a node or an address of the next hop of a TE link. Optionally, each hop may be specified as a SID value corresponding to the MPLS label to use on a hop. In this case, the whole path must consist of SIDs.
To configure a primary or secondary path to always use a specific link whenever it is up, the strict hop must be entered as an address corresponding to the next hop of an adjacency SID, or the path must consist of SID values for every hop. If the strict hop corresponds to an address of a loopback address, it is translated into an adjacency SID as described below and therefore does not guarantee that the same specific TE link is picked.
MPLS assigns a Tunnel-ID to the SR-TE LSP and a path-ID to each new instantiation of the primary or secondary path. These IDs represent both the MBB path and the original path of an SR-TE LSP, which both exist during the process of an MBB update of the primary path.
The router retains full control of the path of the LSP. The LSP path label stack size is checked by MPLS against the maximum value configured for the LSP after the TE-DB returns the label stack. See Service and shortcut application SR-TE label stack check for more information about this check.
The ingress LER performs the following steps to resolve the user-entered path before programming it in the data path:
MPLS passes the path information to the TE-DB, which uses the hop-to-label translation or the full CSPF method to convert the list of hops into a label stack. The TE database returns the actual selected hop SIDs plus labels as well the configured path hop addresses which were used as the input for this conversion.
The ingress LER validates the first hop of the path to determine the outgoing interface and next hop where the packet is to be forwarded and programs the data path according to the following conditions:
If the first hop corresponds to an adjacency SID (host address of next hop on the link’s subnet), the adjacency SID label is not pushed. In other words, the ingress LER treats forwarding to a local interface as a push of an implicit-null label.
If the first hop is a node SID of some downstream router, then the node SID label is pushed.
In both cases, the SR-TE LSP tracks and rides the SR shortest path tunnel of the SID of the first hop.
In the case where the router is configured as a PCC and has a PCEP session to a PCE, the router sends a PCRpt message to update PCE with the state of UP and the RRO object for each LSP which has the pce-report option enabled. PE router does not set the delegation control flag to keep LSP control. The state of the LSP is now synchronized between the router and the PCE.