Segment Routing Mapping Server Function for IPv4 Prefixes

The mapping server feature allows the configuration and advertisement via IS-IS of the node SID index for prefixes of routers which are in the LDP domain. This is performed in the router acting as a mapping server and using a prefix-SID sub-TLV within the SID/Label binding TLV in IS-IS.

The user configures the SR mapping database in IS-IS using the following CLI command:

configure
        — router
            — [no] isis
                — segment-routing
                — no segment-routing
                    — mapping-server
                        — sid-map node-sid {index 0..4294967295 [range  0..65535]} prefix {{ip-address/mask} | {ip-address}{netmask}} [set-flags {s}] [level {1|2|1/2}]
                        — no sid-map node-sid index 0..4294967295

The user can enter the node SID index for one prefix or a range of prefixes by specifying the first index value and, optionally, a range value. The default value for the range option is 1. Only the first prefix in a consecutive range of prefixes must be entered. The user can enter the first prefix with a mask lower than 32 and the SID/Label Binding TLV is advertised but the routers do not resolve these prefix SIDs and instead originates a trap.

The no form of the sid-map command deletes the range of node SIDs beginning with the specified index value. The no form of the mapping-server command deletes all node SID entries in the IS-IS instance.

By setting the S-flag, the user can indicate to the IS-IS routers in the rest of the network that the flooding scope of the SID/Label binding TLV is the entire domain. In that case, a router receiving the TLV advertisement should leak it between IS-IS levels. If leaked from level 2 to level 1, the D-flag must be set, after which the TLV cannot be leaked back into level 2. Otherwise, the S-flag is clear by default and the TLV must not be leaked by routers receiving the mapping server advertisement.

Note: SR OS does not leak this TLV between IS-IS instances and does not support the multi-topology SID/Label Binding TLV format. In addition, the user can specify the mapping server’s flooding scope for the generated SID/Label binding TLV using the level option. This option allows further narrowing of the flooding scope configured under the router IS-IS level-capability for a one or more SID/Label binding TLVs if required. The default flooding scope of the mapping server is L1/L2, which can be narrowed by what is configured under the router IS-IS level-capability.

The A-flag is used to indicate that a prefix for which the mapping server prefix SID is advertised is directly attached. The M-flag is used to advertise a SID for a mirroring context to provide protection against the failure of a service node. None of these flags are supported on the mapping server; the mapping client ignores them.

Each time a prefix or a range of prefixes is configured in the SR mapping database in any routing instance, the router issues for this prefix, or range of prefixes, a prefix-SID sub-TLV within a IS-IS SID/label binding TLV in that instance. The flooding scope of the TLV from the mapping server is determined as explained above. No further check of the reachability of that prefix in the mapping server route table is performed and no check if the SID index is duplicate with some existing prefix in the local IGP instance database or if the SID index is out of range with the local SRGB.