The mapping server feature configures and advertises the node SID index for prefixes of routers in the LDP domain, via IS-IS. 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 generates 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 leaks 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.
The A-flag indicates that a prefix for which the mapping server prefix SID is advertised is directly attached. The M-flag advertises 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 an IS-IS SID/label binding TLV in that instance. The flooding scope of the TLV from the mapping server is determined as described above. No further check of the reachability of that prefix in the mapping server route table is performed. No check of the SID index is performed to determine whether if the SID index is duplicated with an existing prefix in the local IGP instance database or if the SID index is out of range with the local SRGB.