The following TLVs/sub-TLVs are defined in draft-ietf-ospf-segment-routing-extensions-04 and are required for the implementation of segment routing in OSPF:
prefix SID sub-TLV part of the OSPFv2 Extended Prefix TLV
prefix SID sub-TLVpart of the OSPFv2 Extended Prefix Range TLV
adjacency SID sub-TLV part of the OSPFv2 Extended Link TLV
SID/Label Range Capability TLV
SR-Algorithm Capability TLV
This section describes the behaviors and limitations of the OSPF support of segment routing TLVs and sub-TLVs.
The 7210 SAS originates a single prefix SID sub-TLV for each OSPFv2 Extended Prefix TLV and processes the first one only if multiple prefix SID sub-TLVs are received within the same OSPFv2 Extended Prefix TLV.
The 7210 SAS encodes the 32-bit index in the prefix SID sub-TLV. The 24-bit label or variable IPv6 SID is not supported.
The 7210 SAS originates a prefix SID sub-TLV with the following flag encoding:
The NP-flag is always set, meaning that the label for the prefix SID is pushed by the PHP router when forwarding to this router. 7210 SAS PHP routers process a received prefix SID with the NP-flag set to zero and use implicit-null for the outgoing label toward the router that advertised it.
The M-flag is always unset because the 7210 SAS does not support originating a mapping server prefix-SID sub-TLV.
The E-flag is always set to 0. The 7210 SAS PHP routers properly process a received prefix SID with the E-flag set to 1, and when the NP-flag is also set to 1, they push explicit-null for the outgoing label towards the router that advertised it.
The V-flag is always set to 0 to indicate an index value for the SID.
The L-flag is always set to 0 to indicate that the SID index value is not locally significant.
The algorithm field is always set to 0 to indicate the SPF algorithm is based on the link metric and is not checked on a received prefix SID sub-TLV.
The system resolves a prefix SID received within an extended prefix TLV based on the following route preference:
SID received via an intra-area route in a prefix SID sub-TLV part of Extended Prefix TLV
SID received via an inter-area route in a prefix SID sub-TLV part of Extended Prefix TLV
The 7210 SAS originates an adjacency SID sub-TLV with the following encoding of the flags:
The F-flag is not set to indicate the adjacency SID refers to an adjacency with outgoing IPv4 encapsulation.
The B-flag is set to 0 and is not processed on receipt.
The V-flag is always set.
The L-flag is always set.
The S-flag is not supported.
The weight octet is not supported and is set to all zeros.
The 7210 SAS does not originate the OSPFv2 Extended Prefix Range TLV but can process it if received. The following rules and limitations should be considered:
Only the prefix SID sub-TLV within the TLV is processed, and the ILMs are installed if the prefixes are resolved.
The range and address prefix fields are processed. Each prefix is resolved separately.
If the same prefix is advertised with both a prefix SID sub-TLV in an IP reachability TLV and a mapping server Prefix-SID sub-TLV, the resolution follows the following route preference:
the SID received via an intra-area route in a prefix SID sub-TLV part of Extended Prefix TLV
the SID received via an inter-area route in a prefix SID sub-TLV part of Extended Prefix TLV
the SID received via an intra-area route in a prefix SID sub-TLV part of an OSPFv2 Extended Range Prefix TLV
the SID received via an inter-area route in a prefix SID sub-TLV part of an OSPFv2 Extended Range Prefix TLV
No route leaking of any part of the TLV is allowed between areas. In addition, an ABR does not propagate the prefix-SID sub-TLV from the Extended Prefix Range TLV (received from a mapping server) into an Extended Prefix TLV if the latter is propagated between areas.
The mapping server that advertised the OSPFv2 extended prefix range TLV does not need to be in the shortest path for the FEC prefix.
If the same FEC prefix is advertised in multiple OSPFv2 extended prefix range TLVs by different routers, the SID in the TLV of the first router that is reachable is used. If that router becomes unreachable, the next reachable one is used.
No check is performed to determine whether the contents of the OSPFv2 Extended Prefix Range TLVs received from different mapping servers are consistent.
Any other sub-TLV (for example, the ERO metric and unnumbered interface ID ERO) is ignored, but the user can get a list of the octets of the received but unsupported sub-TLVs using the existing IGP show command.
The 7210 SAS supports the propagation on ABR of an external prefix LSA into other areas with the route type set to 3 as per draft-ietf-ospf-segment-routing-extensions-04.
The 7210 SAS supports the propagation on ABR of external prefix LSAs with route type 7 from the NSSA area into other areas with the route type set to 5, as described in draft-ietf-ospf-segment-routing-extensions-04. The system does not support the propagation of the prefix SID sub-TLV between OSPF instances.
If an OSPF import policy is configured, the outcome of the policy applies to prefixes resolved in RTM and the corresponding tunnels in TTM. A prefix that is removed by the policy is removed as both a route in the RTM and as an SR tunnel in the TTM.