Skip to main content

3.1.3. SRv6

When SR is used over the IPv6 data plane:

  • A Prefix-SID is an IPv6 address.

  • An operator MUST explicitly instantiate an SRv6 SID. IPv6 node addresses are not SRv6 SIDs by default.

A node N advertising an IPv6 address R usable as a segment identifier MUST maintain the following FIB entry:

Incoming Active Segment: R
Ingress Operation: NEXT
Egress interface: NULL

Note that forwarding to R does not require an entry in the FIBs of all other routers for R. Forwarding can be, and most often will be, achieved by a shorter mask prefix that covers R.

Independent of SR support, any remote IPv6 node will maintain a plain IPv6 FIB entry for any prefix, no matter if the prefix represents a segment or not. This allows forwarding of packets to the node that owns the SID even by nodes that do not support SR.

Support of multiple algorithms applies to SRv6. Since algorithm-specific SIDs are simply IPv6 addresses, algorithm-specific forwarding entries can be achieved by assigning algorithm-specific subnets to the (set of) algorithm specific SIDs that a node allocates.

Nodes that do not support a given algorithm may still have a FIB entry covering an algorithm-specific address even though an algorithm-specific path has not been calculated by that node. This is mitigated by the fact that nodes that do not support a given algorithm will not be included in the topology associated with that algorithm-specific SPF; therefore, traffic using the algorithm-specific destination will normally not flow via the excluded node. If such traffic were to arrive and be forwarded by such a node, it will still progress towards the destination node. The next-hop will be either a node that supports the algorithm -- in which case, the packet will be forwarded along algorithm-specific paths (or be dropped if none are available) -- or a node that does NOT support the algorithm -- in which case, the packet will continue to be forwarded along Algorithm 0 paths towards the destination node.

Last updated on