3.8 OSPFv2 to IS-IS Migration Example

Graceful migration from one IGP to another requires coordinated

operation of both protocols during the migration period. Such a

coordination requires identifying a given physical link in both IGPs.

The IPv4 Router-ID provides that "glue", which is present in the Node

Descriptors of the OSPF Link NLRI and in the link attribute of the

IS-IS Link NLRI.

Consider a point-to-point link between two routers, A and B, that

initially were OSPFv2-only routers and then IS-IS is enabled on them.

Node A has IPv4 Router-ID and ISO-ID; node B has IPv4 Router-ID, IPv6

Router-ID, and ISO-ID. Each protocol generates one Link NLRI for the

link (A, B), both of which are carried by BGP-LS. The OSPFv2 Link

NLRI for the link is encoded with the IPv4 Router-ID of nodes A and B

in the local and remote Node Descriptors, respectively. The IS-IS

Link NLRI for the link is encoded with the ISO-ID of nodes A and B in

the local and remote Node Descriptors, respectively. In addition,

the BGP-LS attribute of the IS-IS Link NLRI contains the TLV type

1028 containing the IPv4 Router-ID of node A, TLV type 1030

containing the IPv4 Router-ID of node B, and TLV type 1031 containing

the IPv6 Router-ID of node B. In this case, by using IPv4 Router-ID,

the link (A, B) can be identified in both the IS-IS and OSPF

protocol.