Selection of IGP for IPv6 is agnostic for dual stack scenarios in mpls vpn backbones or ip backbones. Everyone is having a question that we are using OSPF as IGP for IPv4, which protocol is used as IGP for IPv6. No one is actually putting light which one is best for IPv6 where in OSPF already deployed for IPv4 in most of the cases. During reading RFC 4029 “Selection and Analysis For Introducing IPv6 into SP Networks”, I found section 4.3.1 of IGP which is explicitly defining the protocols require for IGP in IPv6 deployments. According to RFC OSPFv3,IS-IS is used but CISCO is also providing IPv6 support for EIGRP and RIPng is clearly renounce. If we are using the same protocol for both IPv6 as well as for IPv6, it may lead to lot of the problems in case of instability of links because every time protocol has to calculate the path for each and every link. But in case of different protocols for IPv4 and IPv6, the problem in one protocol may not affect another but this requires lot of CPU as well as memory.
Possible combinations are as follows:
1. OSPFv2 for IPv4, IS-IS for IPv6 (only)
2. OSPFv2 for IPv4, OSPFv3 for IPv6
3. IS-IS for IPv4, OSPFv3 for IPv6
4. S-IS for both IPv4 and IPv6
When IS-IS is used for both IPv4 as well as for IPv6, the IPv6/IPv6 topologies must be convex, unless multiple-topology IS-IS extensions have been implemented.