When we were writing the MPLS and VPN architectures books, there was a limit on the number of OSPF processes you could configure per PE-router. The limit was based on the fact that IOS supports up to 32 routing information sources. Two of them are static and connected; you also need an IGP and BGP in the MPLS VPN backbone, resulting in 28 OSPF processes that could be configured on a single PE router. This “feature” severely limited OSPF-based MPLS VPN deployments until IOS release 12.3(4)T when the limitation was removed, resulting in the availability of up to 30 routing processes per VRF.

RIP, BGP, and EIGRP never experienced the same limitations as you configure VRF-specific routing instances within address families of a single routing protocol

The details of the actual change that took place are not obvious from the IOS documentation; they simply changed the global allocation of routing process identifiers to per-routing-table identifiers. For example, in the following show ip protocol summary printouts, the two OSPF processes (one in the global routing table, one in the VRF test) share the same index.

router#show ip protocols summary
Index Process Name
0 connected
1 static
2 ospf 112
router#show ip protocols vrf Test summary
Index Process Name
0 connected
1 static
2 ospf 111