{思科CCIE-RS}OSPF external Tag用于MPLS VPN LSA5/LSA7防环（扩展） BY乱雪纷飞

The Domain Tag is applicable only for the OSPF Type 5 and Type 7 LSA. When the VPNv4 routes are redistributed from MP-BGP into OSPF on PE Router, the Domain Tag is set for OSPF External Routes. The tag could either be manuallly set with the domain-tag command under OSPF Process or a 32-bit value can be automatically generated:

Based on the network topology, PE2 sets the Domain Tag for Type 5 and Type 7 LSA when it redistributes the VPNv4 route into OSPF. This LSA is never considered for route calculation because the DN Bit is already set, but it also has the Domain Tag set, so the LSA is ignored because the Domain Tag matches the VPN / VRF Tag. Hence the route is never redistributed into OSPF.

This example shows LSA Type 5 ignored when it is received with the Domain Tag Set the same as the local VRF Domain Tag on PE3 from CE3:

*Jan 31 00:29:23.947: OSPF-2 EXTER:  adv_rtr 10.10.57.5, age 3, seq 0x80000001,

metric 10, metric-type 2, fw-addr 0.0.0.0

*Jan 31 00:29:23.947: OSPF-2 EXTER: Tag equals to VPN Tag, ignoring the LSA

*Jan 31 00:29:23.947: OSPF-2 EXTER: Process partial nssa spf queue

MPLS VPN OSPF PE-CE always includes the loop-prevention mechanism in order to handle issues. In the older Cisco IOS®, the Per original IETF draft Type 3 LSAs use the DN Bit in LSA and Type 5 LSAs use a tag. The newer RFC 4576 mandates use of DN Bit for both Type 3 and Type 5 LSAs.​