Strange OSPF LSA7/5 translation type behavior

[Vadim Semenov]

Hi, all

i have this topology: R2(area1 nssa)<-OSPF->(area1 nssa)R1(area0)<--OSPF-->(area0)R3

I redistribute loopback 22.22.22.22 into  R2 and LSA7/5 translation on R1 proceed correctlly.

But if i want to connect area 2 to R2 -> LSA7/5 translation broken. Could you please explaine why? ( I know what area 2 should be connected to backbone directly or through using virtual-link, which is not supported in nssa area).

Before area 2 connection to R2:

*Oct 29 13:20:20.186: OSPF-1 LSGEN: Build external LSA 22.22.22.22, mask 255.255.255.255, type 7, age 0, seq 0x8000002D to area 1
*Oct 29 13:20:20.186: OSPF-1 LSGEN: MTID Metric Metric-type     FA      Tag Topology Name
*Oct 29 13:20:20.186: OSPF-1 LSGEN:     0      20              2        2.2.2.2     0         Base

R5 has route to 22.22.22.22

After area 2 connection to R2:

*Oct 29 13:28:17.173: OSPF-1 LSGEN: Build external LSA 22.22.22.22, mask 255.255.255.255, type 7, age 0, seq 0x8000002E to area 1
*Oct 29 13:28:17.173: OSPF-1 LSGEN: MTID Metric Metric-type     FA     Tag Topology Name
*Oct 29 13:28:17.173: OSPF-1 LSGEN:    0        20          2           0.0.0.0    0          Base

R5 has not route to 22.22.22.22

[Richard Burts · Accepted solution]

You have already identified correctly what is going on and what causes it. area 2 needs a connection to the backbone. But there is no physical connection to the backbone and you can not configure a virtual link over NSSA.

It would be logical to think of R2 as a Border router if it has interfaces in two areas. But that is not how the Cisco implementation works. If R2 has two areas but no connection to the backbone then R2 does not share routes between its areas (including not doing the type 7 to type 5 translation).

HTH

Rick

[Vadim Semenov]

Thank you, Richard!

i knew what if router has connection to 2 areas (but no backbone) - router stop generate Summary LSA, but i forgot that the same rules apply on LSA7 type.

[Richard Burts]

This aspect of what is required for a Border router is sometimes now well understood. I am glad that my explanation was helpful. Thank you for using the rating system to mark this question as answered. This will help other readers in the forum to identify discussions with helpful information.

HTH

Rick