Ethan Banks On productivity.

NMC DOiT Vol.2 Scenario 7 Day 4-1 – OSPF NSSA 7-to-5 Election + BGP Dual-AS


I didn’t spend any time with the rack last night. I couldn’t face it. I just relaxed, got a good night’s sleep, and I’m spending the afternoon here finishing up scenario 7. This is sort of like what I think the afternoon may be like in the real lab – I’ve got mutual reachability tasks completed, which I would have done in the morning of the real lab, ideally. So now it’s the afternoon, and I’m working through the rest of the tasks, cherry-picking the stuff I know how to do easily, working to get the point value up over 80 as quickly as possible.

There is one OSPF issue I ran into that is worth talking about. One of the OSPF areas was NSSA. I was also redistributing routes into that NSSA area. In principle, an NSSA area takes external routes and makes them “O N2” routes, which are type 7 LSAs. Type 7 LSAs don’t make it out of the NSSA area. Instead, the ABR for that NSSA area converts the type 7 LSAs to type 5’s before sending them into the backbone area.

One of the NSSA routers will be elected to be the “7-to-5” converter. If I understand it correctly, this will be the ABR with the highest RID. That means you may have to manually set the RID if the scenario tells you which router is to be the 7-to-5 converter. I didn’t pay attention to this detail, and ended up with some issues that I think were unique to this scenario. The router that ended up being my 7-to-5 had a connection to area 0 via a virtual link. But, none of my O N2 routes were known to area 0. I’m not sure why that wasn’t working – in theory, the NSSA router connected to area 0 via a virtual link should have been able to handle the 7-to-5 conversion and flood the type 5 LSAs into area 0 over the virtual link. But that wasn’t happening.

Long story slightly shorter – I reset the router RID as required by the scenario to force the 7-to-5 converter role to the proper router. This router was attached to area 0 normally, not via a virtual link. I cleared the OSPF process on the 2 routers and let the adjacencies come back up. Voila! All my NSSA O N2 routes began showing up in area 0 as O E2 routes. Life was good.

Moving away from OSPF to make a brief BGP comment, I learned from this scenario that you can make a router appear to participate in multiple BGP autonomous systems by using the “neighbor local-as” command. That command is a tool to help you transition into a new BGP AS.

Read more about BGP dual-AS support.

Read more about the neighbor local-as command.

By Ethan Banks
Ethan Banks On productivity.

You probably know Ethan Banks because he writes & podcasts about IT. For example, he co-authored "Computer Networks Problems & Solutions" with Russ White.

This site is Ethan on productivity--not tech so much.

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