OSPF V-bit

EdTheLad

I've been playing around with TransitCapabilty and i'm wondering why does the ABR terminating a VL sends router LSAs with the V-bit set. The V-bit is supposedly used to tell other routers within the area that the area is a transit area. Now why do the other routers need to know the area is transit? In my testing as far as i can see the TransitCapability really just applies to the ABR terminating the VL.

So if i have 3 area's 0, 1 and 2

A VL is configured across area 1 to join area 2 to the backbone. Rx and Ry are the VL endpoints. Ry will inject summary type 3 lsa's into Area 1, Ry will also send type 3 lsa's over the VL to R2.

When transit capability is off, Rx will prefer the summaries arriving on the VL, even if the summaries injected into area 1 are lower cost. With TransitCapability enabled, Rx will prefer the lower cost summaries which are injected by Ry.

If i have another router Rz in area 1, it will receive type 3 lsa's from both ABRs Rx and Ry, which ever has the lowest metric is installed into the rib regardless of any V-bit setting.

So can somebody give me a practical example of how this V-bit can influence the routing decision for the area routers without a VL endpoint?

EdTheLad

Finally worked it out, sometimes the hard part is coming up with a topology to test a feature.Anyway i'll leave it open as a quiz question and see if anyone can answer it. I'll give the answer in 12+ hrs .

EdTheLad

No quiz takers, very disappointing , anyway the V-bit only affects other abr's in the transit area.

gorebrush

Aww I was in the middle of trying to lab all this up

I haven't done OSPF in any anger for a while

EdTheLad

You should have sent me a pm, to tell me to hold off, i didn't think anyone was interested.

gorebrush

I'll know for next time.

Reading your posts still makes me go "where's he getting all that from?" I don't think I'm at your knowledge level at this point.