OSPF Down Bit

[ipotts]

Hello,

In "MPLS and VPN Architectures" Volume 1 on p223 it states "The down-bit is necessary only if customer CE-routers have connectivity to each other within area 0 and also have attachment to other non-backbone areas. This is because of the rule in OSPF that states: If an ABR receives a summary LSA from a non-backbone area, it should ignore the summary if it has connectivity to Area 0".

I thought the down bit was set by a PE, so that a remote PE would not redistribute the route back into the MP-BGP cloud causing a loop. Therefore, why does the book discuss how it controls a summary from non-area 0 entering area 0?

Secondly, does this rule actually exist "If an ABR receives a summary LSA from a non-backbone area, it should ignore the summary if it has connectivity to Area 0". If so, where abouts in RFC2328?

Many Thanks

Ian

[Harold Ritter · Accepted solution]

In a multi-home scenario, Any topology where a LSA type 3 generated by one PE can get to the other PE would cause a loop if it wasn't of the DN bit. This includes the CEs and PEs all being in area 0 (with the link between CEs in area 0 as well) or a CE connected to 2 PEs.

As far as your second question is concerned, section 12.4.3. states the follwing:

Note that only intra-area routes are advertised into the backbone.

Hope this helps,

[ipotts]

Thank you, that helps a lot.