Hello, everyone.
I found this topic by accident while studying for SPCOR. I am not talking about having an ABR with interfaces in multiple areas, I am talking about a literal multi-area adjacency (one interface being in multiple areas)
Take this as an example (don’t worry, the red area on the left is Area 0).
P4 is advertising 4.4.4.4/32. This is a loopback enabled for OSPF Area 200.
P5-XR prefers the longer path via Area 200 over Area 100 because O > O IA.
To mitigate this, we can literally configure the interfaces between P4 and P5-XR to be part of both Area 100 and Area 200.
RP/0/RP0/CPU0:P5-XR(config-ospf)#area 200
RP/0/RP0/CPU0:P5-XR(config-ospf-ar)#multi-area-int G0/0/0/2
RP/0/RP0/CPU0:P5-XR(config-ospf-ar-mif)#commit
P4(config)#int G2
P4(config-if)#ip ospf 110 area 100
P4(config-if)#ip ospf multi-area 200
This forms a multiarea adjacency.
I have some questions regarding this.
- Documentation states that “A multi-area interface advertises an unnumbered point-to-point link in the device link-state advertisement (LSA) for the corresponding area”. I haven’t found this kind of entry in the LSDB.
Here is the LSDB for P5-XR
Neither of these have an unnumbered LSA entry.
- Documentation states “It establishes a neighbor relationship with the corresponding multiarea interface on the neighboring device. A mixture of multiarea and primary interfaces is not supported”"
If I perform the following configuration:
RP/0/RP0/CPU0:P5-XR(config-ospf)#area 200
RP/0/RP0/CPU0:P5-XR(config-ospf-ar)#no multi-area-int G0/0/0/2
RP/0/RP0/CPU0:P5-XR(config-ospf-ar-mif)#commit
P4(config-if)#ip ospf 110 area 200
P4(config-if)#ip ospf multi-area 100
P5-XR has no multiarea interface, it only has the primary interface. P4 has a multiarea interface and yet it happily forms the adjacency with P5-XR despite one interface being multiarea and the other one being primary. Wasn’t the documentation saying this wasn’t possible? Or am I misunderstanding this?
Thank you
David