how to avoid routing loops when an area connects with two ABRs with the backbone area

[geo555]

Hello everyone.

We recently switched from eigrp to ospf and I have a question.

Each of our remote site/area has two routers, for redundancy purposes, that connect to the backbone area. 

r1-----r3------r5

|            |             |

r2-----r4------r6

R1-R2-R3-R4 area 0

R3-R4-R5-R6 area 1

R3,R4 are ABRs

is this design valid?

since inter-area ospf is distance vector, do I need to implement area filtering routes to avoid a routing loop via area 1?

[VIvanov]

Good Design, there will be no loop as the SPF protocol will not make a looped network. Generally to loop a network you need a couple different types of routing protocols(or static routes).

[Joseph W. Doherty]

"Is this design valid?"

For avoiding routing loops, yes, at also noted by Vlvanov.

Concerning other possible issues, it depends in which area r3<>r4 link is and whether you're doing summarization on r3 and r4.

[geo555]

R3-R4 is area 0, and yes we are doing summarization on r3 r4

[Joseph W. Doherty · Accepted solution]

Ok, then it's possible to blackhole traffic, again depending how the r3<>r4 link is configured. For example, r3 and r4 summarize area 1, and the r3<>r4 link is in area 0, and r4<>r6 link is down, any area 1 traffic that arrives on r4 will be blackholed.

[geo555]

this is very interesting, can you point me to a document that describes best practices with multi ABRs between regions? I have searched cisco site and I haven't found anything with multi ABRs

[Joseph W. Doherty]

". . . can you point me to a document that describes best practices with multi ABRs between regions?"

Sorry, none that I recall.

My example is due to intra-area routing taking precedence over inter-area routing.