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OSPF - Loop prevention from backbone area back into a nonbackbone area

kylelongtran
Level 1
Level 1

Hi,

Can someone help explain me a question regarding OSPF Loop prevention in inter-area routing?

I understand the two rules about originating and processing type 3 LSAs on ABRs. First, when an ABR originates type 3 LSAs, it translates only intra-area routes from a nonbackbone area into type 3 LSAs and floods them into the backbone, and it translates both intra-area and inter-area routes from the backbone area into type 3 LSAs and floods them into nonbackbone areas. Second, when an ABR runs the SPF algorithm, it ignores all type 3 LSAs received over nonbackbone areas.
These two rules can help prevent a nonbackbone area tries to advertise inter-area rules learned from backbone area back into backbone are.

So my question is what if the situation is reversed: a backbone area tries to advertise inter-area rules learned from nonbackbone area back into that nonbackbone are? What rules can help prevent that?

Thank you.

1 Accepted Solution

Accepted Solutions

Rolf Fischer
Level 9
Level 9

Hi,

A backbone area tries to advertise inter-area rules learned from nonbackbone area back into that nonbackbone area? What rules can help prevent that?

An ABR performs a check which is a logical equivalent of a Distance Vector protocol's split horizon logic (RFC 2328 12.4.3):

Else, if the next hops associated with this set of paths belong to Area A itself, do not generate a summary-LSA for the route.

HTH
Rolf

View solution in original post

3 Replies 3

dukenuk96
Level 3
Level 3

Sorry, can not understand your question - can you draw diagram with routers, interconnections and area numbers, then, based on this diagram ask question referring to specific area numbers and routers.

Rolf Fischer
Level 9
Level 9

Hi,

A backbone area tries to advertise inter-area rules learned from nonbackbone area back into that nonbackbone area? What rules can help prevent that?

An ABR performs a check which is a logical equivalent of a Distance Vector protocol's split horizon logic (RFC 2328 12.4.3):

Else, if the next hops associated with this set of paths belong to Area A itself, do not generate a summary-LSA for the route.

HTH
Rolf

Thank you, Rolf. Your answer is very clear.

Sorry, can not understand your question - can you draw diagram with routers, interconnections and area numbers, then, based on this diagram ask question referring to specific area numbers and routers.

Sorry for not being clear. Please see the topology in the attachment.

R1 and R2 advertise type 3 LSA about 10.1.1.0/24 into area 0. My question is what rule can help prevent R1 and R2 from creating another type 3 LSA and advertise that route back into area 1.

I think Rolf has answered my question correctly.