Suppose ABR1 is an ABR which connects Area 1 to Area 0. Area 1 consists of routers connected via point to point links only (advertised by LSA 1). Is this correct that ABR1 will report all of these links as LSA3 with linkid as router address and data as subnet

| RT1<--p2p-->RT2<--p2p-->ABR1<--->    |
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 14 '17 at 22:41

All the routers in an area will generate Type-1 LSAs to describe their directly attached links in that area. It doesn't matter if they are point-to-point links are not. Every router in an area will know about every other router and its links in an area.

An ABR will convert all the Type-1 LSAs in one area to a Type-3 LSA for the other area to which it is attached. The ABR Router ID will be the advertising router for the Type-3 LSAs which it generates. A Type-3 LSA can contain multiple networks.

From RFC 2328, OSPF Version 2:

When the LSA is describing a network (LS type = 2, 3 or 5), the network's IP address is easily derived by masking the Link State ID with the network/subnet mask contained in the body of the LSA.


What you quoted in your comment is actually on page 30, not 31. If you read it carefully, that area is performing a route summary, not to be confused with a summary LSA. You are confusing two different things. A Type-3 LSA is a summary LSA which contains all the networks for the Type-1 and Type-2 LSAs from an area. A summary route is a single, supernet route containing all the routes behind it. A summary route can only be advertised from an ABR in OSPF, but other routing protocols can use summary routes, and only OSPF uses LSAs.

Yes, they Type-3 LSA would contain all the individual Type-1 LSAs for all the routes covered by the summary route if the Area was not summarizing the routes, but the Type-1 LSAs themselves would not be leaked into Area 0.

  • What I am really interested in is whether eventually all internal Area 1 RT's ip's will be flooded into area0, on contrary to say "regular" situations (in broadcast network with DR) where the internal routers ip's are "hidden" behind LSA 3 network advertisements of the ABR.
    – Boris
    Jul 30 '16 at 21:01
  • The Type-3 LSAs generated by ABRs contain all the information from the Type-1 and Type-2 LSAs in an area. Type-1 and Type-2 LSAs do not cross ABRs.
    – Ron Maupin
    Jul 30 '16 at 21:04
  • If there's a broadcast network segment in Area 1 then LSA 3 advertised by ABR into Area 0 will contain the network and its mask without "revealing" the ip's of internal routers attached to the network (derived from LSA1 records). However this logic would not hold when advertising the point to point links so all routers ip's attached to the ends of the links (advertised in LSA1) should be advertised to the area 0. Is that correct?
    – Boris
    Jul 30 '16 at 21:13
  • That is incorrect. The details of an area are only within an area. Every router in an area knows about all the other routers and networks (including point-to-point links) in that area, but other areas do not. Area 0 (or any other areas attached to Area 0) would know that to get to any of the networks (including the point-to-point links) in your Area 1, they must go through ABR1. No Type-1 (including point-to-point links) or Type-2 LSAs are sent from one area to another area, and that includes Area 0.
    – Ron Maupin
    Jul 30 '16 at 21:22
  • I am not saying that LSA1 would be sent by ABR1. I am trying to understand the content of LSA3 in this case. What stems from your comment is that ABR1 has nothing to advertise as there are only p2p links behind it. Look at page 31 of the RFC. Pay attention to H1 which is connected in p2p manner. It states specifically that 3rd area is configured to condense the H1 p2p link which results in single line in routing table. But that implies that if area 3 wasn't configured in such manner , H1 identity would appear in area 0 routing table as a single line (masked /32).
    – Boris
    Jul 30 '16 at 21:33

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