Why Can't I See My Type 4 LSA?

My network is in the link below (note, R2's RID is actually 22.22.22.22, and NOT 2.2.2.2)

http://ccieblog.co.uk/wp-content/uploads/2015/07/OSPF-Forwarding-Address-With-An-NSSA-Two-ABRs.png

I want to know why I can't see any ASBR summary LSA on R1. Redistribution is bidrectional between OSPF and EIGRP, and R1 has reachability to the 99.0.0.0/24 network. I just can't understand why R1 does not have a Type 4 LSA from R2 or R3. Here is ospf database on R2 and R1

 

R2#sh ip ospf da

OSPF Router with ID (22.22.22.22) (Process ID 1)

Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 370 0x8000000B 0x002BE0 4
3.3.3.3 3.3.3.3 1434 0x80000009 0x006553 2
22.22.22.22 22.22.22.22 291 0x80000007 0x008F95 2

Summary Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum
10.0.24.0 3.3.3.3 671 0x80000005 0x006492
10.0.24.0 22.22.22.22 291 0x80000007 0x00BFF2
10.0.34.0 3.3.3.3 1696 0x80000008 0x008B68
10.0.34.0 22.22.22.22 533 0x80000005 0x00B9E6

Router Link States (Area 1)

Link ID ADV Router Age Seq# Checksum Link count
3.3.3.3 3.3.3.3 1434 0x8000000C 0x003841 2
4.4.4.4 4.4.4.4 1751 0x80000010 0x00247A 4
22.22.22.22 22.22.22.22 291 0x80000007 0x0035C5 2

Summary Net Link States (Area 1)

Link ID ADV Router Age Seq# Checksum
10.0.12.0 3.3.3.3 1435 0x80000009 0x008672
10.0.12.0 22.22.22.22 292 0x80000007 0x00E9CE
10.0.13.0 3.3.3.3 1435 0x80000009 0x0017EA
10.0.13.0 22.22.22.22 294 0x80000007 0x00436A

Type-7 AS External Link States (Area 1)

Link ID ADV Router Age Seq# Checksum Tag
99.0.0.0 4.4.4.4 1754 0x8000000A 0x001FCA 0

Type-5 AS External Link States

Link ID ADV Router Age Seq# Checksum Tag
99.0.0.0 22.22.22.22 1795 0x80000006 0x009D12 0

 

R1#  sh ip ospf database

OSPF Router with ID (1.1.1.1) (Process ID 1)

Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 922 0x8000000B 0x002BE0 4
3.3.3.3 3.3.3.3 1986 0x80000009 0x006553 2
22.22.22.22 22.22.22.22 845 0x80000007 0x008F95 2

Summary Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum
10.0.24.0 3.3.3.3 1223 0x80000005 0x006492
10.0.24.0 22.22.22.22 845 0x80000007 0x00BFF2
10.0.34.0 3.3.3.3 206 0x80000009 0x008969
10.0.34.0 22.22.22.22 1087 0x80000005 0x00B9E6

Type-5 AS External Link States

Link ID ADV Router Age Seq# Checksum Tag
99.0.0.0 22.22.22.22 333 0x80000007 0x009B13 0

 

Comments

  • You dont see any type-4 LSAs because there is no need for them in your network! =) 

    You have an NSSA connected to Area 0...The NSSA is bringing in externals from EIGRP as Type-7s, which get translated into Type-5s by R2 in your case. 

    R1 uses the forwarding address on that Type-5, which will be one of the interfaces on R4 (depending on your config). The ABRs to get into Area 1 (R2 and R3) are in the same area as R1, so R1 does not need a Type-4 as part of the recursive lookup to get to R4.

    If you had another area behind R1, say for example area 2, routers in that area would need Type-4 LSAs, as they would not be in the same area as R2 and R3. The Type-4 LSA would be generated by R1, and it essentially makes R1 act as a proxy, in that it would tell routers in area 2 that R2 and R3 are reachable via R1 (R1, generating the Type-4, is acting as a sort of proxy).

    Does this help?

  • Ok so let me get this right. Tell me if any of my points are wrong below. I will try to do them in the right sequence.

    1. R4 redistributes EIGRP routes into the NSSA with a Type 7 LSA
    2. R2 does the Type 7 to 5 translation and forwards a type 5 LSA to R1
    3. R1 receives the LSA 5 and checks the forwarding address. 
    4. The forwarding address is 10.0.34.4, so R1 checks the shortest path to the node who owns this IP.
    5. R1 doesn't find it in a Type 1 LSA, so checks his type 3 LSA's and finds a match via R2 and R3
    6. R3 has a shorter path (lower cost), so R3 is the winner. 
    7. R1 then checks for his type 1 LSA to reach the node R3. It finds the next hop from this information and routes via R3.

    Is this right?


     

  • The important thing to note about your scenario is that R4 is an NSSA, meaning that Type-7s are originated, not Type-5. R2 (or R3) are the devices that actually do the Type-5 origination, and it is due to this reason that Type-4 is not needed in Area 0. However, if you change Area 1 to be a normal area, then type-5s will be originated by R4. In this case, Type-4s will be needed within Area 0, as the originator of the Type-5 is not in the same area as R1.

    In the case where you have Area 100 attached to R1:

    Area 100 routers receive the flooded type-5, since it has domain-wide flooding scope, the forwarding address is set to 10.0.34.4, with advertising router-id of 22.22.22.22. Since 22.22.22.22 is not in their area, they need a type-4 in order to hint them where traffic should be forwarded. This is the proxy behavior I described in my last comment. 

    The best way to fully grasp this concept is to see this in action by trying it out in your lab. 

  • Hi,

    Curious, shouldn't there be a LSA type 5 99.0.0.0 from 3.3.3.3 too?

     

  • No. There is only 1 Type-7 to Type-5 translator. In this case it is R2 as it has the highest router-id, manually configured to 22.22.22.22

     

  • Great. Thanks!! (here I go re-read)

  • Thanks. I have done a lot more reading and now I understand it.

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