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OSPF Redistribution / Down Bit LSA Type3 or 5

Hi,


I'm trying to figure out something here and not sure which one is correct. I am currently doing MPLS VPN with 3 PEs. One of them is the hub and the other two are just stub centres connected to MPLS-VPN backbone. I'm using OSPF as the routing protocol on both spoke sites between PE and CE then the main hub just have a loopback interface being advertised to the VPN for now for testing purposes. However, I'm wonderin why there's a different behaviour on each spoke sites although they have the same configuration (different IPs of course). When I advertised 1.1.1.1/32 network from the hub PE using a network statement under MP-BGP, the result in spoke centres are not the same.

Spoke1-PE1:

show ip ospf database external 1.1.1.1

            OSPF Router with ID (10.200.21.33) (Process ID 65020)

                Type-5 AS External Link States

  LS age: 1307

  Options: (No TOS-capability, DC, Downward)

  LS Type: AS External Link

  Link State ID: 1.1.1.1 (External Network Number )

  Advertising Router: 10.200.21.33

  LS Seq Number: 80000002

  Checksum: 0x5486

  Length: 36

  Network Mask: /32

        Metric Type: 1 (Comparable directly to link state metric)

        MTID: 0

        Metric: 15

        Forward Address: 0.0.0.0

        External Route Tag: 3489725933

Spoke2-PE1:

sh ip ospf database external 1.1.1.1

            OSPF Router with ID (10.200.21.18) (Process ID 65020)

                Type-5 AS External Link States

  LS age: 1254

  Options: (No TOS-capability, DC)

  LS Type: AS External Link

  Link State ID: 1.1.1.1 (External Network Number )

  Advertising Router: 10.200.21.18

  LS Seq Number: 80000002

  Checksum: 0x3634

  Length: 36

  Network Mask: /32

        Metric Type: 1 (Comparable directly to link state metric)

        MTID: 0

        Metric: 15

        Forward Address: 0.0.0.0

        External Route Tag: 3489725933


As you can see, Spoke1-PE1 has a downbit set on its Type5 LSA. I read on several articles that Type5 LSAs shouldn't have down bit as it uses domain-tag as the loop prevention mechanism but it isn't the case here. Spoke2-PE1 does not have downhard in the output so I suspect this type5 entry in the its database does not have downbit.

The other difference is the IOS, one is using 15.3(1)T and the other one is 15.2(1)T3a. The 15.3 is the one running type5 down bit and the 15.2 does not have.

Is this a bug or something or am I doing something wrong?

Thanks,

John

1 ACCEPTED SOLUTION

Accepted Solutions

DearsFigured out that old

Dears

Figured out that old implementation is to use domain-tag and new implementation is to use down-bit also with LSA-5

http://mellowd.co.uk/ccie/?p=4814

Regards

Sherif Ismail

3 REPLIES

Re: OSPF Redistribution / Down Bit LSA Type3 or 5

I downgraded the higher version router to 15.2 and the downbit LSA in Type5 is now gone. Not sure if this is a bug or there's a change with the OSPF process on 15.3.

show ip ospf database external 1.1.1.1

            OSPF Router with ID (10.200.21.33) (Process ID 65020)

                Type-5 AS External Link States

  LS age: 214

  Options: (No TOS-capability, DC)

  LS Type: AS External Link

  Link State ID: 1.1.1.1 (External Network Number )

  Advertising Router: 10.200.21.33

  LS Seq Number: 80000001

  Checksum: 0xDD7E

  Length: 36

  Network Mask: /32

        Metric Type: 1 (Comparable directly to link state metric)

        MTID: 0

        Metric: 15

        Forward Address: 0.0.0.0

        External Route Tag: 3489725933

Hi JohnI am having same issue

Hi John

I am having same issue also but with XRv .. Not sure If this is bug or XR behavior is different
Any ideas ?

Thanks

|||||||||

RP/0/0/CPU0:XR2#show ospf database external
Mon Apr 28 10:53:37.481 UTC


            OSPF Router with ID (20.20.20.20) (Process ID 100)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 956
  Options: (No TOS-capability, DC, DN)
  LS Type: AS External Link
  Link State ID: 10.1.1.1 (External Network Number)
  Advertising Router: 19.19.19.19
  LS Seq Number: 80000002
  Checksum: 0xc9c6
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 3489661028

 

Regards

Sherif Ismail

DearsFigured out that old

Dears

Figured out that old implementation is to use domain-tag and new implementation is to use down-bit also with LSA-5

http://mellowd.co.uk/ccie/?p=4814

Regards

Sherif Ismail

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