Routing

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  • 1.  OSPF external routes tag within VRF

     
    Posted 12-06-2016 01:58

    I am redistributing a VRF learnt route (0/0) into OSPF configured within a VRF as an external route. The OSPF domain is area 0. I have noticed a tag on the route as shown below. No tags have been set by the export policy.

     

    {master}
    lab@MX480-0_re0> show ospf database instance VRF-BGP-1 external lsa-id 0.0.0.0 extensive
    OSPF AS SCOPE link state database
    Type ID Adv Rtr Seq Age Opt Cksum Len
    Extern *0.0.0.0 xxx.10.15.22 0x800000c6 2054 0xa2 0xc012 36
    mask 0.0.0.0
    Topology default (ID 0)
    Type: 2, Metric: 0, Fwd addr: 0.0.0.0, Tag: 208.0.97.126
    Gen timer 00:15:45
    Aging timer 00:25:45
    Installed 00:34:14 ago, expires in 00:25:46, sent 00:34:12 ago
    Last changed 6d 20:44:14 ago, Change count: 1, Ours
    Standby flood status Idle

     

    This number does not mean anything to me and is not in the routing table.

     

    Can anyone explain where this has come from?



  • 2.  RE: OSPF external routes tag within VRF
    Best Answer

    Posted 12-06-2016 03:18

    Hello,

    RFC 4577 section 4.2.5.2  https://tools.ietf.org/html/rfc4577#section-4.2.5.2 

     

           0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|1|0|1|     ArbitraryTag      |       AutonomousSystem        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       1 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 _AS number of the VPN Backbone_

    FYI, if You google "OSPF VRF route tag", then RFC 4577 is the 3rd result from top.

    HTH

    Thx
    Alex



  • 3.  RE: OSPF external routes tag within VRF

     
    Posted 12-06-2016 03:21

    Hi,

     

    This is as per RFC4577:

    If a particular VRF in a PE is associated with an instance of OSPF,
   then by default it MUST be configured with a special OSPF route tag
   value, which we call the VPN Route Tag.  By default, this route tag
   MUST be included in the Type 5 LSAs that the PE originates

     

    https://tools.ietf.org/html/rfc4577#section-4.2.5.2

     

    The domain-vpn-tag can also be disabled on Junos if D/N bit is used for loop prevention.

    https://www.juniper.net/documentation/en_US/junos15.1/topics/reference/configuration-statement/no-domain-vpn-tag-edit-protocols-ospf-2.html

     

    In ospf neighbor routing tables, the tag would be present for that ospf route as 3489685886.

     

    Cheers,

    Ashvin