Cisco Mpls VPN OSPF down-bit, domain-tag and domain-id

Down Bit:

If a OSPF route has been advertised from a PE router into an OSPF area, the down bit is set. Another PE router in the same area does not redistribute this route into iBGP of the MPLS VPN network if down is set.

We can see this bit like below:

CPE-2#show ip ospf 10 database  summary

            OSPF Router with ID (8.8.8.8) (Process ID 10)

                Summary Net Link States (Area 0)

  LS age: 2

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

  LS Type: Summary Links(Network)

  Link State ID: 1.1.1.111 (summary Network Number)

  Advertising Router: 100.100.100.3

  LS Seq Number: 80000001

  Checksum: 0x1CFB

  Length: 28

  Network Mask: /32

        TOS: 0  Metric: 2

  Domain Tag:

If a  PE router that is connected to the same site or another site that is connected through a backdoor link receives Ospf route and it matches the configured domain tag, the route is not redistributed into iBGP.By default this value is BGP AS number of PE routers.

We can see this value like below:

PE-1(config)#router ospf 10

PE-1(config-router)#domain-tag 100

PE-1#show ip ospf 10 database external

            OSPF Router with ID (100.100.100.1) (Process ID 10)

                Type-5 AS External Link States

  LS age: 30

  Options: (No TOS-capability, DC)

  LS Type: AS External Link

  Link State ID: 80.80.80.80 (External Network Number )

  Advertising Router: 100.100.100.1

  LS Seq Number: 80000002

  Checksum: 0x17B4

  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: 100

Domain ID:

By default, it is equal to the Ospf process id.If domain-id of the route received by a PE

router does not match the OSPF process ID of the particular VRF, the route is advertised as an

OSPF external route (Type-5 E2). If the domain ID does match the OSPF process ID, the

route is advertised as an internal route.