OSPF Domain-Tag or Domain-Id

[saurabh joshi]

Hello Experts,

Can somebody brief on why Domain-Tag or Domain-Ids are required when we use OSPF as CE-PE protocol.

Also my 30 sites connected to two different MPLS providers. Say Provider-A and B.

Provider-A is primary for all traffic and B links are backup.

Now when i use common OSPF process ID at CE there are chances of advertising routes learned from ISP-A to ISP-B via CE and this leads to outage scenario.

I read somewhere that OSPF-Tag can be configured at PEs under particular vrf ospf process to avoid such issues.

Need technical explanation what exactly causing this issue and how we can avoid these.

Let me know if further details required.

Thanks

[Mahesh Gohil]

Hi Saurabh,

OSPF domain-id required at PE end  and the reason is ....

say one PE has 4 customers and everyone is given a process id in incremental order like

router ospf 1 vrf ABC
router ospf 2 vrf ABCD
router ospf 3 vrf ABCDE
router ospf 4 vrf ABCDEF

Not customer ABC want to connect to some different PE but there we already have configure three ospf customer like

router ospf 1 vrf ABCD
router ospf 2 vrf ABCDE
router ospf 3 vrf ABCDEF

Now it is obvious that process id 1 is already engaged with customer ABCD. so to ensure optimal routing provider assign
one domain-id per customer vrf so that this problem can be overridden.

meaning if we do not give common domain-id to vrf then routes from one domain will be installed to
other domain as an external route

Now regarding your next question..

ospf has two facility ..downbit and tag

Downbit is used to avoid redistribution of ospf routes back to ospf domain from mp-bgp to avoid loops.
Tag is by default is set as bgp as number so if ISPA is receiving routes advertised by same ISPA somewhere is not installed in mp-bgp..this is used specifically for external routes.

is your requirement is ISPA should not allow routes advertised CE which is received from ISPB. I don't think this is possible unless
provider ISPA manually match tag (ISPB bgp AS number ) and deny those prefixes from advertising to mp-bgp.

hope this is helpful to you

Regards
Mahesh