Posted on December 21, 2020 4:37 am
 |  Asked by jaxk Panther
 |  131 views
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Hi Guys

I’m struggling to build an ospf superbackbone on arista vEOS platform. I have ospf peering between CE and PE and stable MPLS route for VPN between PE. But something goes sideway that I didn’t expect. I can’t see ospf domain value on PE when hitting ” show bgp vpn-ipv4 <specific route> detail”.
My perspective is it should come out as a bgp communication. Isn’t it? or probably something missing in a part of configuration?
Additional info, I’ve tried to find a resource in Arista site but Unfortunately, No direction.

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Posted by Massimo Magnani
Answered on January 4, 2021 4:09 pm

Hi Jaxk,

In order to achieve connectivity between CEs using OSPF as PE-CE protocol, from a pure configuration standpoint, you must redistribute the local OSPF into BGP on one side and the VPNv4 received BGP route into the local OSPF on the other side (and the other way around). In your diagram, for instance, you need to redistribute OSPF to BGP on PE1 and BGP into OSPF on PE2 and viceversa. When local OSPF routes are advertised into BGP, they may or may not be tagged with a domain identifier extended community. EOS as of today only supports the NULL domain-id, therefore as per RFC4577 Section 4.2.4:

"If the OSPF instance's Domain Identifier is NULL, the Domain Identifier Extended Communities attribute MAY be omitted when routes from that OSPF instance are distributed by BGP"

so BGP VPNv4 NLRIs originated by EOS devices don't carry the Domain ID Extended Community. Just for your information, if the domain-id was present, it would be reflected into the "External Route Tag" field which you can check using the command "show ip ospf database [external|network] $ROUTE vrf $VRF_NAME". As EOS doesn't support it, this field is always "0". As an example, take a look at the following CLI output from vEOS 4.25.0F:

R1#show ip ospf database external 10.200.0.2 vrf TEST

OSPF Router with ID(172.16.0.2) (Instance ID 10) (VRF TEST)

Type-5 AS External Link States

LS Age: 1685
Options: (E DC)
LS Type: AS External Links
Link State ID: 10.200.0.2
Advertising Router: 172.16.0.2
LS Seq Number: 0x80000001
Checksum: 0x6cb1
Length: 36
Network Mask: 255.255.255.255
Metric Type: 2
Metric: 1
Forwarding Address: 0.0.0.0
External Route Tag: 0

Hope this helps.

Max

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