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Two ASN and two subnets over single link to same ISP

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  • 1.  Two ASN and two subnets over single link to same ISP

    Posted 08-01-2019 12:31

    Hello all,

     

    I'm trying to determine if this is even possible, and if so, is there a good document or configuration example out there?  We just received a /24 from our ISP along with our own ASN.  Currently we still have a /26 through the same ISP using a different ASN that will eventually go away once we are fully transitioned to the /24.  For now the /26 must be active until we move some VPN tunnels and change vendor whitelists of our current IP address.  Is there a way to advertise both networks over the same link?  I was reading about local-as.  Would that apply to this case?

     

    Thank you in advance.



  • 2.  RE: Two ASN and two subnets over single link to same ISP

     
    Posted 08-01-2019 13:21

    Hello,

    Yes, you can leverage local-as knob in this situation.

    The new ASN you can cofigure under routing-option autonomous-system hierarchy while the the old ASN you can confgure as local-as under the old eBGP neighbor configuration.

    https://www.juniper.net/documentation/en_US/junos/topics/topic-map/bgp-local-as.html

    In this link you can see many options to make local-as be present or removed from the AS path of the route, which can also be used based on your requirement.

    If you have specific query, a network diagram would help to provide a detailed response.

    Please accept my answer as solution if it resolves your query, kudos are appreciated too!

    Thanks
    Vishal



  • 3.  RE: Two ASN and two subnets over single link to same ISP

    Posted 08-01-2019 21:42

    Hello,

    Let me see if I get this right:

    0/ You have ASN-A and prefix-1/26 with eBGP towards ISP-1. You eBGP-peer with ISP-1 using ASN-A. But what is the link subnet-1?

    1/ You also got ASN-B with prefix-2/24 from same ISP-1. You haven't got any new link subnets, or have You?

    2/ Your goal is :

    2a/ to advertise to ISP-1 prefix-1/26 with leftmost ASN-A over eBGP peering with ISP-1 across link subnet-1

    2b/ to advertise to ISP-1 prefix-2/24 with leftmost ASN-B over same eBGP peering with ISP-1 across link subnet-1

    2c/ to establish another peering session with ISP-1 over link subnet-1 using ASN-B and advertise prefix-2/24.

    2d/  anything else?

    2a is what You already have.

    2b is not supported since own ASN-A will be prepended to the AS_PATH for prefix-2/24 before it is advertised out.

    2c is not supported either unless:

    [i] You'd get a brand new link-subnet-2 which You can then add as secondary to Your link and use for peering with ISP-1 using ASN-B; or

    [ii] if existing link subnet-1 is large enough (/29 or shorter) at least Your ISP-1 must get a new IP for this peering, but You can reuse Your own link subnet-1 IP for new peering if You want to. 

    [iii] You establish multihop eBGP peering using ASB-B towards brand new ISP-1 IP using the same link subnet-1 IP on Your side.

    And then You'd need "local-as" once You are past [i], [ii], or [iii] above.

    HTH

    Thx

    Alex

     



  • 4.  RE: Two ASN and two subnets over single link to same ISP

    Posted 08-02-2019 07:57

    Thanks Vishal.

     

    Hi Alex,

     

    I have a single /30 link subnet to ISP-1.  According to what they told me, that will work for both the /24 and /26.



  • 5.  RE: Two ASN and two subnets over single link to same ISP
    Best Answer

    Posted 08-02-2019 20:23

    Hello,

     

    I hope Your ISP-1 knows what they are doing but in JUNOS You cannot advertise BOTH /26 and /24 over THE SAME EBGP SESSION with different leftmost ASNs. "local-as" won't help here.

     

    HTH

    Thx
    Alex