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  • 1.  BGP AS23456 questions

    Posted 9 days ago

    Hi Team,

    I met a very strange issue and couldn't find out the root cause.

    The BGP session can not be established and my ISP shared this log message with us.

    Reserved AS_TRANS(23456) cannot be used as the AS of any BGP Speaker (Used for 4 byte AS extension only)

    However, we use 2 bytes AS to peer with our ISP, and our ISP also uses 2 bytes AS

    More interesting is we have another router using the same BGP configuration to peer with the same router of our ISP and it's successfully established.

    Anywhere I should check further about this issue?

    Thank you.

    Best regards,



  • 2.  RE: BGP AS23456 questions

    Posted 8 days ago


    AS23456 is 2-byte AS, which is used as 4-byte AS substitute. The first peer supports 4-byte AS. The second one – doesn't. You need to setup a proper 2-byte AS for the second peering on your side.


    Best e-gards, 

    Georgi Sinapov 

    Core Network Architect


  • 3.  RE: BGP AS23456 questions

    Posted 8 days ago

    Hi Georgi,

    Thank you for the response.

    I just noticed that I forgot to mention that the problematic router is running JUNOS 21.4r3 which I think the 4-byte ASN is supported.

    The router that can establish BGP peer without a problem also runs the same version of JUNOS.

    Nothing related to BGP is different.

    The BGP configuration is pretty simple. 

    Just Like this

    edit protocol bgp group ISP
    set local-address [our router IP]
    set neighbor [our ISP IP]
    set peer-as [2-byte AS of ISP]
    set local-as [2-byte AS of us]

    This is not the first time I have encountered this problem.

    I also have this problem when peering with the member in IX sometimes.

    Best regards,



  • 4.  RE: BGP AS23456 questions

    Posted 8 days ago


    have you tried to disable 4Byte support with this peer?


    Andrei Cebotareanu