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  • 1.  check AS Path attribute?

    Posted 11-12-2011 22:54

    Hi！

     

    I have a question about BGP advertisement.

    Here‘s the diagram.

    R1[AS65531]----------------R2[AS65532]----------------R3[AS65531]


    R1 and R2 in AS65531 and R2 in AS65532.

    eBGP session between R2 and R1

    eBGP session between R2 and R3

    No BGP session between R1 and R3


    R1 advertised a route 172.16.1.0/24 to its eBGP peer R2. Does R2 examine the contents of the AS Path attribute of the prefix before advertising it to R3? If it does, R2 does not advertise the prefix to R3, Right?

     

    ThankS!



  • 2.  RE: check AS Path attribute?

    Posted 11-13-2011 00:37

    Hi

     

    By default, R2 advertises the prefix to R3 but R3 discards it due to looped AS-PATH

    attribute. If you enable "loops" option at routing-options on R3, see

     

    http://www.juniper.net/techpubs/software/junos/junos94/swconfig-routing/autonomous-system.html

     

    then R3 will accept the route. Also, for R2 to advertise the route to R3 with AS65531

    substituted with its own AS65532, you can use as-override option

     

    http://www.juniper.net/techpubs/en_US/junos10.2/topics/reference/configuration-statement/as-override-edit-protocols-bgp.html

     

    In this case R3 will accept the route (no "loops" option will be needed).



  • 3.  RE: check AS Path attribute?

    Posted 11-13-2011 01:33

    >By default, R2 advertises the prefix to R3 but R3 discards it due to looped AS-PATH attribute.

     

    yeah, but  when i use "show route advertising-protocol bgp 172.16.1.3" command on R2, there's nothing. It means that R2 did not advertise the prefix to R3. That's why i asked this question. Is there something wrong?

     

    ThankS!



  • 4.  RE: check AS Path attribute?
    Best Answer

    Posted 11-13-2011 02:22

    Hi

     

    You are right...

     

    "The JUNOS software does not advertise the routes learned from one external BGP (EBGP) peer back to the same EBGP peer. In addition, the software does not advertise those routes back to any EBGP peers that are in the same AS as the originating peer, regardless of the routing instance."

     

    http://www.juniper.net/techpubs/software/junos/junos94/swconfig-routing/disabling-suppression-of-route-advertisements.html

     

    I think this was different some time ago, but now it behaves this way.

     

    So what you see is a normal default behavior. But if you enable the advertise-peer-as option,

    then it will be working as I described above...



  • 5.  RE: check AS Path attribute?

    Posted 11-13-2011 03:34

    Smiley HappyThankS!