BGP routes NOT advertising

Answered Question
Nov 25th, 2013
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Hi All,


I have created a simple topology in GNS3.  ( Please refer attached config and topology )


EBGP between R1 and R2 ,  R1 and R3.


Routes are NOT advertised to R3 and R2 from R1 ,which are learnt from R2 and 3.


R1 AS number is 65000

R2 and R3 AS number is 65457


R1 NOT advertising routes to R2 which comes from R3 ( may be AS number is the same)


Please advise , should I configure some special command to achieve this.


Thanks

Sri

Correct Answer by cadet alain about 3 years 8 months ago

Hi,

in BGP the loop prevention mechanism is the AS_PATH so when R1 sends the BGP update to R2 it contains R3 AS which is also R2 AS so when R2 sees its own AS in the AS_PATH it considers there was a loop and doesn't accept the update.

You can use the allow-as-in feature to disable the loop prevention mechanism per neighbour.

http://ccieblog.co.uk/bgp/bgp-as-override-vs-allow-as-in


Regards


Alain



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Correct Answer
cadet alain Mon, 11/25/2013 - 23:49
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Hi,

in BGP the loop prevention mechanism is the AS_PATH so when R1 sends the BGP update to R2 it contains R3 AS which is also R2 AS so when R2 sees its own AS in the AS_PATH it considers there was a loop and doesn't accept the update.

You can use the allow-as-in feature to disable the loop prevention mechanism per neighbour.

http://ccieblog.co.uk/bgp/bgp-as-override-vs-allow-as-in


Regards


Alain



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s.kanth Tue, 11/26/2013 - 02:31
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Thank you very much Alain, I will test it out tomorrow and get back to you.


I guessed, it blocked due to SAME AS but did NOT know how to block it.


Thanks

Sri

s.kanth Tue, 11/26/2013 - 21:46
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Alain, it works! thanks once again :-) - Sri

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