Re: BGP peculiarity

That is all well ..

My query is regarding the inconsistent behaviour of BGP, irrespective
of "AS-OVERRIDE" or "ALLOWAS-IN" commands being used.

In some cases, it is observed that BGP follows its standard rule for
route updates and does not advertise routes learned from an e-BGP peer
back to the same neighbor.

But in some other cases, i have observed that BGP does actually
advertise routes back to an e-BGP peer from which they were learnt in
the first place. Why this dichotomy ?

It is a different matter that the BGP AS originating those routes will
reject them if they are advertised back to it because it will see its
own AS # in the AS-Path. That part is understood. My query is
regarding the BGP policy for the routes which will not be included in
updates to e-BGP peers. As i understand, BGP should not advertise to
an e-BGP peer those routes for which the AS-path contains AS# of that
peer. But i have seen his rule violated in some scenarios. Why is that
?

Regards,
Amit

On Fri, Sep 16, 2011 at 6:24 PM, Narbik Kocharians <narbikk_at_gmail.com> wrote:
> Amit,
> When an e-bgp neighbor advertises a prefix to you, it will prepend its AS
> number, and if you ever advertise it back to him, he will see his AS number
> in the update and reject the route. Unless you have "AS-OVERRIDE" or he has
> "ALLOWAS-IN".
>
> On Fri, Sep 16, 2011 at 5:09 AM, Amit Kumar Lohumi <getakl_at_gmail.com> wrote:
>>
>> Hi All,
>>
>> Can anyone pls explain ....
>>
>> why is it that in some cases a BGP router would not advertise e-BGP
>> routes learned from a peer back to the same peer (sticking to the
>> standard rule on BGP updates).
>>
>> ... while in other cases, BGP will infact advertise the e-BGP routes
>> learned from a peer back to the same peer (although they do get
>> rejected by the originating peer because of ASN match ).
>>
>> I have made these dichotomous BGP observations on a couple of
>> different scenarios.
>>
>> Regards
>>
>>
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>
>
>
> --
> Narbik Kocharians
> CCSI#30832, CCIE# 12410 (R&S, SP, Security)
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Received on Fri Sep 16 2011 - 19:50:26 ART