Re: BGP peculiarity

I have never seen that. Or I am missing something

CCIE # 23962 (SP)

Sent from my iPhone 4

On 16 Sep 2011, at 4:20 PM, Amit Kumar Lohumi <getakl_at_gmail.com> wrote:

> 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)
>> www.MicronicsTraining.com
>> Sr. Technical Instructor
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Received on Fri Sep 16 2011 - 16:35:36 ART