RE: Cisco Route filtering [was Re: OPS: BGP spew from ASN 7374]

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1

I've had a question on this for some time that maybe can get answered
here.

Let's assume for a moment that I have a network of 209.123/18
(1/4 of a
classful B).

Let's further assume that my upstream filters me with:

access-list 2002 permit ip host 209.123.0.0 host 255.255.0.0
neigh 2.3.4.5 distrib 2002 in

Will my /18 announcement make it through this /16 filter?

No. The ACL specifies an exact match.

Would the same be true with a prefix-length filter, which I assume the
syntax would be:

ip prefix-list 1 permit 209.123.0.0/16
neigh 2.3.4.5 prefix-list 1 in

Same deal.

Chris

2

> Let's assume for a moment that I have a network of 209.123/18
> (1/4 of a
> classful B).
>
> Let's further assume that my upstream filters me with:
>
> access-list 2002 permit ip host 209.123.0.0 host 255.255.0.0
> neigh 2.3.4.5 distrib 2002 in
>
> Will my /18 announcement make it through this /16 filter?

No. The ACL specifies an exact match.

You should change 'host 255.255.0.0' to '255.255.0.0 0.0.255.255' to
allow any _more specific_ mask. Read CISCO docs.

> Would the same be true with a prefix-length filter, which I assume the
> syntax would be:
>
> ip prefix-list 1 permit 209.123.0.0/16
> neigh 2.3.4.5 prefix-list 1 in
>

Same deal.

Chris

Aleksei Roudnev, Network Operations Center, Relcom, Moscow
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