[c-nsp] /18 and multihomed BGP

[Adam Armstrong]

On 12/08/2010 13:21, RAZAFINDRATSIFA Rivo Tahina wrote:
> Hi all,
>
> I'm BGP multihomed with 2 ISPs and have a /18 subnet.
> I declared the /18 block in RIPE database as 64 /24.
> I'm adding a new ISP and he asked me to modify the 64 /24 in RIPE to a
> unique /18.
> How does this change affect the existing routing with 2 former ISPs?
> How can I announce a /24 prefix from one ISP to another? He is talking
> about "no export", what is the purpose of this attribute?

Your new ISP thinks you're being incredibly stupid by announcing 64 /24s 
(thereby taking up 64 times as much space in the routing table as you 
should be, could you imagine if the global table was 64x300k?), It's a 
justified reaction, in some parts of the UK you'd be hung upside down by 
your feet for splitting a /18 into 64 /24s.

If you haven't done this to do some form of fisherprice traffic 
engineering, you need to stop announcing the /24s to the existing ISPs 
and just announce the /18 supernet to them (otherwise all traffic will 
come in via them, as they'll have more specifics).

It's rare to have a good reason to split netblocks up like that, but if 
you do, you could try splitting it into less tiny blocks, like 2 /19s or 
4 /20s.

no-export is a well-known BGP community which tells the router not to 
announce that route outside of the AS (or confederation).

adam.