[c-nsp] /18 and multihomed BGP

[RAZAFINDRATSIFA Rivo Tahina]

Thanks Adam,

in fact, I'm already with 2 upstreams, each one with 2 links, I do 
load balancing with them, announcing a /24 from one link to another 
according to the traffic, these 2 existing ISPs have /24 filters.
I'm adding a new ISP with 3 links, and this 3rd ISP wants to change a 
unique /18 filter; we plan to use all 3 ISPs at the same time, not 
for backup only.

BR

At 15:42 12/08/2010, Adam Armstrong wrote:
>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.