[c-nsp] /18 and multihomed BGP

[Heath Jones]

Robert's response pretty much covers it all really.

If those 2 ISP's are advertising /24's out to the wider internet, your
screwed. Traffic will never come down via this new ISP.
The way to solve it, is to advertise the /24's as you already are, but get
those 2 ISP's to summarize into a /18 before they export, or you pass them a
/18 as well and mark the /24's as no export. Then you pass just the /18 to
this new ISP which they will then advertise.


So as far as the wider internet, it will just be /18's to your network.
As far as the 2 existing ISP's you will have load balanced links as they are
now.
As far as the new ISP, you won't have load balancing but you will still see
some traffic.


Hope this makes sense...

Heath





On 13 August 2010 13:02, RAZAFINDRATSIFA Rivo Tahina <r.tahina at moov.mg>wrote:

> 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.
>>
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