[c-nsp] BGP long prefix ads

[Jon Lewis]

On Fri, 11 Jan 2008, Roman Bestuzhev wrote:

> I am thinking about load balancing between both ISPs. I have read about a
> technique when you divide your block to several pieces, for example to two
> /22 blocks and advertise one of them to one ISP and other block to another
> ISP and at the same time advertise whole prefix to both ISPs, /21 in this
> case. This leads to getting incoming traffic trough both providers and you
> can control which subnets in your AS connect to Internet trough which ISP.

You can probably achieve some level of load sharing without resorting to 
polluting the DFZ.  First thing to look at is relative as-path length.  If 
provider A is more of a "Tier 1" provider than provider B, then you might 
try prepending your ASN once on announcements to A.

If A supports it, you may be able to use community tagging to tell them to 
do some prepending when your route is propogated to certain peers, thus 
causing those peers to be more likely to use your B provider path.

If you do resort to chopping up your CIDR, make sure you announce both the 
aggregate and subnets...so those who filter those sorts of "garbage 
routes" can do so without losing connectivity to your network.

> My question: is this scheme used widely and would this scheme work well in

Used widely and correct/advisable are different things.  Lots of networks 
announce their CIDRs as individual /24s.  It doesn't mean you should.

----------------------------------------------------------------------
  Jon Lewis                   |  I route
  Senior Network Engineer     |  therefore you are
  Atlantic Net                |
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