Re: [isp-bgp] Route advertisement problem

[James <haesu@towardex.com>]

On Tue, Oct 26, 2004 at 12:36:07PM -0600, Craig Baird wrote:

[ snip ]

> 
> Now here's the dilemma: If Wireless link 1 goes down, things will be fine.  
> Edge router A will stop announcing 1.1.1.0/24 to AT&T because it will no 
> longer have a route to any 1.1.1.x networks.  However, in the event that 
> Wireless link 2 goes down, Edge router A will continue to announce the 
> 1.1.1.0/24 route to AT&T because it can still see the 1.1.1.0/26 network.  
> Even though it cannot see 1.1.1.64/26 or 1.1.1.128/25.  So basically, half of 
> my network will be down if Wireless link 2 dies.  Instead, what I would like 
> to see is traffic to 1.1.1.64/26 and 1.1.1.128/25 come in via MCI, Qwest or 
> Sprint, while still having traffic to 1.1.1.0/26 come in through AT&T.  But 
> the only way I can think of for that to happen would be to announce networks 
> smaller than /24 to my upstreams (which obviously cannot and would not be 
> allowed).

Two ways:

	(a) I assume the way router A is announcing 1.1.1.0/24 is that you
	probably have "aggregate-address" command. If not, you probably at
	least have "network 1.1.1.0 mask 255.255.255.0" command which would
	announce that /24 if route(s) are provided via IGP or connected/static.

	Easiest way of me recommending this is to not advertise 1.1.1.0/24 at
	all on router A. Remove any network statements for that /24, remove
	the aggregate-address statement if its used on A.

	Then, announce 1.1.1.0/24 on the intermediate router that sits between
	w_link1 and w_link2 using network statement in your iBGP configuration.

	This method however won't help you if w_link2 goes down, but at least
	more than half of the network is not down. So this doesn't solve all
	of your problems.


	(b) You can also use 'conditional advertisement' feature in Cisco IOS.
	I personally don't like it b/c to me its more or less of a quick
	workaround that won't scale as network grows large later.. But it could
	help you.

	In this case, you will need intermediate routers to announce the more
	specifics using iBGP, then have conditional route-maps listen for those.

	But like you said, this probably won't solve all the problems.


Usually, split-AS situation doesn't work well, it is pain in the neck for even
the largest Tier1 backbones today. That's why you need stable and redundant
paths between core routers participating in backbone BGP, at the topology
design level.

HTH,

-J


-- 
James Jun                                            TowardEX Technologies, Inc.
Technical Lead                       IPv4 and Native IPv6 Colocation, Bandwidth,
james@towardex.com             and Web Hosting Services in the Metro Boston area
cell: 1(978)-394-2867           web: http://www.towardex.com , noc: www.twdx.net

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