[AusNOG] Multiple IP Transit BGP

[Sean K. Finn]

Are you Inbound-Heavy or Outbound-Heavy?



For Inbound-Heavy..

I’ve seen it done where you run something  monitoring what your link to trigger when you consider it ‘full’, such as Cacti with THRESHOLD module with alerting turned on, or otherwise a timed event trigger to run, at, lets say, 30 minutes before you regularly hit peak time.

e.g. a cron-job on a linux box somewhere, or a triggered script, SSHing / (Or Telnetting) into a server, running an ‘expect’ script, and then running the commands that will either ‘turn on’ bgp, or change your route maps.


The real fun will be in HOW you balance your traffic across these links.

I would be of a mind to run both as active-active, and to use route-maps in all of their glory and configuration hell to try to balance your traffic as best as possible.

If you have multiple /24’s you could preference some over one, and others over the other, and shuffle them around until you approximate some sort of balance,
Or,

On your least-preferential route, add an AS path prepend or two to make it look like a longer path, and somehow achieve some sort of lucky balance.


Adding a second carrier is really where the fun begins, and, being the wild internet, you really just have to come up with a configuration or two, deploy, and suck-it-and-see.


For outbound-heavy traffic, you should start by profiling which AS’s you send the most amount of traffic to, then, figure out which ones are ‘closest’ on the network to each of your two connections.

If your routers are big enough, you could take two default-routes via BGP from both providers, AND two full-global or full-national routing tables.
Be careful with taking too many routes, if your routers don’t have enough ram to hold it all BGP will die and you’ll just stop routing, period.

If you can take two full global tables, I’d suggest at least 2GB of ram (Others on the list feel free to chip in at this point what’s enough ram), then you can construct some route-maps to tag certain prefixes learnt from certain networks with local-preferences, to push traffic out an appropriate link.



In essence though, even if you turn on your second link when your first one is maxing out, you’re still going to need route maps, one way or another, to control which of your BGP neighbors see’s what.

Welcome to BGP Hell ☺
(Otherwise known as Job Security).

S.

From: ausnog-bounces at lists.ausnog.net [mailto:ausnog-bounces at lists.ausnog.net] On Behalf Of James Mcintosh
Sent: Thursday, April 26, 2012 3:47 PM
To: ausnog at lists.ausnog.net
Subject: [AusNOG] Multiple IP Transit BGP

Hi Noggers,

We currently advertise our whole IP range via a single IP Transit upstream carrier who supplies us 150Mbps of transit.

We are are adding another 70Mbps but from a different carrier.

I've Googled high and low but can't seem to find the answer and I'm personally a bit rusty on my BGP. Really hoping some of the pro's on the list can help.

How would we advertise make use of the the new 70Mbps IP transit but only once the existing 150Mbps was full. As an overflow type of set up.

Is this even possible? Or are there better ways to manage multiple upstream carriers?

I'm using a Cisco 7206 VXR.

Thanks and really appreciate the help...


-James
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