Friday, July 13, 2007

Lab 3 BGP

Last night I completed IE Lab 3 BGP. The section was relatively easy until the last part. I was led down a path where I had to override BGP weight on an inbound path. Huh? I didn't think this was possible. Remote outbound always gets precedence over local inbound. That's why weight and local preference are above AS path and med. I was going to go down a path of advertising two /25's subnetted from the /24--longest match would definitely take precedence over weight. I know that would have worked, but if this were the real lab I'd definitely have to ask the proctor if that were allowed or not.

The more I thought about it, the more I assumed it would not be allowed, so I caved and checked the answers. Sure enough, it was something I did not know--BGP conditional advertisement. There's a command in BGP which allows you to only advertise a route when another route is not in the network. VERY HANDY! I can't wait to start using this in the real world. I always have ISPs choosing the wrong path even when the AS-path is longer. Usually because the remote host's ISP is the same as the low bandwidth link.

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