flipping over traffic to different circuit with BGP

[Amafsha1]

Let's say we have 2 sites(main and Dr) in 2 different cities that have 1 circuit each from the same ISP.  Lets say we get a default route for both, and lets say we also advertise the same /24 through both.  currently the primary circuit is the main site and I can see the main site does not pre-pend AS instances in a route-map to the world, so when I look at the metrics on looking glass they are very cheap.   The DR site however, uses the same route-map but does some instances of pre-pending so it's not as cheap the metrics.

Now, lets say I wanted to failover to the DR side so it becomes the main guy for the /24.  Is it as theoretically as easy as just adding a bunch of instances of AS prepending to the route-map on the

main side, so that now the DR sides metrics are cheaper to the world? I was thinking about removing pre-pending on the DR side, and adding some pre-pending on the main side to be 100% sure that it's cheaper to get to DR instead of main.  I would also check on looking glass after making the flip.  I understanding I didn't post up the configs, but I guess I'm kinda looking for a general answer.  Thank you

[Dennis Mink · Accepted solution]

is your /24 a public range?  if so dont trust AS path too much as ISPs sometimes do not honor these.

You probably might be better of withdrawing advertisement from your primary site and only advertising the same range from DR, then swap back.

cheers

[Amafsha1]

That is correct, it is publicly accessible.  As a matter of fact, today I was given the script to change the route-maps to stop advertising our /24 at main site and start advertising at dr site.  i guess what you described is exactly what happened...