bryanlloydharris
asked on
cisco 7200 router config
What is the functionality of the commands? This question is in my quest to understand how bgp works along with how route maps work, and what route maps really do. I don't really understand it much, and the repeating AS numbers are confusing to me. Any explanation or a point in the right direction is greatly appreciated.
neighbor 64.240.32.41 route-map savvis-out out
route-map savvis-out permit 10
set as-path prepend 23376 23376 23376 23376
neighbor 64.240.32.41 route-map savvis-out out
route-map savvis-out permit 10
set as-path prepend 23376 23376 23376 23376
Each BGP router on the Internet follows a prescribed route selection algorithm. See http://tinyurl.com/ya2yt9 for Cisco's algorithm. If a router has learned two paths for a prefix and both paths have reachable next hop, achieved synchronization (or had sync disabled, often the case), have the same weight, have the same local preference, and neither were originated on the local router, it will prefer a prefix if it has a shorter AS path. This command is changing the AS path from "23376" to "23376 23376 23376 23376 23376" so that most likely 'the other' path will win.
However, since you are (as I recall) purchasing transit from Savvis, they're putting a higher local preference on the prefix they're learning directly from you, and therefore Savvis is prefering the path straight to you. Other networks may prefer the AS path through 2828, though, since the path will likely be '2828 23376' and therefore much shorter than '3561 23376 23376 23376 23376 23376'.
However, since you are (as I recall) purchasing transit from Savvis, they're putting a higher local preference on the prefix they're learning directly from you, and therefore Savvis is prefering the path straight to you. Other networks may prefer the AS path through 2828, though, since the path will likely be '2828 23376' and therefore much shorter than '3561 23376 23376 23376 23376 23376'.
ASKER
In your third sentence, what is a prefix? Is a prefix the same as an AS number? I know that our router is AS number 23376, what is 2828?
ASKER
I think I understand. Is it like this... please let me know if I worded it incorrectly:
* Some router outside is learning a new path to our router, but already had one working path.
* It's going to compare the total hops and discover that the new path has more hops because 23376 is prepended 4 more times.
* Then it's going to simply choose the route it already had memorized?
* Some router outside is learning a new path to our router, but already had one working path.
* It's going to compare the total hops and discover that the new path has more hops because 23376 is prepended 4 more times.
* Then it's going to simply choose the route it already had memorized?
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ASKER
Wow. I have so much to learn... Of course this will bring me to ask so many more questions. Thanks for all your help.
ASKER