BGP: Difference between revisions
→ASPath Prepend
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= ASPath Prepend =
Source: [https://www.noction.com/blog/as-path-and-as-path-prepending noction.com]
* AS path is a well-known mandatory attribute, which means that it’s present for all prefixes exchanged between BGP neighbors.
* When a BGP router sends out an update to a neighbor in a different autonomous system (i.e., an external or eBGP neighbor), it adds its own AS number to the front (left side) of the AS path.
* So the AS path lists all the ASes that need to be traversed to reach the location where the prefix that the path is attached to is advertised from.
* As such, a traceroute should encounter those same ASes.
* The main purpose of the AS path is to avoid loops.
* Prepending means adding one or more AS numbers to the left side of the AS path.
* Normally this is done using one’s own AS number, using someone else’s AS number for this can have unintended side effects.
Config:
router bgp 65123
neighbor 198.51.100.90 remote-as 65456
neighbor 198.51.100.90 description IX peer
neighbor 198.51.100.90 route-map prepend out
!
route-map prepend permit 10
set as-path prepend 65123
= BGP Route Dampening =
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