To prepend one or more AS numbers to the AS path in a BGP route, use the set as-path command in Route Map Configuration mode. This command is normally used to insert one or more instances of the local AS number at the beginning of the AS_PATH attribute of a BGP route. Doing so increases the AS path length of the route. The AS path length has a strong influence on BGP route selection. Changing the AS path length can influence route selection on the local router or on routers to which the route is advertised.
When prepending an inbound route, if the first segment in the AS_PATH of the received route is an AS_SEQUENCE, as-path-string is inserted at the beginning of the sequence. If the first segment is an AS_SET, as-path-string is added as a new segment with type AS_SEQUENCE at the beginning of the AS path. When prepending an outbound route to an external peer, as-path-string follows the local AS number, which is always the first ASN.
| Format | set as-path prepend as-path-string |
| Mode | Route Map Configuration |
| Parameter | Description |
|---|---|
| as-path-string | A list of AS path numbers to insert at the beginning of the AS_PATH attribute of matching BGP routes. To prepend more than one AS number, separate the ASNs with a space and enclose the string in quotes. Up to ten AS numbers may be prepended. |
The following example prepends three instances an external peer‘s AS number to paths received from that peer, making routes learned from this peer less likely to be chosen as the best path.
(Extreme 220) (Routing)# config (Extreme 220) (Routing)# route-map ppAsPath (Extreme 220) (Routing)# set as-path prepend “2 2 2” (Extreme 220) (Routing)# exit (Extreme 220) (Routing)# router bgp 1 (Extreme 220) (Routing)# neighbor 172.20.1.2 remote-as 2 (Extreme 220) (Routing)# neighbor 172.20.1.2 route-map ppAsPath in
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