BGP AS_PATH Prepend

Fri, 05 Jun 2026 21:31:13 +0200

Lab Data

BGP ASN topology

R1-AS17 <—> R2-AS22

R1-AS17 <—> R4-AS22

WAN Subnets

R1:.1 <- 10.10.10.0/30 -> R2: .2

R1:.9 <- 10.10.10.8/30 -> R4: .10

LAN Subnets

R1: .69 <- 172.17.99.0 -> LAN1

Purpose

When R1 advertises the network 172.17.99.0 in BGP, R2 and R4 receive the route with the default AS_PATH attribute value, which is the ASN of R1. I want to make the 172.17.99.0 route received by R2 and R4 a bit ‘unattractive’, by making R1 send it with a longer AS_PATH attribute value. Since the AS_PATH attribute is a BGP non-transitive attribute, this modification will only impact the AS that are immediate neighbors of R1’s AS.

Microlab on

BGP AS_PATH Prepend

Lab Data

BGP ASN topology

WAN Subnets

LAN Subnets

Purpose