Information Technology Reference
In-Depth Information
Example 4-43
BGP RIB on R2 (Continued)
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* i172.15.0.0 192.168.12.1 0 100 0 i
*> 0.0.0.0 0 32768 i
* 172.16.0.0 192.168.23.3 0 2 300 400 i
* i 192.168.14.4 100 0 200 400 i
*> 192.168.25.5 0 300 400 i
Now AS 100 and AS 2 decide to merge into a single AS 2. All BGP speakers in AS 100 are
to be migrated to AS 2. Because a common IGP must be used in the same AS, IGP must be
migrated first (migrating the IGP is outside the scope of this topic and thus isn't covered
here). To reduce migration risk and the impact on the peers, migration is to take a gradual
approach, with R2 being migrated first.
Local AS is configured on R2 on the session with R5. To maintain the current forwarding
architecture, a higher WEIGHT is set on R2 to prefer the path from R5. The outbound
AS_PATH is prepended twice on R3 toward R6 and once on R1 toward R4. The
no-
prepend
option on R2 is needed so that R1 accepts the path via R5, because now there
is an eBGP session between R1 and R2.
Examples 4-44, 4-45, and 4-46 show the configurations on R1, R2, and R3, respectively.
Example 4-44
BGP Configuration on R1
router bgp 100
network 172.15.0.0
neighbor 192.168.12.2 remote-as 2
neighbor 192.168.14.4 remote-as 200
neighbor 192.168.14.4 route-map Path-200 out
!
route-map Path-200 permit 10
set as-path prepend 100
Example 4-45
BGP Configuration on R2
router bgp 2
network 172.15.0.0
neighbor 192.168.12.1 remote-as 100
neighbor 192.168.23.3 remote-as 2
neighbor 192.168.25.5 remote-as 300
neighbor 192.168.25.5 local-as 100 no-prepend
neighbor 192.168.25.5 weight 100
