Case Study: AS Integration via the Local AS
This case study shows you how to integrate two existing autonomous systems (AS 100 and AS 2) into one AS (AS 2) using the Local AS feature. A simple topology is shown in Figure 4-11. AS 100 is multihomed to three different autonomous systems: 200, 300, and 2. The prefix 172.15.0.0/16 is generated and advertised to neighboring autonomous systems. AS 100 also receives the prefix 172.16.0.0/16 generated by AS 400.
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Figure
4-11 Network Topology for the Case Study
For the purposes of this case study, the last octet of an IP address indicates the router number. Basic BGP configurations for R1 and R2 are shown in Examples 4-40 and 4-41, respectively.
Example 4-40 BGP Configuration on R1
router bgp 100 no synchronization bgp log-neighbor-changes network 172.15.0.0 neighbor 192.168.12.2 remote-as 100 neighbor 192.168.14.4 remote-as 200 no auto-summary
Example 4-41 BGP Configuration on R2
router bgp 100 no synchronization bgp log-neighbor-changes 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 no auto-summary
Examples 4-42 and 4-43 show the BGP RIB.
Example 4-42 BGP RIB on R1
R1#show ip bgp
BGP table version is 3, local router ID is 192.168.14.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* i172.15.0.0 192.168.12.2 0 100 0 i
*> 0.0.0.0 0 32768 i
* i172.16.0.0 192.168.25.5 100 0 300 400 i
*> 192.168.14.4 0 200 400 i
Example 4-43 BGP RIB on R2
R2#show ip bgp
BGP table version is 3, local router ID is 192.168.25.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure
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 book 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
Example 4-46 BGP Configuration on R3
router bgp 2 neighbor 192.168.23.2 remote-as 2 neighbor 192.168.36.6 remote-as 300 neighbor 192.168.36.6 route-map Path-300 out ! route-map Path-300 permit 10 set as-path prepend 2 2
The new BGP RIB on R1, R2, and R7 is shown in Examples 4-47, 4-48, and 4-49, respectively.
Example 4-47 BGP RIB on R1
R1#show ip bgp
BGP table version is 3, local router ID is 192.168.14.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* 172.15.0.0 192.168.12.2 0 0 2 i
*> 0.0.0.0 0 32768 i
* 172.16.0.0 192.168.12.2 0 2 300 400 i
*> 192.168.14.4 0 200 400 i
Example 4-48 BGP RIB on R2
R2#show ip bgp
BGP table version is 5, local router ID is 192.168.25.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* 172.15.0.0 192.168.12.1 0 0 100 i
*> 0.0.0.0 0 32768 i
*> 172.16.0.0 192.168.25.5 100 300 400 i
* 192.168.12.1 0 100 200 400 i
* i 192.168.36.6 100 0 300 400 i
Example 4-49 BGP RIB on R7
R7#show ip bgp
BGP table version is 4, local router ID is 192.168.57.7
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* 172.15.0.0 192.168.57.5 0 300 100 2 i
*> 192.168.47.4 0 200 100 100 i
*> 172.16.0.0 0.0.0.0 0 32768 i
The resulting topology is shown in Figure 4-12.
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Figure
4-12 Topology After R2 Is Migrated to AS 2
The next step is to migrate R1 to the new AS. Local AS is configured on R1 on the session with R4. AS_PATH prepending is now removed on R1. The LOCAL_PREF is modified to prefer the path via R4. The reason that LOCAL_PREF is used instead of WEIGHT is that R2 would also prefer the path via R1 for 172.16.0.0/16 if the link between R2 and R5 failed. The new BGP configurations on R1 and R2 are shown in Examples 4-50 and 4-51, respectively.
Example 4-50 BGP Configuration on R1
router bgp 2 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 local-as 100 neighbor 192.168.14.4 route-map Set-lpref in ! route-map Set-lpref permit 10 set local-preference 120
Example 4-51 BGP Configuration on R2
router bgp 2 network 172.15.0.0 neighbor 192.168.12.1 remote-as 2 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
The BGP RIB is shown in Examples 4-52, 4-53, and 4-54 for R1, R2, and R7, respectively.
Example 4-52 BGP RIB on R1
R1#show ip bgp
BGP table version is 3, local router ID is 192.168.14.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* i172.15.0.0 192.168.12.2 0 100 0 i
*> 0.0.0.0 0 32768 i
*> 172.16.0.0 192.168.14.4 120 0 100 200 400 i
* i 192.168.25.5 100 0 300 400 i
Example 4-53 BGP RIB on R2
R2#show ip bgp
BGP table version is 5, local router ID is 192.168.25.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure
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
* i172.16.0.0 192.168.14.4 120 0 100 200 400 i
*> 192.168.25.5 100 300 400 i
* i 192.168.36.6 100 0 300 400 i
Example 4-54 BGP RIB on R7
R7#show ip bgp
BGP table version is 5, local router ID is 192.168.57.7
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 172.15.0.0 192.168.57.5 0 300 100 2 i
* 192.168.47.4 0 200 100 2 i
*> 172.16.0.0 0.0.0.0 0 32768 i
Now AS 2 can convince AS 300 to change its peering and, thus, R5's configuration. Local AS is not needed on R2. However, AS 200 will only honor its previous peering agreement with AS 100. Local AS is still needed between R1 and R4. To maintain the same forwarding policy, R2 now needs to prepend its AS_PATH outbound to R5. The final configuration of R2 is shown in Example 4-55. The BGP RIB on R7 is shown in Example 4-56.
Example 4-55 BGP Configuration on R2
router bgp 2 network 172.15.0.0 neighbor 192.168.12.1 remote-as 2 neighbor 192.168.23.3 remote-as 2 neighbor 192.168.25.5 remote-as 300 neighbor 192.168.25.5 weight 100 neighbor 192.168.25.5 route-map Path-300 out ! route-map Path-300 permit 10 set as-path prepend 2
Example 4-56 BGP RIB on R7
R7#show ip bgp
BGP table version is 10, local router ID is 192.168.57.7
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* 172.15.0.0 192.168.47.4 0 200 100 2 i
*> 192.168.57.5 0 300 2 2 i
*> 172.16.0.0 0.0.0.0 0 32768 i
Figure 4-13 shows the final topology.
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Figure
4-13 Final Topology