step 12 - minimum cluster list length ( 13 step selection route BGP)

R1#show run | se router bgp router bgp 1  no synchronization  bgp log-neighbor-changes  network 8.8.8.0 mask 255.255.255.0  neighbor 10.1.12.2 remote-as 1  neighbor 10.1.13.3 remote-as 1

R2#show run | se router bgp router bgp 1  no synchronization  bgp log-neighbor-changes  neighbor 10.1.12.1 remote-as 1  neighbor 10.1.12.1 route-reflector-client  neighbor 10.1.25.5 remote-as 1  no auto-summary

R3#show run | se router bgp router bgp 1  no synchronization  bgp log-neighbor-changes  neighbor 10.1.13.1 remote-as 1  neighbor 10.1.13.1 route-reflector-client  neighbor 10.1.34.4 remote-as 1  no auto-summary

R4#show run | se router bgp router bgp 1  no synchronization  bgp log-neighbor-changes  neighbor 10.1.34.3 remote-as 1  neighbor 10.1.45.5 remote-as 1  no auto-summary !  interface f0/1  ip ospf cost 2

R5#show run | se router bgp router bgp 1  no synchronization  bgp log-neighbor-changes  neighbor 10.1.25.2 remote-as 1  neighbor 10.1.25.2 route-reflector-client  neighbor 10.1.45.4 remote-as 1  no auto-summary

R4#show ip route

           1.0.0.0/32 is subnetted, 1 subnets

O        1.1.1.1 [110/4] via 10.1.45.5, 00:06:21, FastEthernet0/0

                       [110/4] via 10.1.34.3, 00:06:21, FastEthernet0/1

           8.0.0.0/24 is subnetted, 1 subnets

B        8.8.8.0 [200/0] via 10.1.13.1, 00:09:42

           10.0.0.0/8 is variably subnetted, 7 subnets, 2 masks

O        10.1.12.0/24 [110/3] via 10.1.45.5, 00:06:31, FastEthernet0/0

O        10.1.13.0/24 [110/3] via 10.1.34.3, 00:06:21, FastEthernet0/1

R4#show ip bgp 8.8.8.0

BGP routing table entry for 8.8.8.0/24, version 7

Paths: (2 available, best #2, table default)

  Not advertised to any peer

  Local

    10.1.12.1 (metric 3) from 10.1.45.5 (5.5.5.5)

      Origin IGP, metric 0, localpref 100, valid, internal

      Originator: 8.8.8.8, Cluster list: 5.5.5.5, 2.2.2.2

  Local

    10.1.13.1 (metric 3) from 10.1.34.3 (3.3.3.3)

      Origin IGP, metric 0, localpref 100, valid, internal, best

      Originator: 8.8.8.8, Cluster list: 3.3.3.3

Nhận xét: R4 học 8.8.8.0 qua R3.

Chúng ta đi kiểm tra từng bước sau:

1. Weight: bằng nhau 0

2. Local prefer: bằng nhau 100

3. Perfer the path was locally..: bằng nhau

4. AS-path: bằng nhau

5. Origin type : bằng nhau

6. Med: bằng nhau

7. Prefer eBGP over iBGP: cùng iBGP

8. Prefer with lowes IGP metric to BGP next-hop : bằng nhau = 3 ( điều chỉnh cost IGP bằng nhau như bảng RIB ở trên)

9. Không xét ở đây.

10. The path are external: cùng học iBGP

11. Router-id: đều học từ R1 -> cùng Router-id

12. minimum cluster list length: BREAK.

=> cluster length qua next-hop 10.1.13.1 (Cluster list: 3.3.3.3) = 1 < next-hop 10.1.12.1 = 2 (Cluster list: 5.5.5.5, 2.2.2.2)