Dans cet exemple de configuration :
- BGP est utilisé sur la partie Wan
- EIGRP est utilisé sur un lien LAN entre 2 AS (eBGP)
la distance administrative de eBGP (20) est préféré à celle de EIGRP (90)
- R2 utilisera donc le lien Wan pour atteindre 3.0.0.0 /8 et 150.1.3.0 /24
- R3 utilisera donc le lien Wan pour atteindre 2.0.0.0 /8 et 150.1.2.0 /24
Le routage entre R2 et R3 n’est pas optimal : « a sub-optimal path »
R2#show ip route B 1.0.0.0/8 [20/0] via 10.1.12.1, 00:12:22 C 2.0.0.0/8 is directly connected, Loopback0 B 3.0.0.0/8 [20/0] via 10.1.12.1, 00:12:22 10.0.0.0/24 is subnetted, 2 subnets R 10.1.13.0 [120/1] via 10.1.12.1, 00:00:07, Serial0/0.201 C 10.1.12.0 is directly connected, Serial0/0.201 150.1.0.0/24 is subnetted, 3 subnets C 150.1.23.0 is directly connected, FastEthernet0/0 B 150.1.3.0 [20/0] via 10.1.12.1, 00:01:34 C 150.1.2.0 is directly connected, Loopback1 R2#show ip bgp Network Next Hop Metric LocPrf Weight Path *> 1.0.0.0 10.1.12.1 0 0 100 i *> 2.0.0.0 0.0.0.0 0 32768 i *> 3.0.0.0 10.1.12.1 0 100 300 i *> 150.1.2.0/24 0.0.0.0 0 32768 i *> 150.1.3.0/24 10.1.12.1 0 100 300 i
R3#show ip route B 1.0.0.0/8 [20/0] via 10.1.13.1, 00:11:57 B 2.0.0.0/8 [20/0] via 10.1.13.1, 00:10:55 C 3.0.0.0/8 is directly connected, Loopback0 10.0.0.0/24 is subnetted, 2 subnets C 10.1.13.0 is directly connected, Serial1/0.301 R 10.1.12.0 [120/1] via 10.1.13.1, 00:00:00, Serial1/0.301 150.1.0.0/24 is subnetted, 3 subnets C 150.1.23.0 is directly connected, FastEthernet0/0 C 150.1.3.0 is directly connected, Loopback1 B 150.1.2.0 [20/0] via 10.1.13.1, 00:01:04 R3#show ip bgp Network Next Hop Metric LocPrf Weight Path *> 1.0.0.0 10.1.13.1 0 0 100 i *> 2.0.0.0 10.1.13.1 0 100 200 i *> 3.0.0.0 0.0.0.0 0 32768 i *> 150.1.2.0/24 10.1.13.1 0 100 200 i *> 150.1.3.0/24 0.0.0.0 0 32768 i
Configuration
La configuration « backdoor » permet de aux routeurs d’utiliser les routes BGP en dernier recours (lien de backup)
R2(config)#router bgp 200 R2(config-router)#network 150.1.3.0 mask 255.255.255.0 backdoor R3(config)#router bgp 300 R3(config-router)#network 150.1.2.0 mask 255.255.255.0 backdoor
Vérifications
R2#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route B 1.0.0.0/8 [20/0] via 10.1.12.1, 00:13:52 C 2.0.0.0/8 is directly connected, Loopback0 B 3.0.0.0/8 [20/0] via 10.1.12.1, 00:13:52 10.0.0.0/24 is subnetted, 2 subnets R 10.1.13.0 [120/1] via 10.1.12.1, 00:00:13, Serial0/0.201 C 10.1.12.0 is directly connected, Serial0/0.201 150.1.0.0/24 is subnetted, 3 subnets C 150.1.23.0 is directly connected, FastEthernet0/0 D 150.1.3.0 [90/156160] via 150.1.23.3, 00:00:46, FastEthernet0/0 C 150.1.2.0 is directly connected, Loopback1 R2#show ip bgp BGP table version is 18, local router ID is 150.1.2.2 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 1.0.0.0 10.1.12.1 0 0 100 i *> 2.0.0.0 0.0.0.0 0 32768 i *> 3.0.0.0 10.1.12.1 0 100 300 i *> 150.1.2.0/24 0.0.0.0 0 32768 i r> 150.1.3.0/24 10.1.12.1 0 100 300 i
Le prefix 150.1.3.0 /24 est bien appris par BGP
- il a le status « r RIB-failure »
- il n’est pas installé dans la table de routage
En cas de coupure du lien direct LAN :
R2#show ip route B 1.0.0.0/8 [20/0] via 10.1.12.1, 23:42:48 C 2.0.0.0/8 is directly connected, Loopback0 B 3.0.0.0/8 [20/0] via 10.1.12.1, 23:42:48 10.0.0.0/24 is subnetted, 2 subnets R 10.1.13.0 [120/1] via 10.1.12.1, 00:00:19, Serial0/0.201 C 10.1.12.0 is directly connected, Serial0/0.201 150.1.0.0/24 is subnetted, 3 subnets C 150.1.23.0 is directly connected, FastEthernet0/0 B 150.1.3.0 [200/0] via 10.1.12.1, 00:02:31 C 150.1.2.0 is directly connected, Loopback1