Architecture :

BGP backdoor

Configuration :

R1

 1 router bgp 1
 2  no synchronization
 3  bgp router-id 1.1.1.1
 4  bgp log-neighbor-changes
 5  neighbor 10.1.3.3 remote-as 3
 6  no auto-summary
 7 
 8 router bgp 1
 9  no synchronization
10  bgp router-id 1.1.1.1
11  bgp log-neighbor-changes
12  neighbor 10.1.3.3 remote-as 3
13  no auto-summary

R2

1 router eigrp 12
2  redistribute connected
3  network 10.1.2.0 0.0.0.255
4  no auto-summary
5  eigrp router-id 2.2.2.2

R3

1 router bgp 3
2  no synchronization
3  bgp router-id 3.3.3.3
4  bgp log-neighbor-changes
5  neighbor 10.1.3.1 remote-as 1
6  neighbor 10.3.4.4 remote-as 4
7  no auto-summary

R4

1 router bgp 4
2  no synchronization
3  bgp router-id 4.4.4.4
4  bgp log-neighbor-changes
5  network 10.2.4.0 mask 255.255.255.0
6  neighbor 10.3.4.3 remote-as 3
7  no auto-summary

Pour R1, le réseau d'interconnexion entre R2 et R4 et joignable via EIGRP et BGP.
Il serait logique de passer par R2 car il est plus près.
Mais l'Administrative Distance de BGP (ici eBGP) est de 20 qui prend le main sur EIGRP qui est de 170 (car elle est redistibuée).

Table de routage R1

 1 R1#sh ip route
 2 Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
 3        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
 4        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
 5        E1 - OSPF external type 1, E2 - OSPF external type 2
 6        i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
 7        ia - IS-IS inter area, * - candidate default, U - per-user static route
 8        o - ODR, P - periodic downloaded static route
 9 
10 Gateway of last resort is not set
11 
12      10.0.0.0/24 is subnetted, 3 subnets
13 C       10.1.3.0 is directly connected, FastEthernet0/1
14 C       10.1.2.0 is directly connected, FastEthernet0/0
15 B       10.2.4.0 [20/0] via 10.1.3.3, 00:04:26

On vérifie bien que R1 utilise BGP car son Administratice Distance et inférieure à EIGRP.

Il est possible de changer ce comportement juste pour le réseau choisi.

Configuration :

On va utiliser la fonction backdoor de BGP.

R1

1 router bgp 1
2  no synchronization
3  bgp router-id 1.1.1.1
4  bgp log-neighbor-changes
5  network 10.2.4.0 mask 255.255.255.0 backdoor
6  neighbor 10.1.3.3 remote-as 3
7  no auto-summary

Il faut en revanche le faire par réseau.

Vérification :

 1 R1#sh ip route 
 2 Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
 3        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
 4        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
 5        E1 - OSPF external type 1, E2 - OSPF external type 2
 6        i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
 7        ia - IS-IS inter area, * - candidate default, U - per-user static route
 8        o - ODR, P - periodic downloaded static route
 9 
10 Gateway of last resort is not set
11 
12      10.0.0.0/24 is subnetted, 3 subnets
13 C       10.1.3.0 is directly connected, FastEthernet0/1
14 C       10.1.2.0 is directly connected, FastEthernet0/0
15 D EX    10.2.4.0 [170/284160] via 10.1.2.2, 00:01:36, FastEthernet0/0
16 R1#sh ip bgp 
17 BGP table version is 3, local router ID is 1.1.1.1
18 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
19               r RIB-failure, S Stale
20 Origin codes: i - IGP, e - EGP, ? - incomplete
21 
22    Network          Next Hop            Metric LocPrf Weight Path
23 r> 10.2.4.0/24      10.1.3.3                               0 3 4 i

Test de perte EIGRP :

Nous allons couper l'interface f0/0 et vérifier que la route BGP prenne la main.

 1 R1#conf t
 2 Enter configuration commands, one per line.  End with CNTL/Z.
 3 R1(config)#int f0/0
 4 R1(config-if)#shutdown 
 5 R1(config-if)#end
 6 R1#
 7 *Mar  1 00:10:36.095: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 12: Neighbor 10.1.2.2 (FastEthernet0/0) is down: interface down
 8 R1#sh ip bgp 
 9 *Mar  1 00:10:37.563: %SYS-5-CONFIG_I: Configured from console by console
10 *Mar  1 00:10:38.039: %LINK-5-CHANGED: Interface FastEthernet0/0, changed state to administratively down
11 R1#sh ip bgp 
12 *Mar  1 00:10:39.039: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to down
13 R1#sh ip route 
14 Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
15        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
16        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
17        E1 - OSPF external type 1, E2 - OSPF external type 2
18        i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
19        ia - IS-IS inter area, * - candidate default, U - per-user static route
20        o - ODR, P - periodic downloaded static route
21 
22 Gateway of last resort is not set
23 
24      10.0.0.0/24 is subnetted, 2 subnets
25 C       10.1.3.0 is directly connected, FastEthernet0/1
26 B       10.2.4.0 [200/0] via 10.1.3.3, 00:00:06