6-1 distance administrative-distance
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6-1 distance administrative-distanceSyntax Description:
Purpose: If a router has learned about a network from more than one routing protocol, then the administrative distance is used to select the best route. The best route is the route that will be installed in the IP routing table. It is the route with the lowest administrative distance. The default administrative distances for the IP routing protocols are as follows :
Initial Cisco IOS Software Release: 10.0 Configuration Example: Adjusting the Administrative Distance to Influence Route SelectionIn Figure 6-1, Router B is receiving the route 3.3.3.0/24 from Router A via OSPF and Router C via EIGRP. Because EIGRP has a lower administrative distance than OSPF, the EIGRP route will be installed in the routing table on Router B. A loopback address on Routers A and C is used to simulate the network that is being propagated by OSPF and EIGRP. The ip ospf network point-to-point interface command (see Section 19-17) on Router A is used so the loopback is advertised as a /24 network and not a /32 network. Figure 6-1. When a Router Learns the Same Route via Two Different Routing Protocols, the Administrative Distance Is Used to Select the Best Route Router A interface Loopback0 ip address 1.1.1.1 255.255.255.255 ! interface Loopback1 description Simulate the network 3.3.3.0/24 ip address 3.3.3.3 255.255.255.0 ip ospf network point-to-point ! interface Ethernet0/0 ip address 172.16.1.1 255.255.255.0 ! router ospf 1 router-id 1.1.1.1 network 1.1.1.1 0.0.0.0 area 0 network 3.3.3.0 0.0.0.255 area 0 network 172.16.1.0 0.0.0.255 area 0 ________________________________________________________________________________ Router B interface Loopback0 ip address 2.2.2.2 255.255.255.255 ! interface Ethernet0 ip address 172.16.1.2 255.255.255.0 ! interface Serial1 bandwidth 64 ip address 10.1.1.5 255.255.255.252 clockrate 64000 ! router eigrp 1 network 10.0.0.0 router ospf 1 network 2.2.2.2 0.0.0.0 area 0 network 172.16.1.0 0.0.0.255 area 0 ________________________________________________________________________________ Router C interface Loopback0 description Simulate the network 3.3.3.0/24 ip address 3.3.3.3 255.255.255.0 ! interface Serial0 bandwidth 64 ip address 10.1.1.6 255.255.255.252 ! router eigrp 1 network 3.0.0.0 network 10.0.0.0 no auto-summary Verify that Routers A and B have established an OSPF neighbor relationship. rtrA# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 1 FULL/BDR 00:00:30 172.16.1.2 Ethernet0/0 ________________________________________________________________________________ rtrB# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 1.1.1.1 1 FULL/DR 00:00:38 172.16.1.1 Ethernet0 Verify that Routers B and C have formed an EIGRP relationship. rtrB# show ip eigrp neighbors IP-EIGRP neighbors for process 1 H Address Interface Hold Uptime SRTT RTO Q Seq (sec) (ms) Cnt Num 0 10.1.1.6 Se1 10 00:13:27 399 2394 0 4 ________________________________________________________________________________ rtrC# show ip eigrp neighbors IP-EIGRP neighbors for process 1 H Address Interface Hold Uptime SRTT RTO Q Seq (sec) (ms) Cnt Num 0 10.1.1.5 Se0 11 00:13:53 28 2280 0 6 Verify that Router B is installing the 3.3.3.0/24 network learned via EIGRP from Router C. rtrB# show ip route Codes: C - connected, S - static, I - IGRP, 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, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default U - per-user static route, o - ODR Gateway of last resort is not set 1.0.0.0/32 is subnetted, 1 subnets O 1.1.1.1 [110/11] via 172.16.1.1, 00:13:37, Ethernet0 2.0.0.0/32 is subnetted, 1 subnets C 2.2.2.2 is directly connected, Loopback0 3.0.0.0/24 is subnetted, 1 subnets D 3.3.3.0 [90/40640000] via 10.1.1.6, 00:13:38, Serial1 172.16.0.0/24 is subnetted, 1 subnets C 172.16.1.0 is directly connected, Ethernet0 10.0.0.0/30 is subnetted, 1 subnets C 10.1.1.4 is directly connected, Serial1 Modify the configuration on Router B to set the administrative distance of all OSPF routes to 80. Because this value is less than the administrative distance for EIGRP (90), the OSPF route for 3.3.3.0/24 should be installed in the IP routing table on Router B. Router B router ospf 1 network 2.2.2.2 0.0.0.0 area 0 network 172.16.1.0 0.0.0.255 area 0 distance 80 VerificationVerify that the OSPF route for 3.3.3.0/24 has been installed in the routing table on Router B. rtrB# show ip route Codes: C - connected, S - static, I - IGRP, 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, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default U - per-user static route, o - ODR Gateway of last resort is not set 1.0.0.0/32 is subnetted, 1 subnets O 1.1.1.1 [80/11] via 172.16.1.1, 00:01:22, Ethernet0 2.0.0.0/32 is subnetted, 1 subnets C 2.2.2.2 is directly connected, Loopback0 3.0.0.0/24 is subnetted, 1 subnets O 3.3.3.0 [80/11] via 172.16.1.1, 00:01:23, Ethernet0 172.16.0.0/24 is subnetted, 1 subnets C 172.16.1.0 is directly connected, Ethernet0 10.0.0.0/30 is subnetted, 1 subnets C 10.1.1.4 is directly connected, Serial1 Troubleshooting
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