2023年7月4日发(作者：)

OSPF协议中cost值的计算

在 ospf协议中每个接⼝根据其速率不同，都有⼀个默认的公式计算的cost值，ospf协议采⽤cost值来决定到达⽬的地的最佳路径，如果存在多条到达⽬的地的路径，优先选⽤cost值最低的路径。Cost值的计算是沿⽬的地途中所有转发数据的接⼝的cost值的总和，下⾯通过实验来验证⼀下。如下拓扑图：

在 RT11上的路由表如下：rt11#sh ip routeCodes: 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

Gateway of last resort is not set

172.19.0.0/24 is subnetted, 1 subnetsO 172.19.10.0 [110/3] via 10.1.1.6, 00:00:12, FastEthernet2/0 10.0.0.0/30 is subnetted, 2 subnetsO 10.3.3.4 [110/2] via 10.1.1.6, 00:00:12, FastEthernet2/0C 10.1.1.4 is directly connected, FastEthernet2/0C 192.168.1.0/24 is directly connected, FastEthernet0/0C 192.168.2.0/24 is directly connected, FastEthernet0/1在这台路由器上到达 172.19.10.0的度量值为3，计算⽅式为：RT11的数据转发接⼝ f2/0的cost值“1”+RT12的数据转发接⼝f2/0的cost值“1”+sw2的⽬标接⼝vlan1的cost值“1”=3。为了证明，将 RT12的数据流⼊接⼝f0/0的cost值改为50，数据转发接⼝f2/0的cost值改为60，sw2的vlan1的cost改为70，再来看看路由表：rt11#sh ip routeCodes: 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

Gateway of last resort is not set

172.19.0.0/24 is subnetted, 1 subnetsO 172.19.10.0 [110/131] via 10.1.1.6, 00:00:00, FastEthernet2/0 10.0.0.0/30 is subnetted, 2 subnetsO 10.3.3.4 [110/61] via 10.1.1.6, 00:00:00, FastEthernet2/0C 10.1.1.4 is directly connected, FastEthernet2/0C 192.168.1.0/24 is directly connected, FastEthernet0/0C 192.168.2.0/24 is directly connected, FastEthernet0/1这时到达 172.19.10.0的度量值为131了，由1+60+70计算得来，RT12上的数据流⼊接⼝f0/0的cost值没有参与计算。那么再看从 SW1上的路由表：sw2#sh ip routeCodes: 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

Gateway of last resort is not set

172.19.0.0/24 is subnetted, 1 subnetsC 172.19.10.0 is directly connected, Vlan1 10.0.0.0/30 is subnetted, 2 subnetsC 10.3.3.4 is directly connected, Vlan2O 10.1.1.4 [110/51] via 10.3.3.5, 00:00:00, Vlan2O 192.168.1.0/24 [110/52] via 10.3.3.5, 00:00:00, Vlan2O 192.168.2.0/24 [110/52] via 10.3.3.5, 00:00:00, Vlan2从这个路由表上就更加验证了 ospf协议中度量值的计算，从sw1到192.168.1.0的度量值计算为：Sw1的数据转发接⼝ vlan2的cost值“1”+RT12的数据转发接⼝f0/0的cost值“50”+RT11的⽬的接⼝的cost值“1”=52那么针对重发布的路由的度量值⼜是如何计算的呢，在 RT11上将两个直连⽹络192.168.1.0和192.168.2.0改为重发布：配置命令：access-list 10 permit 192.168.1.0access-list 20 permit 192.168.2.0!route-map setmetric permit 10 match ip address 10 set metric 5!route-map setmetric permit 20 match ip address 20 set metric 20

router ospf 100 log-adjacency-changes redistribute connected subnets route-map setmetric这样就将 192.168.1.0的度量值改为了5,192.168.2.0的度量值改为了20，在rt12和SW1上查看路由表：rt12#sh ip routeCodes: 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

Gateway of last resort is not set

172.19.0.0/24 is subnetted, 1 subnetsO 172.19.10.0 [110/130] via 10.3.3.6, 00:04:08, FastEthernet2/0 10.0.0.0/30 is subnetted, 2 subnetsC 10.3.3.4 is directly connected, FastEthernet2/0C 10.1.1.4 is directly connected, FastEthernet0/0O E2 192.168.1.0/24 [110/5] via 10.1.1.5, 00:04:08, FastEthernet0/0O E2 192.168.2.0/24 [110/20] via 10.1.1.5, 00:04:08, FastEthernet0/0

sw2#sh ip routeCodes: 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

Gateway of last resort is not set

172.19.0.0/24 is subnetted, 1 subnetsC 172.19.10.0 is directly connected, Vlan1 10.0.0.0/30 is subnetted, 2 subnetsC 10.3.3.4 is directly connected, Vlan2O 10.1.1.4 [110/51] via 10.3.3.5, 00:04:32, Vlan2O E2 192.168.1.0/24 [110/5] via 10.3.3.5, 00:04:32, Vlan2O E2 192.168.2.0/24 [110/20] via 10.3.3.5, 00:04:32, Vlan2RT12和 SW1的路由表中，到达192.168.1.0和192.168.2.0的度量值和重发布时指定的度量值是⼀致的，那么就说明重发布时的度量值与接⼝的cost值是⽆关的。了解了 cost值的计算，可以在具有冗余的路径的环境下合理的修改cost值，来控制数据的转发路径。