static route (ECMP) の設定（回答）

回答

RT1(config)#ip route 192.0.2.4 255.255.255.255 ethernet0/0 10.1.1.2
RT1(config)#ip route 192.0.2.4 255.255.255.255 ethernet0/1 10.1.3.3

RT4(config)#ip route 192.0.2.1 255.255.255.255 ethernet0/0 10.1.2.2
RT4(config)#ip route 192.0.2.1 255.255.255.255 ethernet0/1 10.1.4.3

　

　

解説

この問題では、RT1 の Lo0 と RT4 の Lo0 が通信する際、RT2 と RT3 の両方を経由できることを求められています。

これは ECMP (Equal Cost Multi Path) といい、ネットワーク機器では同一の宛先に対する経路情報が同じ cost (metric) で複数存在する場合、以下のように複数経路が Routing table に登録されます。

RT1#show ip route
Codes: L - local, 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, m - OMP
       n - NAT, Ni - NAT inside, No - NAT outside, Nd - NAT DIA
       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
       H - NHRP, G - NHRP registered, g - NHRP registration summary
       o - ODR, P - periodic downloaded static route, l - LISP
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR
       & - replicated local route overrides by connected

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C        10.1.1.0/24 is directly connected, Ethernet0/0
L        10.1.1.1/32 is directly connected, Ethernet0/0
C        10.1.3.0/24 is directly connected, Ethernet0/1
L        10.1.3.1/32 is directly connected, Ethernet0/1
      192.0.2.0/32 is subnetted, 2 subnets
C        192.0.2.1 is directly connected, Loopback0
[[red:S        192.0.2.4 [1/0] via 10.1.3.3, Ethernet0/1]]
[[red:                  [1/0] via 10.1.1.2, Ethernet0/0]]

RT4#show ip route
Codes: L - local, 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, m - OMP
       n - NAT, Ni - NAT inside, No - NAT outside, Nd - NAT DIA
       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
       H - NHRP, G - NHRP registered, g - NHRP registration summary
       o - ODR, P - periodic downloaded static route, l - LISP
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR
       & - replicated local route overrides by connected

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C        10.1.2.0/24 is directly connected, Ethernet0/0
L        10.1.2.4/32 is directly connected, Ethernet0/0
C        10.1.4.0/24 is directly connected, Ethernet0/1
L        10.1.4.4/32 is directly connected, Ethernet0/1
      192.0.2.0/32 is subnetted, 2 subnets
[[red:S        192.0.2.1 [1/0] via 10.1.4.3, Ethernet0/1]]
                   [1/0] via 10.1.2.2, Ethernet0/0]]
C        192.0.2.4 is directly connected, Loopback0

　

上記は回答を設定後の RT1, RT4 の Routing table だが、RT1 は 192.0.2.4 宛てに2つ経路情報が、RT4 は 192.0.2.1 宛てに2つの経路情報が登録されていることがわかる。

ECMP を使用すると特定の宛先に対して複数の経路で通信できるため、負荷分散や可用性の向上を見込むことができる。