Monday, May 22, 2006

 

Routing Problems with VLSM. (Problem #67)



Consider the network of four routers shown the figure above. (Click on figure to see larger version.) The administrator of this network was given the 192.168.10.128/25 prefix to use within his network. As a first step, she created subnets of the 192.168.10.128/25 prefix and assigned them to the router interfaces as shown in the figure. To verify physical connectivity, she pinged the neighboring IP address on each link and alll the pings were successful. Next she turns on RIPv2 because she knows that RIPv2 supports VLSM and she is indeed using variable length subnet masks in the network.

Once routing is turned on, she finds that pings between R1, R2 and R3 over the ethernet interfaces have stopped working. Upon debugging this further, she finds that all pings from R1 and R2 to the rest of the network are failing. R3 can ping all addresses on R4 but cannot ping anything on R1 and R2. Finally, she finds that if the loopback interface on R4 is shut down, all connectivity in the rest of the network is restored and all pings work fine.

Given the following output from "show ip route" on all the routers, your task is to figure out what the problem is and how it can be fixed.


R1#sh 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, E - EGP
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

192.168.10.0/24 is variably subnetted, 5 subnets, 3 masks
R 192.168.10.224/29 [120/2] via 192.168.10.227, 00:00:20, Ethernet0
C 192.168.10.224/28 is directly connected, Ethernet0
C 192.168.10.240/29 is directly connected, Loopback0
R 192.168.10.248/29 [120/1] via 192.168.10.226, 00:00:09, Ethernet0
R 192.168.10.200/30 [120/1] via 192.168.10.227, 00:00:20, Ethernet0

R2#sh 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, E - EGP
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

192.168.10.0/24 is variably subnetted, 5 subnets, 3 masks
R 192.168.10.224/29 [120/2] via 192.168.10.227, 00:00:07, Ethernet0
C 192.168.10.224/28 is directly connected, Ethernet0
R 192.168.10.240/29 [120/1] via 192.168.10.225, 00:00:25, Ethernet0
C 192.168.10.248/29 is directly connected, Loopback0
R 192.168.10.200/30 [120/1] via 192.168.10.227, 00:00:07, Ethernet0

R3#sh 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, E - EGP
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

192.168.10.0/24 is variably subnetted, 5 subnets, 3 masks
R 192.168.10.224/29 [120/1] via 192.168.10.202, 00:00:17, Serial2
C 192.168.10.224/28 is directly connected, Ethernet0/0
R 192.168.10.240/29 [120/1] via 192.168.10.225, 00:00:00, Ethernet0
R 192.168.10.248/29 [120/1] via 192.168.10.226, 00:00:27, Ethernet0
C 192.168.10.200/30 is directly connected, Serial2/0

R4#sh 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, E - EGP
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

192.168.10.0/24 is variably subnetted, 5 subnets, 3 masks
C 192.168.10.224/29 is directly connected, Loopback0
R 192.168.10.224/28 [120/1] via 192.168.10.201, 00:00:08, Serial2
R 192.168.10.240/29 [120/2] via 192.168.10.201, 00:00:08, Serial2
R 192.168.10.248/29 [120/2] via 192.168.10.201, 00:00:08, Serial2
C 192.168.10.200/30 is directly connected, Serial2


Comments:
Hi

I believe the problem here may well be a discontiguous network. With the addressing scheme shown the loop interface on R4 is encroaching on the R1,R2,R3 ethernet subnet of 192.168.10.224/28.

If the R4 loop interface is changed to say use 192.168.10.193 in the 192.168.10.192 225.225.225/248 subnet i think this may solve the problem??
 
The problem is that the common ethernet network for R1, R2 and R3 shown here:

C 192.168.10.224/28 is directly connected, Ethernet0

includes the loop0 of R4 which is 192.168.10.224/29

One fix would be to change the R4 loopback address to 192.168.10.217 255.255.255.248
 
Yes, the loopback address of R4 is indeed the problem. Changing the address of R4's loopback interface such that it does not overlap with any of the other subnets used in the network will fix the problem.
 
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