Discontinuous Subnet Fault

Publication Date:  2012-07-27 Views:  306 Downloads:  0
Issue Description

As shown in Figure 1-5, the RIP routing protocol is configured.

Figure 1-5  Networking diagram of RIP


After the configuration, run the display ip routing-table command to the routing table.

The display shows:

  • Only one route to 137.99.0.0 exists in the routing table of USG A.
  • The next hop of the route is 192.168.10.1 or 193.168.10.1.
[USGA] display ip routing-table
Routing Tables: Public net
         Destinations : 9        Routes : 10

Destination/Mask    Protocol  Pre  Cost       NextHop         Interface

 127.0.0.0/8   Direct         0    0         127.0.0.1       InLoopBack0
 127.0.0.1/32  Direct         0    0         127.0.0.1       InLoopBack0
 137.99.0.0/16  RIP           100  1         192.168.10.1    Vlanif4
                RIP           100  1         193.168.10.1    Vlanif5
 192.168.10.0/24  Direct      0    0         192.168.10.2    Vlanif4
 192.168.10.1/32  Direct      0    0         192.168.10.1    Vlanif4
 192.168.10.2/32  Direct      0    0         127.0.0.1       InLoopBack0
 193.168.10.0/24  Direct      0    0         193.168.10.2    Vlanif4
 193.168.10.1/32  Direct      0    0         193.168.10.1    Vlanif4
 193.168.10.2/32  Direct      0    0         127.0.0.1       InLoopBack0

In Figure 1-5, USG A should have two routes:

  • 137.99.88.0/24 that is forwarded to USG B.
  • 137.99.99.0/24 that is forwarded to USG C.
Alarm Information
None.
Handling Process
  1. Using the display ip routing-table command on USG A, you can find only one route 137.99.0.0/16.
  2. Using the debugging rip send command on USG B and USG C, you can view the route sent to USG A. Check the routing table on USG A and configuration files on each router to locate the fault.
  3. Configure RIP-2 in the network and cancel the automatic aggregation of RIP-2 on USG B and USG C.
Root Cause

Run the debugging rip send command on USG B and USG C respectively. Then by observing the RIP packet that is sent from Vlanif4, you can find:

  • USG B sends classful 137.99.0.0 to USG A.
  • USG C sends classful 137.99.0.0 to USG A.

The routing table of USG A shows that USG A receives only one of the two routes.

The cause may be that RIP-1 does not support discontinuous subnets.

The discontinuous subnets refer to several subnets belonging to the same network that are segmented by different networks.

In Figure 1-5, the network 137.99.0.0 is segmented by the network 192.168.10.0 and the network 193.168.10.0.

Configure RIP as shown in Figure 1-5.

# Configure USG B.
[USGB] rip
[USGB-rip] network 192.168.10.0
[USGB-rip] network 137.99.0.0
# Configure USG C.
[USGC] rip
[USGC-rip] network 193.168.10.0
[USGC-rip] network 137.99.0.0
# Configure USG A.
[USGA] rip
[USGA-rip] network 193.168.10.0
[USGA-rip] network 192.168.10.0

Thus, the cause of the fault is that RIP-1 does not support discontinuous subnets.

To remove the fault, configure RIP-2 in the network and cancel the automatic aggregation of RIP-2 on USG B and USG C.

By checking the routing table again, you can find route 137.99.88.0/24 and route 137.99.99.0/24 are in the routing table of USG A.

[USGA-rip] display ip routing-table
         Destinations : 11       Routes : 12

Destination/Mask    Protocol  Pre  Cost       NextHop         Interface

 127.0.0.0/8         Direct    0    0        127.0.0.1       InLoopBack0
 127.0.0.1/32        Direct    0    0        127.0.0.1       InLoopBack0
 137.99.0.0/16       RIP       100  1        193.168.10.1    Vlanif5
                     RIP       100  1        192.168.10.1    Vlanif4
 137.99.88.0/24      RIP       100  1        192.168.10.1    Vlanif4
 137.99.99.0/24      RIP       100  1        193.168.10.1    Vlanif5
 192.168.10.0/24     Direct    0    0        192.168.10.2    Vlanif4
 192.168.10.1/32     Direct    0    0        192.168.10.1    Vlanif4
 192.168.10.2/32     Direct    0    0        127.0.0.1       InLoopBack0
 193.168.10.0/24     Direct    0    0        193.168.10.2    Vlanif5
 193.168.10.1/32     Direct    0    0        193.168.10.1    Vlanif5
 193.168.10.2/32     Direct    0    0        127.0.0.1       InLoopBack0
Suggestions

RIP-1 does not support the discontinuous subnets. The configuration of the assistant address, namely the addresses with the same mask that belongs to the same network can solve the problem. This solution requires more bandwidth and may cause unnecessary errors. It is therefore, not advisable to use this method.

END