Variable Length Subnet Mask Issues & Troubleshooting

Recognize when VLSM is in Use

Identify if VLSM is being used by examining the subnet masks of all subnets.

VLSM in use example /24

VLSM not in use

VLSM is in use when multiple subnet masks are used for different subnets of a single classful network

Determine the routing protocol being used

Only certain routing protocols support VLSM ( RIP-2, EIGRP, OSPF )

RIP-1 does not support VLSM so if that is in use VLSM is not in play.

Look for Overlapping VLSM subnets

After figuring out if VLSM is in use look for overlaps in the subnets

Cisco IOS can recognize VLSM overlaps when an IP address command implies an overlap with another existing IP address, but only on that router. If the interface is not shutdown the router will reject the command. If the interface is down it will accept it, but will refuse to bring up the interface until the overlap is resolved.

Cisco IOS cannot detect an overlap when the IP address is configured on a different router.

VLSM Troubleshooting summary

  • Identify if VLSM is in use
  • Look for VLSM overlaps
  • Symptoms of overlaps are: some hosts in a subnet can communicate with the network and some may not. Hosts in the smaller subnets will be the ones communicating
  • Use traceroute to located problem hops between routers and hosts