On Cisco switches that do not perform routing, setting up a default route or a default gateway can be a little bit confusing. I’ve seen some configuration drift in our network recently were incorrect settings caused some switches to become unreachable for management. Read on to find out the details.
ip default-gatewayand the
ip routingsetting enabled. On some switches,
ip routingis enabled by default but does not show up in the running config. Only “
no ip routing” will show up. I’ll explain why this combination of settings causes switches to unreachable below.
ip routing setting will put the switch in L3 mode, making the switch use the routing table for lookups. This fact also makes the switch ignore the
ip default-gateway. If the routing table does not have a default route or a specific route to the destination you need to reach, reachability will be broken, even with a default gateway specified. Configuring
no ip routing removes the routing table. You can view the difference:
ip default-gatewaysetting present won’t show any routes because the routing table will not be used. You’ll need to decide for your use case which configuration option makes sense. It seems logical to just use the default gateway option for pure L2 switches, as long as you make sure you don’t enable routing at the same time and keep the config on all switches consistent. More detailed information about this subject can be found in Cisco’s documentation.