Why does a switch need a default gateway and how do you configure it?

A Layer 2 switch needs a default gateway only so its own management traffic (SSH/Telnet/HTTP) can reach an admin on another network; set it with the ip default-gateway ip-address global command.

Default Gateway on a Switch:

A Layer 2 switch needs a default gateway to be remotely managed from another network.

Why switches need a default gateway:

• Switch management traffic (Telnet, SSH, HTTP) originates from the switch
• Without a gateway, management is only possible from the local network
• The switch SVI (VLAN interface) needs an IP for management

Configuration command:

Switch(config)# ip default-gateway ip-address

Example:

S1(config)# interface vlan 1
S1(config-if)# ip address 192.168.10.50 255.255.255.0
S1(config-if)# no shutdown
S1(config-if)# exit
S1(config)# ip default-gateway 192.168.10.1

Important distinction:

• Routers do NOT use ip default-gateway - they use default routes (ip route 0.0.0.0 0.0.0.0 next-hop)
• Switches use ip default-gateway for management traffic only

Go deeper:

• Default gateway (Wikipedia) — the gateway concept a switch needs so its own management traffic can leave the local network.
• Static routing — default route (Wikipedia) — the 0.0.0.0/0 default route routers use, contrasted with a switch's ip default-gateway.