IP Routing, IP Default Gateway and DHCP Relay Summary

IP Routing 

The Cisco IOS software uses both administrative distance and metric information to determine the default route. Multiple ip default-network commands can be given. All candidate default routes, both static (that is, flagged by the ip default-network command) and dynamic, appear in the routing table preceded by an asterisk.

If the IP routing table indicates that the specified network number is subnetted and a nonzero subnet number is specified, then the system will automatically configure a static summary route. This static summary route is configured instead of a default network. The effect of the static summary route is to cause traffic destined for subnets that are not explicitly listed in the IP routing table to be routed using the specified subnet.


The following example defines a static route to network as the static default route:

ip route
ip default-network

If the following command was issued on a router not connected to network, the software might choose the path to that network as a default route when the network appeared in the routing table:

ip default-network


The following example chooses an administrative distance of 110. In this case, packets for network will be routed to a router at if dynamic information with an administrative distance less than 110 is not available.

ip route 110

IP Default Gateway (Switch)

Enter the IP address of the next-hop router interface that is directly connected to the switch where a default gateway is being configured. The default gateway receives IP packets with unresolved destination IP addresses from the switch.

Once the default gateway is configured, the switch has connectivity to the remote networks with which a host needs to communicate.

Note When your switch is configured to route with IP, it does not need to have a default gateway set.

DHCP Relay

You must configure a relay device when a switch sends broadcast packets that need to be responded to by a host on a different LAN. Examples of broadcast packets that the switch might send are DHCP, DNS, and in some cases, TFTP packets. You must configure this relay device to forward received broadcast packets on an interface to the destination host.

If the relay device is a Cisco router, enable IP routing (ip routing global configuration command), and configure a helper addresses by using the ip helper-address interface configuration command.

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