Cisco Systems 3560-X, 3750-X Routing Assistance When IP Routing is Disabled

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Catalyst 3750-X and 3560-X Switch Software Configuration Guide

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Chapter 42 Configuring IP Unicast Routing

Configuring IP Addressing

By default, the switch uses proxy ARP to help hosts learn MAC addresses of hosts on other networks or

subnets.

Beginning in privileged EXEC mode, follow these steps to enable proxy ARP if it has been disabled:

To disable proxy ARP on the interface, use the no ip proxy-arp interface configuration command.

Routing Assistance When IP Routing is Disabled

These mechanisms allow the switch to learn about routes to other networks when it does not have IP

routing enabled:

• Proxy ARP, page 42-12

• Default Gateway, page 42-12

• ICMP Router Discovery Protocol (IRDP), page 42-13

Proxy ARP, the most common method for learning about other routes, enables an Ethernet host with no

routing information to communicate with hosts on other networks or subnets. The host assumes that all

hosts are on the same local Ethernet and that they can use ARP to learn their MAC addresses. If a switch

receives an ARP request for a host that is not on the same network as the sender, the switch evaluates

whether it has the best route to that host. If it does, it sends an ARP reply packet with its own Ethernet

MAC address, and the host that sent the request sends the packet to the switch, which forwards it to the

intended host. Proxy ARP treats all networks as if they are local and performs ARP requests for every

IP address.

Proxy ARP is enabled by default. To enable it after it has been disabled, see the “Enable Proxy ARP”

section on page 42-12. Proxy ARP works as long as other routers support it.

Another method for locating routes is to define a default router or default gateway. All nonlocal packets

are sent to this router, which either routes them appropriately or sends an IP Control Message Protocol

(ICMP) redirect message back, defining which local router the host should use. The switch caches the

redirect messages and forwards each packet as efficiently as possible. A limitation of this method is that

there is no means of detecting when the default router has gone down or is unavailable.

Command Purpose

Step 1 configure terminal Enter global configuration mode.

Step 2 interface interface-id Enter interface configuration mode, and specify the Layer 3

interface to configure.

Step 3 ip proxy-arp Enable proxy ARP on the interface.

Step 4 end Return to privileged EXEC mode.

Step 5 show ip interface [interface-id]Verify the configuration on the interface or all interfaces.

Step 6 copy running-config startup-config (Optional) Save your entries in the configuration file.