Configuring IP
December 2005 © Foundry Networks, Inc. 16 - 27
NOTE: If you want to change a previously configured the ARP rate limiting policy, you must remove the
previously configured policy using the no rate-limit-arp <num> command before entering the new policy.
Changing the ARP Aging Period
When the Layer 3 Switch places an entry in the ARP cache, the Layer 3 Switch also starts an aging timer for the
entry. The aging timer ensures that the ARP cache does not retain learned entries that are no longer valid. An
entry can become invalid when the device with the MAC address of the entry is no longer on the network.
The ARP age affects dynamic (learned) entries only, not static entries. The default ARP age is ten minutes. On
Layer 3 Switches, you can change the ARP age to a value from 0 – 240 minutes. You cannot change the ARP age
on Layer 2 Switches. If you set the ARP age to zero, aging is disabled and entries do not age out.
To globally change the ARP aging parameter to 20 minutes, enter the following command:
FESX424 Router(config)# ip arp-age 20
Syntax: ip arp-age <num>
The <num> parameter specifies the number of minutes and can be from 0 – 240. The default is 10. If you specify
0, aging is disabled.
To override the globally configured IP ARP age on an individual interface, enter a command such as the following
at the interface configuration level:
FastIron SuperX Router(config-if-e1000-1/1)# ip arp-age 30
Syntax: [no] ip arp-age <num>
The <num> parameter specifies the number of minutes and can be from 0 – 240. The default is the globally
configured value, which is 10 minutes by default. If you specify 0, aging is disabled.
Enabling Proxy ARP
Proxy ARP allows a Layer 3 Switch to answer ARP requests from devices on one network on behalf of devices in
another network. Since ARP requests are MAC-layer broadcasts, they reach only the devices that are directly
connected to the sender of the ARP request. Thus, ARP requests do not cross routers.
For example, if Proxy ARP is enabled on a Layer 3 Switch connected to two sub-nets, 10.10.10.0/24 and
20.20.20.0/24, the Layer 3 Switch can respond to an ARP request from 10.10.10.69 for the MAC address of the
device with IP address 20.20.20.69. In standard ARP, a request from a device in the 10.10.10.0/24 sub-net cannot
reach a device in the 20.20.20.0 sub-net if the sub-nets are on different network cables, and thus is not answered.
NOTE: An ARP request from one sub-net can reach another sub-net when both sub-nets are on the same
physical segment (Ethernet cable), since MAC-layer broadcasts reach all the devices on the segment.
Proxy ARP is disabled by default on Foundry Layer 3 Switches. The feature is not supported on Foundry Layer 2
Switches.
To enable IP proxy ARP, enter the following command:
FESX424 Router(config)# ip proxy-arp
To again disable IP proxy ARP, enter the following command:
FESX424 Router(config)# no ip proxy-arp
Syntax: [no] ip proxy-arp
Enabling Local Proxy ARP
Foundry devices support Proxy Address Resolution Protocol (Proxy ARP), a feature that enables router ports to
respond to ARP requests for subnets it can reach. However, router ports will not respond to ARP requests for IP
addresses in the same subnet as the incoming ports. Software release 02.3.03 resolves this issue with the
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