IP Routing Features

Overview of IP Routing

If the cache contains an entry with the destination IP address, the device uses the information in the entry to forward the packet out the ports listed in the entry. The destination IP address is the address of the packet’s final destination. The port numbers are the ports through which the destination can be reached.

 

If the cache does not contain an entry, the software can create an entry

 

in the forwarding cache.

 

Each entry in the IP forwarding cache has an age timer. If the entry remains

 

unused for five minutes, the software removes the entry. The age timer is not

 

configurable.

 

 

N o t e

You cannot add static entries to the IP forwarding cache.

 

 

IP Global Parameters for Routing Switches

The following table lists the IP global parameters and the page where you can find more information about each parameter.

Table 7-1. IP Global Parameters for Routing Switches

Parameter

Description

Default

See page

 

 

 

 

Address

A standard IP mechanism that routers use to learn the Media Access

Enabled

7-8

Resolution

Control (MAC) address of a device on the network. The router sends

 

 

Protocol (ARP)

the IP address of a device in the ARP request and receives the device’s

 

 

 

MAC address in an ARP reply.

 

 

 

 

 

 

ARP age

The amount of time the device keeps a MAC address learned through

20 minutes

7-10

 

ARP in the device’s ARP cache. The device resets the timer to zero each

 

 

 

time the ARP entry is refreshed and removes the entry if the timer

 

 

 

reaches the ARP age.

 

 

 

 

 

 

Proxy ARP

An IP mechanism a router can use to answer an ARP request on behalf

Disabled

7-12

 

of a host. It replies with the router’s own MAC address instead of the

 

 

 

host’s.

 

 

 

 

 

 

Time to Live

The maximum number of routers (hops) through which a packet can

64 hops

7-11

(TTL)

pass before being discarded. Each router decreases a packet’s TTL by

 

 

 

1 before forwarding the packet. If decreasing the TTL causes the TTL

 

 

 

to be 0, the router drops the packet instead of forwarding it.

 

 

 

 

 

 

Directed

A directed broadcast is a packet containing all ones (or in some cases,

Disabled

7-13

broadcast

all zeros) in the host portion of the destination IP address. When a router

 

 

forwarding

forwards such a broadcast, it sends a copy of the packet out each of its

 

 

 

enabled IP interfaces.

 

 

 

Note: You also can enable or disable this parameter on an individual

 

 

 

interface basis. See table 7-2 on page 7-7.

 

 

 

 

 

 

 

 

 

 

7-6