Technical data

Brocade 6910 Ethernet Access Switch Diagnostic Guide 19
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Chapter
4Layer 2 Protocol Diagnostics
This chapter describes Layer 2 troubleshooting and diagnostic processes for the Brocade 6910
switch.
MAC address learning
In MAC address learning, the source MAC address of each received packet is stored so that future
packets destined for that address can be forwarded only to the interface where that address is
located. (Packets destined for unrecognized addresses are forwarded out every bridge interface.)
MAC address learning, defined in the IEEE 802.1 standard, helps minimize traffic on the attached
LANs.
Address Resolution Protocol
Switches use Address Resolution Protocol (ARP) to learn the MAC addresses of devices on the
network. The switch sends an ARP request that contains the IP address of a device, and receives
the MAC address for that device in an ARP reply. These dynamically learned entries are stored in
the ARP cache. You can also manually configure MAC addresses, which are called static entries.
Dynamic and static ARP entries in the ARP cache resemble the following:
Console# show arp
ARP Cache Timeout: 1200 (seconds)
IP Address MAC Address Type Interface
--------------- ----------------- --------- -----------
192.168.0.2 74-8E-F8-68-02-30 other VLAN1
192.168.0.9 11-22-33-44-55-66 static VLAN1
192.168.0.99 00-E0-29-94-34-64 dynamic VLAN1
Total entry : 3
ARP age
The ARP age is the amount of time the device keeps a learned MAC address in the 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. The default ARP age is 20 minutes.
Changing the ARP aging period
When the switch places an entry in the ARP cache, it 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aging timer ensures that the ARP cache does not retain learned entries that are no longer valid.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.
ARP age affects dynamic entries only.