ARP

carl_townshend · ‎05-30-2008

Hi all, what will be the implications of clearing the arp on my l3 switch, can you clear it per vlan ? also do the arp entries have a lifetime ?

ralphcarter · ‎05-30-2008

#clear arp-cache [interface xxx]

The [interface xxx] is optional where you can clear arps from an interface.

If you clear the arp cache, you will have a slight delay as your L3 routing engine forwards new arp requests to its connected vlans to locate mac-addresses.

Its good to clear the arp when troubleshooting problems related to reconfiguration.

CCIE 26175
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shiva_ial · ‎06-01-2008

in normal cases arp cache timeout period is 4 hrs

you can also get confirmed by using show int (your interface) command.

rgds/shiva

bvsnarayana03 · ‎06-01-2008

As mentiond in previous post, arp lifetime is default of 4 hrs. Chek the output below.

MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation ARPA, loopback not set

Keepalive not supported

ARP type: ARPA, ARP Timeout 04:00:00

ARP cache is cleared when existing devices are replaced with similar IP address. e.g. you replaced a switch with new onw but same UIP adress. in this case the new mac-add has to be updated in the arp but it doesnt get reflected automaticlly due to the large timeout period. In such cases, clear arp cache is a good option.