Cisco HSRP CAM vs ARP Timeout – Network Configuration Guide

arpciscofhrphsrpNetwork

Today we received email from our ISP saying they are performing maintenance on their network so if you have HSRP connectivity with them in that case you need to clear ARP table or reduce ARP timeout

We do have HSRP connectivity with our ISP and following i found where i got confused. 

#show mac address-table aging-time
Vlan    Aging Time  Configured Aging Time
----    ----------  ---------------------
Global Vlan Admin Age: 300
   1    300         300
   2    300         300

On interface: ARP timeout 4 hour 

#sh int te1/50
TenGigabitEthernet1/50 is up, line protocol is up (connected)
  Hardware is Ten Gigabit Ethernet Port, address is 68ef.bd75.7370 (bia 68ef.bd75.7370)
  Description: ISP_A
  MTU 1500 bytes, BW 10000000 Kbit, DLY 10 usec,
     reliability 255/255, txload 33/255, rxload 36/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 10Gb/s, link type is auto, media type is 10GBase-LR
  input flow-control is on, output flow-control is off
  ARP type: ARPA, ARP Timeout 04:00:00
  • what are those two timeout?
  • in Document saying you should keep CAM timeout and ARP timeout equal but how?
  • When ISP say you should reduce ARP timeout does that means only for specific interface which is connected to their router or whole switch?

Best Answer

What the ISP means is that as they make changes, you may lose connectivity for a time because the ARP table entries your router has with them will not match the changes it is making. If you reduce the ARP timeout, you reduce the time you may have lost connectivity.

You really only need to change arp timeout on the interface used to connect to the ISP.

Router(config)# interface Ethernet0/0
 arp timeout <seconds>

The ARP timeout deals with the amount of time an ARP (layer-3 to layer-2 address resolution) entry remains in the ARP table before it is aged away.

The CAM table used by a switch deals with the MAC address to interface resolution. Layer-2 switches don't care about layer-3 or layer-3 addresses, so they don't use ARP, but they do care about which interface should be used for a MAC address, so they have a table to resolve that.

These two table deal with very different things. It may be desirable, on a layer-3 switch, to keep the two tables coordinated. The switch CAM timeout is set by:

mac address-table aging-time <seconds>
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