what updates arp table in router

carl_townshend · ‎11-26-2008

Hi all, when I have a router with an arp entry for a device, firstly how does the router leran this, does the router need to arp for it itself? or does it learn it as traffic flows through it?

also if I plug one device in with an ip address, then move this ip address to another machine, how does the router update the arp ?

crow930us · ‎11-26-2008

ARP works on layer 2.

When a switch is first turned on it has no knowledge of any devices. As the first device sends a packet in to be delivered elsewhere the only thing the switch knows is the mac address and the port number that received that ticket. The switch then floods a request out all of it's other ports asking where to send the packet. Which ever device responds will also send it's mac address and the port it's on. In this method the ARP table gets filled. So a router does not need to arp for it's self.

Arp tables are refreshed every 30 or 60 seconds.

EDWIN.GONZALEZ · ‎11-29-2008

Carl,

As frames are recevied the router copies the source MAC address, makes note of the port on which it was received and places it in the ARP table, if the entry already exist it updates the ARP counter.

If the router needs to forward a frame to a local network and it does not have an entry for the destination address in the ARP table it then will ARP for the destination.

If you move an IP address to another machine the router has to update the IP to MAC address association, you can either:

1. Wait for the ARP entry to expire

2. Manually clear the arp table ( clear arp)