Router sends an arp packet when the interface is brought up

[payas60]

Hi all ,

When I am an bringing up the router interface is brought up , i see an arp packet growing from the router to the connected switch(the one connected to the interface that was brought up) with the destination IP set as the same router interface ip that was brought up.

This way the switch seems to know the Mac address of the connected interface .

Can anyone please explain what is happening behind the scene and why the router is sending the arp packet on e ghe interface was brought up ?

[balaji.bandi]

its all depends on what router acting as  ?

you need to provide the model of router and switch, and what IOS code running ?

give some MAC address entries the device seeing ?

[MHM Cisco World]

This called GARP.

Not only router but almost all devices send this garp to inform the other host and device about the IP it have and it MAC.

The MAC use by SW to add it to mac table with port it receive from and vlan if there.

MHM

[M02@rt37]

Hello @payas60,

When a router interface is brought up, it might initiate an ARP request to learn the MAC address corresponding to a specific IP address within the same network. This is typically done to update or refresh the ARP cache of the router and other devices in the network.

1/ Interface Brought Up: When a router interface is activated, it needs to communicate with devices on the local network.

2/ ARP Request: The router sends an ARP request asking, "Who has this IP address?" The target IP address in this case is the router's own interface IP that was just brought up.

3/ MAC Address Resolution: The connected switch receives the ARP request. If the switch doesn't know the MAC address associated with the requested IP, it broadcasts the ARP request to all devices on the local network.

4/ Router Responds: The router, being the owner of the IP address in question, responds to the ARP request with its own MAC address.

5/ Switch Updates ARP Cache: The switch now knows the MAC address corresponding to the router's interface IP. This information is cached in the switch's ARP table, allowing it to forward future frames more efficiently.

Note that this process ensures that devices in the local network are aware of the MAC address associated with the router's interface IP, facilitating proper communication within the network. It's a common behavior in networking protocols to dynamically learn and update address mappings to ensure accurate and efficient data transmission.