Catalyst 9200 arp request question

[jason-shirley]

I have a basic question about arp.  Lets say there are two networks, A and B, and a layer 3 switch is performing the routing between them. Host A is 10.1.1.1/16 on network A and host B is 10.2.1.1/16 on network B.  Lets say host A knows its gateway's IP and MAC address. So if I ping host B from host A, and host B's IP address is not in the switch's arp table, then wouldn't the switch have to arp for host B's MAC address?  Would have to send a broadcast out all of its ports?

[Asemmoqbel · Accepted solution]

Hi,

Basically the Switch when it receives the frame from Host A (Src:10.1.1.1, Dst:10.2..1.1)

will check its arp table against the frame and do the following.

1- the switch will store arp entry for host A if it doesn't have one bound to the vlan or interface it received the frame from.

2- the switch will forward the frame to host B mac address if there's arp entry and if not, it will send arp broadcast out of all ports asking (who has 10.2.1.1, gimme your mac address) except the port it received the frame from. 

I hope this clear to you.

Please rate if you find this helpful.

[jason-shirley]

That was extremely helpful, thank you.

[jason-shirley]

This advice led me to solve my network problem today. Thanks again.

[Asemmoqbel]

You are welcome and I am glad that you solved the problem.

Best regards ,,

Asem

[Giuseppe Larosa]

Hello @jason-shirley ,

a L3 switch uses mutiple VLANs to accomodate different IP subnets and usually with a one to one corrispondence .

IF the L3 switch does not know the MAC address of Host B it will send an ARP request out all ports in Vlan 20 that is associated to IP subnet 10.2.0.0/16 (this is just an example)

It does not need to send the broadcast ARP request out of all ports in the switch only of those ports members of VLAN 20

Hope to help

Giuseppe