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Community Member

There is something wrong with ARP table!!!

Here is my network diagram...

Server----3750---3560---2950---Host1

When i check arp table on 3750, i saw Host1 MAC address... I think i should see 3560 MAC address, right?

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Hall of Fame Super Gold

Re: There is something wrong with ARP table!!!

Routers and other layer 3 forwarding devices do rewrite the MAC address in the header. Switches and layer 2 devices forward frames without changing the MAC addresses.

HTH

Rick

6 REPLIES
Community Member

Re: There is something wrong with ARP table!!!

Wrong, you would see the mac adresses of server and host one if you send a ping between them.

Re: There is something wrong with ARP table!!!

If the 3750 have L3 interface (interface vlan) that is in the same subnet as host1, it is normal that you will see it's mac address in 3750's arp table. The router or L3 device will always have arp entry for the hosts in the subnet that routre is default gateway for or the router have interface for that subnet.

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Community Member

Re: There is something wrong with ARP table!!!

Can you guys correct me?

WHen i ping to server, the host put its mac address to ping packet.. When this packet arrives to the first switch which 2950, switch will change the source MAC address as the port mac address which is connected to 3560 and so on... So when this packet arrives to 3750, how does 3750 knows the host MAC address... As i know, every switch and router change MAC address...

Community Member

Re: There is something wrong with ARP table!!!

Im really stupid...:) The switches are on the Same VLAN... So it is normal to see them...

Thank you guys again...

Hall of Fame Super Gold

Re: There is something wrong with ARP table!!!

Routers and other layer 3 forwarding devices do rewrite the MAC address in the header. Switches and layer 2 devices forward frames without changing the MAC addresses.

HTH

Rick

Community Member

Re: There is something wrong with ARP table!!!

Depends if it is layer 3 of layer 2. When speaking strictly layer 2, the mac address of the host 1 distributed across the layer 2 network. Only at layer 3 does a packet require a rewrite. If this design you have here, is one flat Vlan, or that host resides in a Vlan on the 3750, that it would still be considered in the same L2 network, in which you will see the Mac address of the end device.

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