Do you know what would cause a consistently empty ARP table?

Answered Question
Apr 14th, 2010

Hi all.

I have a 3750 switch stack that has had very minimal config changes over the past year.

In an attempt to locate user ports by cross-referencing the ARP and MAC tables I noticed that I only have 2 entries in the ARP table.

Both are for SVIs. However I do not see any ARP entries for roughly 100 devices. This has not always been the case, as I have used this method in the pass to locate devices' switchports. Does any know what would cause this lack of ARP entries? The MAC table is full.

Thanks in advance.

PS...I have not cleared ARP at all.

I have this problem too.
0 votes
Correct Answer by Jon Marshall about 6 years 6 months ago

Is the 3750 stack responsible for inter-vlan routing ?


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Correct Answer
Jon Marshall Wed, 04/14/2010 - 09:37

Is the 3750 stack responsible for inter-vlan routing ?


geotech333 Wed, 04/14/2010 - 10:41

No. It is not.

I have a 6509 with all our SVIs.

But your question led me to check the 6509 and all the ARP entries I am looking for are there.

Still weird that in the past I had not have had to do this.

Thanks for your help!

Shahaludeen N Wed, 04/14/2010 - 12:59

I think you understood what caused the arp entries not in the ARP table of 3750 Switch. If not let me explain.

Had you pinged the specific IP's from the 3750 Switch and if it had got an SVI created for that specific subnet ( I mean if that subnet is a connected one), then your ARP table would have been populated because then the 3750 SW has to create the Layer2 header ( for the ping packet) and it needs to find the mac-address of  the end device. It will look into the ARP table to find the mac-address and if it doesn't find the entry, it will initiate an ARP request and the get the mac-address and completed the layer2 header as well as populating the ARP table. If it doesn't have a connected subnet it will try to route it using the routing table and only the gateway's address you will find it in the ARP table.

Here in your case the 6500 is doing the routing and since it finds the destination IP as one of its connected network, it can send the packet directly to the end device (without taking the help of another gateway) and so it resolves(get the mac-address) the IP address of the end device and sends the packet directly. On the way to the destination all the switches in between just switches the packet because the packet is not destined to them ( the mac-address is not one of their's but of the end device). For switching it just needs to know through which port that destination can be reached, and it will check the mac-address-table. If it doesn't find the entry it will flood it to all ports in that vlan and as and when the first packet comes from the destination enters the switch it will populate the mac-address-table.

So as long as there is constant communication the mac-address table will be kept populated but the ARP table need not be. In the past you might have initiated some packets from that switch ( assuming it has a connected subnet) for testing purpose or so and then have checked the ARP table.




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