Arp table is not populating**

[grahamhyland]

Hi folks

I have a 3560 with many devices connected on 2 subnets/vlans

when I sh arp table there are only 5 ip's listed. There is actually about 20 devices with Ip addresses on the switch.

Strangely the 5 ip's are all in the same vlan and no ip from the second vlan appear, I have increased the timeout and ran an ip scanner but still no luck.

Can anyone help me with this

Graham

[Chad Peterson]

Hi Graham,

Is this switch the device that will do the routing between the two vlans?  I assume it is, but just want to check.

Also try pining from the switch to a device that you don't see in the ARP table, then look at the ARP table.  Depending on the design the switch may not have a reason to ARP for the hosts in the VLAN. 

I'm guessing that an upstream device is routing the traffic out towards the internet, if that is the case this switch would never really need the ARP entries of these hosts, unless devices from one vlan want to talk to the other vlan (And the switch is set to be their default gateway)

Chad

[grahamhyland]

Hi Chad,

No this switch does no routing it acts as an access switch, with a port channel up to the routing 3560.

I have tried the suggestion of pinging devices on the switch and it did not populat, the gateway of both devices are on the routing 3560

The reason 'm trying to populate the arp is so I can run a port mapper. and ID which devices are on what ports, without any entries in the arp table it cant resolve ip from mac

?

Graham

[Chad Peterson · Accepted solution]

Ahh,

So on this switch it doesn't have an IP address in the vlans where the hosts reside? If it doesn't you won't be able to see any ARP entries you are looking for. You should check on the routing 3560 for the ARP information.

Chad

[grahamhyland]

Thats correct,I have no vlan IP address on this switch, I can however see the arp info on the other switch, but I cannot determine the port that the devices are connected to.

Graham

[Chad Peterson · Accepted solution]

Ahhh okay. You'll have to do this in 2 parts then. First look at the ARP table on the routing switch, note the MAC address of the device you are looking for. Then on the other 3560 you will need to look up which port that MAC resides at 'show mac-address address x.x.x.x'

[grahamhyland]

Thanks For that,

I got what I needed from the switch as you suggested, But could you clarify if my understanding of how this works.

The routing switch receives a packet to an IP address

Because the switch is directly connected to the devices switch (access switch) it knows the mac address of the destination.

Drops the IP and forwards packets to the mac address at L2 ?

Graham

[Chad Peterson]

So on any Layer 2 devices, it will only look at the destination MAC address to determine where the frame needs to go. So if your server is sending traffic towards the routing switch (L3core in my pic below), the access switch will see the destination MAC is the L3core. It will forward it out the port that has learned the L3core mac address.

Once the frame hits the L3core its going to look at the destination see that the MAC is for itself...it will now look at the IP destination to see if it knows how to route the packet to there, if it does it routes the packet.

Server