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ARP table on 3550

We have a vlan creative in a catalyst 3550.

To find all that ip in arp table int that switch, I use ping sweep the whole ip segement of the vlan.

unfornately, I dont get all the ip,

what is the reason?


Re: ARP table on 3550

Waht do you mean you don't get all the ip? Do you mean some ip address did not respond? Would it be possible that those devices are not up? And in addition, you created vlan in 3550, what does that involve? did you create vlan in the vlan database? or did you create vlan, you meant you create interface vlan? did the interface vlan have an ip address? Maybe my questions will give you the reason.

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Community Member

Re: ARP table on 3550

Hi there,

Layer 2 switches don't keep an arp table of IP addresses to MAC addresses for traffic not destined for its management interface. They don't need to, since IP is a layer 3 function. What you will see is a mapping of MAC addresses to switchports. To see this, type:

show mac-address-table

This will give you the listing of MAC-to-port mappings.

You might want to look at the ARP table on your local PC in order to get this information, if the PC you are doing the pingsweep from is on the same subnet. The command for this on a Windows PC is:

ping -a

If you're on a remote subnet, you'll want to look at the arp cache on the router acting as the gateway for the subnet you're scanning.

Hope that helps,


Re: ARP table on 3550

I disagree. L2 keeps an ARP table, just like a PC would keep an ARP table. PC is not an L3 device, but it has an IP stack. Switches have an IP stack as well, otherwise, what will it use to ping. You can do a ping sweep (ping -s on the subnet the interface of Catalyst running CatOS the SC0 belongs to and you see an ARP entry for all that responds. I used CatOS as an example since that is an L2 for sure. However, on a catalyst running IOS from 2950 on up, they will have an ARP table. See below for the ARP table, it's modified a little bit to hid ip address but it's a true ARP table of an L2 device nonetheless:

sc0: flags=63

vlan 1 inet x.x.x.44 netmask broadcast x.x.x.63

CatOS> (enable) sh arp

ARP Aging time = 1200 sec

+ - Permanent Arp Entries

* - Static Arp Entries

x.x.x.43 at xx-xx-xx-xx-58-00 port 3/1 on vlan 1

x.x.x.41 at xx-xx-xx-xx-e8-80 port 3/1 on vlan 1

x.x.x.33 at xx-xx-xx-xx-93-fc port 3/1 on vlan 1

x.x.x.38 at xx-xx-xx-xx-30-c0 port 3/1 on vlan 1

x.x.x.59 at xx-xx-xx-xx-a9-c8 port 3/1 on vlan 1

x.x.x.58 at xx-xx-xx-xx-fd-d0 port 3/1 on vlan 1

x.x.x.57 at xx-xx-xx-xx-24-60 port 3/1 on vlan 1

x.x.x.56 at xx-xx-xx-xx-56-e0 port 3/1 on vlan 1

x.x.x.54 at xx-xx-xx-xx-a0-00 port 3/1 on vlan 1

CatOS> (enable)

Re: ARP table on 3550


I think that he meant that the ARP table is not used unless needed by the L2 switch IOS based itself.

So, uplinks IPS, hsrp IPs are usually on the ARP table.

but unless you ping the connected hosts from the L2 switch , their IP wont be on the table.

L2 switch do need an ARP table to route packets source from itself, but not for traffic from hosts connected on its ports that is not going to the L2 itself.


Community Member

Re: ARP table on 3550


Yes, that's what I was getting at, thanks for clarifying. I may have been a little unclear in stating my original reply.

The arp table on a layer 2 switch will only have entries for traffic originating from the management interface on the device. Therefore, if you were to manually ping a device from the switch, it would have the arp entries in its arp table.

However, if you ping host "A" from host "B", and both are on the same subnet and connected to the switch on the same vlan, host "A"'s arp entry will not be on the switch, because the switch doesn't care about A's IP address, only its MAC address to port number mapping, which is in the CAM table of the switch.

Hope that clears up my previous entry.


Community Member

Re: ARP table on 3550

bosalaza, or whoever interested:

Please ignor the last post of mine. I here show all the detail and what my questions are:

1. Pinged from out side the vlan. no update on arp table.

2. Pinged individual ips from switch, the same vlan, arp update .

3. Pinged from other pc in the same vlan, no arp updated.

4. nowhere, including on the switch, can get the arp updated when I pinged the broadcast address. when i ping outside to the broadcase add, got timeout.

My question is--what is the ARP table used for? for me the ip field is not use.



Community Member

Re: ARP table on 3550


1. This is correct. In this case, the arp table updates would be on whatever device you are using to route between the vlans.

2. This is also correct, Since you're pinging from the switch, the switch updates its own arp table.

3. The switch will not update its arp table in this case, but the PC's arp table will update.

4. Ping acts weird when pinging the broadcast address, sometimes it works sometimes it doesn't. Also, broadcasts don't go across a router, so pinging the broadcast from outside the subnet (vlan) would timeout, as you experienced.

An ARP table is a listing of Layer 2 (MAC) address to Layer 3 (IP) address mappings. When you ping an IP address from a PC, the PC performs an ARP to find out the MAC address for the IP it's pinging. Once it gets that mapping, it puts the mapping in its arp table, local to that PC. The next time you contact that IP, it just uses the arp table instead of sending an arp broadcast.

When you ping from the switch, it updates its own arp table, because it needs to know the layer 3 mapping in this instance. When you aren't pinging from the switch, the switch is acting strictly on Layer 2, which means it just needs to know MAC addresses, not IP addresses. That's why the switch doesn't update its ARP table in this case.

Hope that helps,


Community Member

Re: ARP table on 3550

thank you!

Your post confirm my knowledge about arp in my points.

However, when I ping from the same Vlan, the IP appears after.

If i ping from a different Vlan, the pinged IP wont show up.

Does it mean that the arp table only used for local traffic--the traffic in the same vlan on the switch?

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