ARP & Ping

[snarayanaraju]

Hi Friends,

I am facing an issue which is very strange to me. The Goal is to find out the Interface/port  of the switch named SW-1 where the Host is connected.

I have a host connected to one of the access layer switch SW-1. I am able to ping the host using its IP address 10.2.48.1 and using "show arp" / "show mac address" command from any of the distribution switch I am getting the MAC address of the host as  c0-62-6b-e3-5d-40 to uplink port connecting to the access layer switch SW-1

Login into the particular SW-1 switch, I am able to ping IP address 10.2.48.1. But whereas I am not able to see its mac address using the commands "show arp" / "show mac address". No entries is found for this IP address /MAC address

thanks in advance

sairam

[The_guroo_2]

Its a layer 2 swicth which doesnt have ip to mac mapping

[snarayanaraju]

Hi Thanks for your time.

I am seeing the IP to MAC mapping in any 2 switch

SW-1#sh arp

Protocol  Address               Age (min)  Hardware Addr   Type   Interface

Internet  10.206.11.126        175         001c.f9cd.4e41    ARPA   Vlan2

Internet  10.206.8.1              3            0050.5680.4813   ARPA   Vlan2

Internet  10.206.11.2             -            04c5.a4d7.f6c1    ARPA   Vlan2

[snarayanaraju]

Hi,

Can anybody help me on this.

[milan.kulik]

Hi,

I suppose your L2 switch management address is within another subnet (10.206.x.x?)?

So you will never see an ARP cache entry for a host 10.2.48.1 on the switch.

You will see only an ARP entry for the default gateway you are uisng to reach all other subnets from you management subnet.

But id you know the MAC address assigned to 10.2.48.1 from other L3 device ARP cache, you should be able to find a port on the L2 switch to which a device with this MAC address is connected to.

HTH,

Milan

[Lee Smitherman]

If you ping the IP address from any other subnet than the subnet that has the host i.e 10.2.48.1, your only going to get the MAC address of the next hop Device.  You need to either jump on a server in the 10.2.48 subnet, ping 10.2.48.1 and from a dos prompt do a "Arp -a" OR jump on the Layer 3 device that routes to the 10.2.48 Subnet(has an interface in that very subnet), perform a ping and "show ip arp". Once you have confirmed thats the correct host mac, jump onto the switch and look at the mac address table there.

Lee.

[snarayanaraju]

Thanks all for your time.

As mentioned earlier, my intention is to find out the port in which the host is connected. I know the IP address & MAC address from the uplink switches connected to the said access switch SW-1

SW-1 is a 6509 switch working in L3 mode. By issuing the command "show mac address" i am not seeing the entry for the MAC address of the host. neither "show arp" shows the entry. However I am able to ping the IP Address.

Is this a known behaviour.

Your comments are highly appreciated

[milan.kulik]

Hi,

is the IP subnet to which the address 10.2.48.1 configured on the SW-1 on some L3 (SVI probably) interface?

If not, your L3 switch might be routing some subnets but having some other VLANs configured as L2 only.

I believe it might be your case.

HTH,

Milan

[nisagar]

Hi,

Please add static arp entry on both DS:

arp 10.2.48.1 c062.6be3.5d40 arpa

And then issue those commands sh arp & sh mac address to see from what trunk port the DS is learning about this mac address.

It should provide the interface of trunk if its able to ping the ip address of the machine.