How to see mac-ip correspondence

Unanswered Question
Nov 29th, 2007

I need to know what IP address have a pc knowing it MAC address.

I know in what switch it is in my net but I type sh arp but it doesn't appear this MAC and its associated IP address.

Can anybody say me how I can see it?.

It is a Cisco Catalyst 3750 with 12.2(25) IOS version.

Thanks.

I have this problem too.
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MUHAMMAD SHAHEEN Thu, 11/29/2007 - 04:52

show arp as well as show ip arp should give you this information. I have same version as your switch and it gives me "protocol, ip address, age(min), hardware address, type (L2 address protocol), and VLAN interface".

to get the IP addresses your switch should be running L3 routing as L2 switches like 2950's cannot give you this info....

show mac-address dynamic should give you VLAN, Mac-Address, Type and Port

HTH

Shaheen

mirtenmous Thu, 11/29/2007 - 05:15

With sh ip arp and sh arp output is the same so mac address that I am looking for doesn't appear yet.

MUHAMMAD SHAHEEN Thu, 11/29/2007 - 05:23

Please check the following items...

is your C3750 running Vlans ?

is you PC connected to this 3750's switch port ?

is that pc connected above have an IP address from one of the VLAN you have SVI on this Switch ?

Regards

Shaheen

mirtenmous Thu, 11/29/2007 - 07:14

This pc is connected to this switch and I have three or four vlan I don't remember well.

I don't what is SVI

Kevin Dorrell Thu, 11/29/2007 - 05:30

The answer is that you cannot do it on a switch that is operating at layer-2, unless the management of that switch is in the same VLAN as the PC you are interested in.

The best way to find out is to go to the router or layer-3 switch that is routing this VLAN. You should find the PC in the ARP cache there (provided the PC has sent any packets to outside its VLAN in the past 4 hours). OTOH, if the PC is not talking to anything outside its own VLAN, then you are out of luck.

Here is another trick: from the router, (or from the switch if it has management in the PC's VLAN) ping the subnet directed-broadcast address, e.g. 192.168.42.255. If the PC responds, then it will have been caught in the ARP cache of the route.

Kevin Dorrell

Luxembourg

mirtenmous Thu, 11/29/2007 - 07:17

This pc is not in the same VLAN that management VLAN so I must to investigate what router in my net is routing this VLAN because I don't know it now.

Thanks.

MUHAMMAD SHAHEEN Thu, 11/29/2007 - 08:11

If you have access to that PC then do IPconfig/all and that shoud tell you the default gateway which will be the router/SVI interface??

HTH

Shaheen

hobbe Thu, 11/29/2007 - 08:19

Well the real answer is basically you cant.

Not with 100% certainty

The reasons for this are as follows.

1) a mac-address doesnt need to have an ip

2) a mac-address can have several ip's.

In general what you can do to find out what it is is to follow the cabeling.

Find out wich port the mac address is attached to.

sh mac address | include the_mac_address

or if you have cdp on

traceroute mac macaddress_1 macaddress_2

and last but not least there are some nasty bugs in 12.2(25) it might be wise to upgrade

Good luck

mirtenmous Fri, 11/30/2007 - 03:30

I have the port that it is connected this pc.

I think I must will go to rack and see it.

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