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Question on ARP table on 3750 switch

Hi,

So I have a cisco 3750 switch directly connected to a 2851 router gig 0/0 interface.

Should the show arp command on the switch show an entry for the IP and mac address of the routers gig 0/0 interface?

I dont see one in there now and was just wondering.

Connectivity between the switch and router work fine right now.

thanks

2 ACCEPTED SOLUTIONS

Accepted Solutions
VIP Purple

Re: Question on ARP table on 3750 switch

HI Bill,

if u can ping from Swicth ro router,,,,then please check with this command: sh ip arp.

Just for Info;

To be able to ARP a device, you must have an interface (SVI) on that switch on that network. To ARP a device you must have an interface with a layer 3 IP on the same layer 2 vlan.

You will need to connect to a switch that has an interface on that vlan on it. I would traceroute to the host, and hopefully the next to last hop is the layer 3 switch, or if it's a router, connect to it and do a show cdp neighbors and see if you can find the switch that way. (If you do connect to the router, you'll find the ARP entries there, if it's a layer 3 switch, then it's both a router and switch in one box)

"sh mac address-table". This will give which MAC is connected to which port.

"sh ip device tracking interface gigabitEthernet ". This will give which IP is connectd to a port.

"sh ip arp" will give you a IP to MAC table

Regards

Dont forget to rate helpful posts.

Hall of Fame Super Blue

Re: Question on ARP table on 3750 switch

Bill

Depends if the switch is acting as L2 device or a L3 device. If it is L3 then you should see an entry for routers gi0/0 IP address. If it is L2 then -

1) to ping from the actual switch it needs a L3 SVI for management. But if the L3 SVI is not in the same subnet as the routers gi0/0 IP address you will not see an arp entry for the router

2) if you are simply connecting from clients connected to the switch you will not see an arp entry either.

Jon

2 REPLIES
VIP Purple

Re: Question on ARP table on 3750 switch

HI Bill,

if u can ping from Swicth ro router,,,,then please check with this command: sh ip arp.

Just for Info;

To be able to ARP a device, you must have an interface (SVI) on that switch on that network. To ARP a device you must have an interface with a layer 3 IP on the same layer 2 vlan.

You will need to connect to a switch that has an interface on that vlan on it. I would traceroute to the host, and hopefully the next to last hop is the layer 3 switch, or if it's a router, connect to it and do a show cdp neighbors and see if you can find the switch that way. (If you do connect to the router, you'll find the ARP entries there, if it's a layer 3 switch, then it's both a router and switch in one box)

"sh mac address-table". This will give which MAC is connected to which port.

"sh ip device tracking interface gigabitEthernet ". This will give which IP is connectd to a port.

"sh ip arp" will give you a IP to MAC table

Regards

Dont forget to rate helpful posts.

Hall of Fame Super Blue

Re: Question on ARP table on 3750 switch

Bill

Depends if the switch is acting as L2 device or a L3 device. If it is L3 then you should see an entry for routers gi0/0 IP address. If it is L2 then -

1) to ping from the actual switch it needs a L3 SVI for management. But if the L3 SVI is not in the same subnet as the routers gi0/0 IP address you will not see an arp entry for the router

2) if you are simply connecting from clients connected to the switch you will not see an arp entry either.

Jon

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