Why is Layer 2 Switch having ARP Entries For All Host Connected ?

[Mitesh Manwatkar]

Hi,

I have a 2960-S switch with ports in vlan 102.

I have defined a interface vlan 102 (172.19.102.241/24) for switch management purpose such as SSH and a default gateway is 172.19.102.3/24

Since its a layer 2 device it should not look beyond layer 2 info.

Generally it should maintain ARP entry for interface vlan IP and gateway IP.

However the switch maintains the ARP entries for the hosts connected to it in vlan 102.

Why is so?

Regards,

Mitesh Manwatkar.

[InayathUlla Sharieff]

Only the mac address of the PC would only be learn when the packet goes out of the PC.

If someone might have ping to any address from the PC the mac address from the pc would be learn on the port connected to it.

HTH

Regards

Inayath

[Mitesh Manwatkar]

Hi,

PFA below to understand my query.

As you can see the switch is maintaining the mac addresses of the hosts connected to it  along with the IP address.

Regards.

[InayathUlla Sharieff]

Hi,

Thats the expected. Please find example:

Attach is the packet tracer file.

I have switch configured with Vlan 10 with ip address 10.0.0.1/24

Conected 4 PC to the switch configured ip address in the same subnet.

I initiated ping from the PC to default gateway after which the mac and ip is been learned. Hence this is expected.

HTH

Regards

Inayath

[Mitesh Manwatkar]

Hi,

My question is exactly the same.

What is technical reason for the same as switch does learn IP address in packet.

Regards.

[InayathUlla Sharieff]

Hi Mitesh,

Though you mention that this switch is a layer 2. You are doing the routing for vlan 102.
You have configured an IP address and assigned it to a layer 3 vlan interface.
So it will by all ways do the arp resolution for all the ip addresses that fall in that subnet.
why do you feel that it should learn the ip address of its own vlan interface and the default gateway?

This is not a one to one mapping. The default gateway will ocme into picture only when it has to route traffic outside of the local subnet.(in your case vlan 102)
For the local subnet traffic it still has to do an arp resolution and find the respective mac address to route traffic.
Even the traffic that passes between any two devices on the same network will build the ARP table.

If you do not want this to happen. Remove the ip address from the switch and then it will be a pure layer 2 switch.
you can see that only mac address table will be built and no arp entries will be there.

HTH

Inayath

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[Joseph W. Doherty]

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Posting

The switch might be saving ARPs it "hears/sees" (i.e but doesn't, itself, request) - "just in case" in needs them (especially if the management IP is on that subnet).

[nkarthikeyan]

Hi Mitesh,

I got your tricky question. When a host connected in the layer 2 switch traverses or initiates traffic.... the  packet or data will have the complete information of the initiator.... arp is a protocol which converts network address or ip address to mac address... i.e ip address..... rather in simple term we can say ARP is layer 2.5.... which resides in between layer 2 and layer 3.... it just uses the layer 2 encapsulation..... all you can consider here is... layer switch will not do any routing.... apart from sending the packets to the gateway defined....

Moreover ARP is not developed in a OSI framework.

Regards

Karthik