ARP operates at which layer?

yuddhidhtir

Its all complicated in the internet,expecting a simple answer.

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Zartanasaurus

ARP is a helper protocol for layer 3 functionality. Devices that use layer 3 need ARP, devices that work solely at level 2 don't. It's generally considered to be a layer 2.5 protocol. Not everything fits neatly into a theoretical OSI box IE ICMP, ARP, IGMP.

Switches don't use ARP to fulfill their L2 duties, although they do need it since they have management interfaces. Heck switches function all the way up to layer 7. You can TFTP and HTTP to a switch.

Turgon

It is indeed a helper for layer 3 functionality. Switches and bridges will learn what is around them and passing through in terms of MAC address on the local segments.

Zartanasaurus

Turgon wrote: »

Only if it's on that segment. Another device may respond on behalf of host B. Proxy ARP.

Trying to keep it simple here. =p

Turgon

hehehe.. I know and that's fine, but yes many possibilities!

Trifidw

Zartanasaurus wrote: »

When host A does an ARP for Host B's mac, the switch will see the response from Host B.

No, you've misunderstood me.

SwitchA - Router - SwitchB

SwitchA# ping switchB

switchA has yet to hear about switchB so does an ARP request (for which the router would respond on SwitchB's behalf). A L2 switch will populate it's mac address table from the traffic coming into its switchport (doesn't matter if it is ARP or FTP traffic).

Forsaken_GA

thedrama wrote: »

i think you are wrong. what l2 switches do there is learning the MAC address which will arrive to its port from the devices connected to it. No ARP process comes up on l2 switches in this situation. what l2 switches do is receiving the source MAC then
according to the situation if they have a match on their filter table, broadcasting the frame out all their ports except the
port which frame is received or finding the correct match on their filter table and communicating the source and dest.
directly.

The management functions on a layer 2 switch need ARP just like any other host. It doesn't use the ARP table in it's forwarding decisions, but if it's going to ping, telnet, or ssh, it needs all the same tools as any host.

But this is a silly argument that can easily be settled - login to a layer 2 switch (assuming Cisco) and issues the following commands:

show arp
show ip arp

If they work, then the layer 2 switch does in fact use ARP.

CodeBlox

I waited until I got home to say exactly that about the show arp command, forsaken. Looks like you beat me to it

I guess it's a moot point but at least it's been clarified. I only brought it up because one of the post said that L2 switches don't use arp.

Zartanasaurus

CodeBlox wrote: »

I waited until I got home to say exactly that about the show arp command, forsaken. Looks like you beat me to it I guess it's a moot point but at least it's been clarified. I only brought it up because one of the post said that L2 switches don't use arp.

(I think) the debate was over whether switches need ARP to forward frames. They don't. But switches also act as hosts on the network since they have a management interface and they can host services (HTTP/telnet/SSH/SNMP) and act as a client (DNS/TFTP/SNMP trap).

Zartanasaurus

Forsaken_GA wrote: »

show arp
show ip arp

If they work, then the layer 2 switch does in fact use ARP.

And to prove the other side (switches don't need ARP to forward frames), there's only an arp table for the management interface VLAN. There are no ARP entries for the other VLANs it forwards frames for.

CodeBlox

Zartanasaurus wrote: »

(I think) the debate was over whether switches need ARP to forward frames. They don't. But switches also act as hosts on the network since they have a management interface and they can host services (HTTP/telnet/SSH/SNMP) and act as a client (DNS/TFTP/SNMP trap).

Thedrama stated that L2 switches don't use ARP. And as far as ARP goes with switches, I was referring to the management aspect the whole time.

thedrama

Forsaken_GA wrote: »

The management functions on a layer 2 switch need ARP just like any other host. It doesn't use the ARP table in it's forwarding decisions, but if it's going to ping, telnet, or ssh, it needs all the same tools as any host.

But this is a silly argument that can easily be settled - login to a layer 2 switch (assuming Cisco) and issues the following commands:

show arp
show ip arp

If they work, then the layer 2 switch does in fact use ARP.

Yeah i saw that on a sample topology when i assigned an IP address to the VLAN 1. Unless you do that, l2 switches only make determination based on l2 addresses however.

Actually my objection was since without assigning IP addresses, i had noticed that i can't display sh arp and nothing
in case of layer 3. Though, CodeBlox got a nice catch to see.

Thanks CodeBlox.