ARP Broadcast done by Router?

Quick question.

When an host is on the same subnet als the destination host it's trying to reach, it know this by looking at the subnetmask. It will then send an ARP broadcast asking for the MAC address of the Destination. This is done by broadcast.

When the destination host is not in the same subnet, the host wil send an ARP to the Gateway. But wath is the difference??? It will still sends an ARP Broadcast, so the amount of traffic/broadcast genereated will still be te same right?? How is this ARP message different from when it send traffic locally? The souce will still send a broadcast to the gateway, so the amount of broadcast is still the same, isn't it?

And one step further. If the destination is not in the same subnet, the packet will arrive at the Router, but when the Router wil send the packet to the next hop, how does it get the MAC address of the next hop. Will it also send a Broadcast ARP to the destination it's trying to reach?? I think so, but need to be sure.

Also are there reason for a Router to generate boadcast?

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Comments

NetworkVeteran

FrankGuthrie wrote: »

It will then send an ARP broadcast asking for the MAC address of the Destination. This is done by broadcast. When the destination host is not in the same subnet, the host wil send an ARP to the Gateway. But wath is the difference??? It will still sends an ARP Broadcast, so the amount of traffic/broadcast genereated will still be te same right??

Nope. If host 10.1.1.3/24 wants to talk to hosts 10.1.1.4, 10.1.1.5, and 10.1.1.6.. that requires 3 ARPs, one to each address. If host 10.1.1.3 wants to talk to hosts 10.10.10.4, 10.10.10.5, and 10.10.10.6.. that requires 1 ARP to the default gateway 10.1.1.1. Quite different, the number of ARPs needed.

networker050184

If the host needs to send something off the subnet it does not need to arp for it. It will send the packet to it's default gateway with the destination MAC of the gateway and a destination IP of the remote host. It does not need to resolve the L2 address of the remote host.

Edit: Beat me to it!

FrankGuthrie

@NetworkVeteran
Good point about the decrease in broadcast traffic, but is the actual ARP broadcast message different (in format)? Don't think so, but just need confirmation.

Also, what about the Router? Is it basically the same (ARP) process? So does the Router bascially do the same as a PC to get the Layer 2 address to create the packet?

@networker050184
But the host still needs to get the MAC adress, of the gateway, by the means of ARP?

NetworkVeteran

If you're trying to wrap your head around ARP, this is a good resource--

The TCP/IP Guide - TCP/IP Address Resolution Protocol (ARP)

ARP over Ethernet should always have the same PDU format, although the values in specific fields may vary.

networker050184

Yes it will need ARP for the default gateway, but only once (or until it times out) compared to ARPing for every remote address it wants to reach. That is a very large reduction in ARP traffic.

What you are thinking of is the main reason NOT to place a static route to an outgoing multi-access interface or you end up with all that ARP traffic for no reason.

atorven

@FrankGuthrie - It's the same ARP process, but the router is ARPing for the MAC address of the next hop to send the packet. And, yes, the host will need the MAC address of the gateway which is acquired through ARP.

xXErebuS

networker050184 wrote: »

Yes it will need ARP for the default gateway, but only once (or until it times out) compared to ARPing for every remote address it wants to reach. That is a very large reduction in ARP traffic.

What you are thinking of is the main reason NOT to place a static route to an outgoing multi-access interface or you end up with all that ARP traffic for no reason.

Has anyone ever seen where someone specified a default route with an outgoing interface?

I could see where if it was a subnet it would be somewhat ok (as long as proxy arp is enabled it probably wouldnt crash the router / fill CAM).

networker050184

Yep I've seen it quite a few times. Especially people setting up home routers. Memory can quickly be eaten up by large arp tables. It's basically the difference between needing one arp entry or an entry for every destination on the internet you want to reach.