Next-hop resolution and point-to-point

I had this blog post lying around as a draft for a long time. I didn't think it was was "meaty" enough yet, but since I'm no longer a network consultant I don't think it'll become any meatier. So here it goes.

Here I will describe the process of L3-to-L2 mapping, or next-hop resolution and how it works with point-to-point circuits like PPP, ATM and Frame relay. It's the process of finding out what to actually do with a packet once the relevant routing table entry has been identified.

It's deceptively simpler than on a LAN segment, but since people generally learn Ethernet before they learn point-to-point nowadays I'm writing it anyway.

When a packet is to be sent to an address on the same subnet a L3-to-L2 mapping is done to look up the L2 destination address (if any) to apply.

The packet is then encapsulated in a L2 frame and sent out the interface.

On a normal Ethernet LAN segment ARP is used to look up L3-to-L2, and the frame will then have that (L2) MAC address as its destination. The frame will then be received by (and only by) the intended destination.

In a point-to-point interface there Continue reading