Category: ARP
Generating layer-2 broadcast from a regular IP packet
The Wake-on-LAN discussion we had a while ago brought us nowhere; there's simply no way to generate UDP packets on the router. I thought I could use Application Performance Monitor's Tcl scripts to generate the packet, but it looks like APM has been removed from recent IOS releases (and it's not clear whether you can use APM without a peer router).
The discussion nonetheless had an interesting side effect. Robert Turnšek sent me an interesting trick: with static ARP you can generate layer-2 broadcasts with a layer-3 unicast packet.
ARP timeout resolution is implemented in minutes
ARP entries are periodically refreshed if you use CEF switching
In a previous post I've been writing about the inability to clean the ARP cache due to cached CEF adjacencies. As it turns out, this behavior has another side effect: the router will automatically refresh all ARP entries (and CEF adjacencies) as they expire from the ARP cache. This might become a problem on high-end devices with a lot of directly connected hosts if you set the arp timeout to a low value.
What is a cached CEF adjacency?
Whenever a router running CEF switching has LAN interfaces (or any other multi-access interfaces), you'll find cached adjacencies for active directly attached IP neighbors in its CEF table. These adjacencies ensure the smooth traffic flow toward the LAN-attached next-hops (preventing the initial packet drop symptom once the next-hop becomes active).