Triggers that toggle Interface is up, line protocol is up

[g.andrus]

I am interconnecting two routers with an ethernet crossover cable.

What are the conditions that toggle the status of the interface and the line protocol.?

[thisisshanky]

ethernet uses keealives. missing keepalives can toggle the line protocol. Unplugging the cable should also bring the line protocol down. Giving a "shutdown" on the interface makes both interface and line protocol down.

[g.andrus]

More information about these messages?

What is their format?

Are they ethernet, ip (icmp,tcp,udp) ?

What are the source and destination (mac addresses or ip addresses?)

[g.andrus]

More information about these messages?

What is their format?

Are they ethernet, ip (icmp,tcp,udp) ?

What are the source and destination (mac addresses or ip addresses?)

[thisisshanky]

Keepalives frames are frames send by router to itself. So souce and destination mac addresses are gonna be the same. This tests the physical link whether its capable of sending frames properly.

If you put a sniffer on ethernet segment containing a cisco router, you will see every 10 seconds a frame looped (src and dst mac are same), like above. The receiving device (switch) usually drops this frame after reception, while it sends its own keepalive.

HTH.