PPP vs HDLC

[tbekdemir]

hi there,

While doing some loop tests on serial interfaces in Cisco router, I noticed ;

When the encapsulation is HDLC,the output of show interface is

Serial3/0 is up, line protocol is up (looped)

On the other hand,When the encapsulation is PPP,the output of show interface is

Serial3/0 is up, line protocol is down (looped) .

What do you think about the reason(s) for being DOWN while it is PPP ?

Also,in CCO, I read it must be HDLC while doing loop tests.But I did not find any information about why it is so.

best regards..

Tahsin.

Network Engineer.

[xavierchang]

Can't do loopback testing with PPP, only HDLC.

The reason is simple. For a PPP interface to be up, PPP negotiation must be completed. When you loopback a PPP interface, you are essentially telling it to negotiate with itself, and you can't do that.