Serial is up, line protocol is up (looped)

[limtohsoon]

Hi Sir,

I have a simple question here.

The problem state "Serial x is up, line protocol is up (looped)" in the interface status line of the "show interfaces serial x" indicates a loop exists in the circuit. Does the line protocol stays up when loop is detected regardless of the link's encapsulation, i.e HDLC or PPP?

Thank you.

B.Rgds,

Lim TS

[dgahm · Accepted solution]

A normal loopback with HDLC will be protocol up (looped), and PPP will be protocol down (looped).

[limtohsoon]

Hi David,

Thanks for the info.

If I want the line protocol to be down when loop exists in the circuit, my only option is to configure encapsulation ppp?

For PPP, the default keepalive interval is 10 seconds. If I'm not mistaken, the line protocol goes down only when the router misses 5 consecutive keepalives. Thus, if one end of the link goes down, the router at the other end takes approx. 50 seconds to detect it. If it's true, to speed it up, shall I configure a shorter keepalive interval?

Do correct me if I'm wrong.

Thank you.

B.Rgds,

Lim TS

[dgahm]

This might achieve what you want:

interface sX/X

down-when-looped

[limtohsoon]

Hi David,

I've checked out the command. It's valid for HDLC or PPP encapsulation on serial and HSSI interfaces. Excellent, that's exactly what I want.

Do you mind to share your thoughts on my earlier question about a router missing five consecutive PPP keepalives before bringing the line down?

Thank you.

B.Rgds,

Lim TS

[dgahm]

Yes, protocol down on 5 missed keepalives is pretty slow, but be careful with tuning keepalives or you might end up with a lot of flapping. An option that is not often used is link quality monitoring, where you set a percentage of successful traffic needed to keep the link up. http://www.faqs.org/rfcs/rfc1989.html

interface sX/X

ppp quality 99

This turns on periodic link quality report packets, and would bring the link down if less than 99% successful. It comes closer to evaluating how the link is doing with real data as opposed to small keepalive packets.

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[maher]

Hi dgahm,

I was wondering too. At my place, we I do loop with telco, especially customers running on encap ppp, i will get up down (looped). Is it because of the telco equiptment or something else? I need to remove encap ppp then I'll get nice up up (looped).

Any articles/documentation on this.

thanks.

regards,

maher