Re: serial is up line protocol down(looped)

tushar47 · ‎03-01-2005

Dear sir,

I am getting serial is up line protocol down(looped) on leased line of 64 k on 1700 series router, when looped from DOT end. Can anybody tell me, what does this mean.

Pleae help.

Regards,

Ashu

dgahm · ‎03-01-2005

That would be normal for a loop with PPP encapsulation.

If using HDLC you should get line up, protocol up (looped).

nbhandarkar · ‎03-02-2005

The above post by dgahm is correct.

The line protocol on an interface configured with PPP comes up only when all LCP and NCP sessions are negotiated successfully.

LCP keepalive packet option called "Magic number" is greatly useful in determining loops.

When LCP keepalive packets with same magic number is received by the router, the router then knows there is a loop. When it repeatedly get the same magic number in every consequent LCP packet, then it marks the interface as . For more details you can read RFC 1661.

Hence LCP will not be established and hence line protocol will be down.

tushar47 · ‎03-03-2005

Thank you very much sir,

Ashu

nbhandarkar · ‎03-06-2005

Hi Ashu,

If you do think that the answer clarifies your doubt, do VOTE as this not only identifies correct solution to your problems but also motivates others to give you and others prompt and precise answers.

Best Regards.

saimbt · ‎03-01-2005

I faced a similar kinda issue.

All local, remote loops were successful. The problem was found at the remote exchange modem settings.

-Sai.

nbhandarkar · ‎03-01-2005

As far as what I have seen, we get line protocol up when looped at DOT end. LP down shouldn't happen.

What encap are you using on LL? default HDLC or PPP.

saimbt · ‎03-01-2005

we are using PPP, are you getting a line protocol UP when you give a remote loop to your router?

-Sai.

Josef Oduwo · ‎03-01-2005

Ashu,

The sequence number in the keepalive packet changes to a random number when a loop is first detected in the circuit. If the same random number is returned over the line, a loop is confirmed.

You could take these steps highlighted here to find out what the problem is:

1. Use the write terminal privileged EXEC command to display any instances of the loopback command. If the router has been configured with this command, enter the no loopback command to remove the loop.

2. Check to see whether the DSU/CSU is configured in manual loopback mode. If it is, disable manual loopback.

3. Reset the DSU/CSU.

These and other troubleshooting steps for the 1700 series are available here http://www.cisco.com/en/US/products/hw/routers/ps221/products_configuration_guide_chapter09186a008007cd3d.html#31068

Cheers,

Josef.

don.fortes · ‎03-07-2005

It means a loop exist. You are getting the down(looped)

because your interface is most likely configured with the "down-when-looped" statement.