Difficult troubleshooting in a Cisco 7507 IOS 11.1(33)CC1

[afranco]

Hi,

we are having a serious problem with our Cisco 7507, IOS 11.1(33)CC1. This router is connected to the Internet through three carriers, which correspond to three serial interfaces in the router: 5/1/0, 6/0/0 and 6/0/1

The problem: router is suffering flapping on these serial interfaces. For example,

Oct 23 02:29:06.348: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial6/0/1, changed state to down

Oct 23 02:29:07.348: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial6/0/1, changed state to up

Oct 23 02:36:05.844: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial6/0/1, changed state to down

Oct 23 02:36:06.844: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial6/0/1, changed state to up

As you can see these little "breaks" have a duration around one second. Flapping is suffered in the three serial interfaces.

We have checked that is not a Port-Adaptor problem. Neither a comunication problem as it could seem (the three serial interfaces use three independent circuits). So, if the router (not the VIPs, not the links) is generating the problem, what kink of things I can do to troubleshoot and demostrate that the problem is in the router and not in the links?.

Do you know about some similar cases?.

Note: the three interfaces have CEF and dCEF activated.

[nf289]

1. Have you done a "show interface s6/0/1"? If so, does it show a high error rate on anything (CRC, etc)?

2. Is this limited to the one interface or all three?

3. Is this occuring all the time, or does it run okay for a period of time, then start flapping again?

I have had a "flapping" situation in several different circumstances, but most involve carrier / line problems, not the router. The most common lately has been timing issues on the circuit - the carrier is supposed to be providing the clock, but is not providing a true, stable clock, so the circuit runs okay until it gets so far out of synch it causes the interface to flap, then drop completely. This usually shows up on the "sho interf" command as high CRC error.

Hope this helps.

[mwinter]

Hello again,

I guess layer 2 protocol is HDLC. Have you let checked the CSU/DSU respective an "end-to-end-check" of the line (if the peer is not located at the same facility).

Try debugging the keepalive messages "debug serial interface" . Here you can see are the hello packets sent and received properly. Maybe it is helpful to get in contact with the provider to check and observe the events on both sides simultaneously.

We also experienced failures caused by external influences, e.g. high frequent equipment.

As mentioned in the first answer already I just can agree most times it is a problem of the line.