I work for a service provider and I have a customer that is complaining of drops and CRC errors on his T1, but only when he's pushing traffic across it. The T1 has been stress tested repeatedly and is clean. When the customer tries to FTP a file from one location to the site in question, he sees CRC errors on his interface as well as overruns. My thought is with the overruns, he's over utilizing the T1, but will the overruns also show up as CRCs on his interface. See his snapshot below:
ip address x
no ip unreachables
no ip proxy-arp
service-module t1 cablelength short 110ft
service-module t1 timeslots 1-24
no cdp enable
Serial0/0/0 is up, line protocol is up
Hardware is GT96K with integrated T1 CSU/DSU
Internet address is x
MTU 1500 bytes, BW 1536 Kbit/sec, DLY 20000 usec,
reliability 255/255, txload 16/255, rxload 4/255
Encapsulation PPP, LCP Open
Open: IPCP, loopback not set
Keepalive set (10 sec)
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters 01:54:36
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 2367
Queueing strategy: fifo
Output queue: 0/40 (size/max)
30 second input rate 27000 bits/sec, 18 packets/sec
30 second output rate 99000 bits/sec, 18 packets/sec
Hi Tater, CRC is kind of a catchall for Cisco. I have seen in some cases links become so saturated that it does cause CRC errors. But if that customer is sending only 99000 bits a second that's like .09 megabit while a T1 is 1.544 megabit. Overrun and Output drop is usually indicative of over utilization but given the amount of traffic on the T1 I may be guessing but you do you have a policy map?
Another good indication of an actual physical issue is going to be the reliability scale 255/255. Repeatedly run a command like
show inter ser0/0/0 | inc (CRC)|(reliability)|input)
Do this many times over for about 5-10 minutes and see if there is a fluctuation in the reliability. I guess the real question is, is there an actual service impact? If it, then I don't think it matters. I know on my network, almost every ATM connection we have be it T1, DS3 or sonet, they all have CRC errors and a lot of time slowly incrementing but it's not any real issue.
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