I've been having a recurring problem with a flapping link on an 881 router at one of our remote sites. The router is connected to a Century Link (Technicolor) DSL modem. About a month ago we started having a problem where the interface connected to the modem started flapping--the log showed the interface dropping/re-connecting and the interface counter showed lost carriers. The ISP came out and replaced the modem, but that didn't solve the problem. I replaced the router and the cable to the modem, but the problem is continuing. The odd thing is that the log shows the interface coming up, but doesn't show it drop:
Apr 17 17:05:23: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
Apr 17 19:04:34: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
Apr 17 20:19:47: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
Apr 17 20:20:28: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
.Apr 17 21:10:30: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
Apr 17 23:20:45: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
Apr 17 23:21:25: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
Apr 18 04:53:00: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
Apr 18 04:53:40: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
Apr 18 05:04:20: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
Apr 18 06:08:43: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
.Apr 18 06:32:14: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
Apr 18 11:25:27: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
Apr 18 15:59:29: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
Apr 18 16:00:59: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
Apr 18 17:34:03: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
Apr 19 02:37:06: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
Apr 19 08:08:20: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
.Apr 19 17:40:11: %LINK-3-UPDOWN: Interface FastEthernet0, changed state to up
.Apr 19 17:40:37: %LINK-3-UPDOWN: Interface FastEthernet0, changed state to up
Apr 19 17:44:56: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
Apr 19 22:57:50: %LINK-3-UPDOWN: Interface FastEthernet4, changed state to up
Interface is configured for auto speed and duplex, and shows multiple lost carrier:
FastEthernet4 is up, line protocol is up
Hardware is PQII_PRO_UEC, address is 1cdf.0f6d.5566 (bia 1cdf.0f6d.5566)
Internet address is
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 100Mb/s, 100BaseTX/FX
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters 2d16h
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 1000 bits/sec, 1 packets/sec
5 minute output rate 1000 bits/sec, 1 packets/sec
11938285 packets input, 3032155040 bytes
Received 182963 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog
0 input packets with dribble condition detected
4974302 packets output, 582701732 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 unknown protocol drops
0 babbles, 0 late collision, 0 deferred
21 lost carrier, 0 no carrier
0 output buffer failures, 0 output buffers swapped out
Beyond changing out the modem again, which the ISP probably won't do, I'm out of things to check. The connection will drop for anywhere from a few minutes to an hour, during which time the DSL modem stays up (our manager at the site can connect to the modem's wifi and have internet access while our router is down).
