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New Member

Line Protocol Flapping

Hi all,

been having an ongoing issue with a particular site having one of the devices line protocol flapping.

I work for a Wireless ISP and the device impacted is the back haul radio into the site. I have swapped from a 2950T to a 3550. Replaced the radio, PoE, patch lead. We have re ran the cable up the tower using shielded outdoor cable with a drain wire which has been earthed to an earthing block in the cabinet.

We have other devices on the tower not experiencing the issue. We suspect cable interference however am now at a loss to diagnose further. If anyone can suggest further troubleshooting at the switch level it would be appreciated.

Please see diagnostics so far:

Aug 28 17:37:09.563 AEST: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/7, changed state to up

Aug 28 18:22:40.563 AEST: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/7, changed state to down

Aug 28 18:22:42.567 AEST: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/7, changed state to up

Aug 28 18:45:15.923 AEST: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/7, changed state to down

Aug 28 18:45:17.927 AEST: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/7, changed state to up

QLD-SBK-3550#show int fa0/7

FastEthernet0/7 is up, line protocol is up (connected)

  Hardware is Fast Ethernet, address is 000e.83f9.3287 (bia 000e.83f9.3287)

  Description: RM5+RD30 to HQ Building

  MTU 1546 bytes, BW 100000 Kbit, DLY 100 usec,

     reliability 255/255, txload 2/255, rxload 4/255

  Encapsulation ARPA, loopback not set

  Keepalive not set

  Full-duplex, 100Mb/s, media type is 10/100BaseTX

  input flow-control is off, output flow-control is unsupported

  ARP type: ARPA, ARP Timeout 04:00:00

  Last input 00:00:15, output 01:43:38, output hang never

  Last clearing of "show interface" counters never

  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 1745000 bits/sec, 421 packets/sec

  5 minute output rate 1107000 bits/sec, 409 packets/sec

     1132370184 packets input, 4139538864 bytes, 0 no buffer

     Received 9667975 broadcasts (0 multicasts)

     0 runts, 0 giants, 0 throttles

     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

     0 watchdog, 2117081 multicast, 0 pause input

     0 input packets with dribble condition detected

     1068411398 packets output, 3034028216 bytes, 0 underruns

     0 output errors, 0 collisions, 1 interface resets

     0 babbles, 0 late collision, 0 deferred

     0 lost carrier, 0 no carrier, 0 PAUSE output

     0 output buffer failures, 0 output buffers swapped out

QLD-SBK-3550#show int fa0/7 counters

Port            InOctets   InUcastPkts   InMcastPkts   InBcastPkts

Fa0/7       674154638736    1122703246       2117081       7550914

Port           OutOctets  OutUcastPkts  OutMcastPkts  OutBcastPkts

Fa0/7       402466413143    1067069897        116175       1226488

QLD-SBK-3550#show int fa0/7 counters error

Port        Align-Err    FCS-Err   Xmit-Err    Rcv-Err UnderSize

Fa0/7               0          0          0          4         0

Port      Single-Col Multi-Col  Late-Col Excess-Col Carri-Sen     Runts    Giants

Fa0/7              0         0         0          0         0         0         0

QLD-SBK-3550#show int fa0/7 counters protocol status

Protocols allocated:

FastEthernet0/7: Other, IP

QLD-SBK-3550#show int fa0/7 stats


          Switching path    Pkts In   Chars In   Pkts Out  Chars Out

               Processor          0          0         67       4020

             Route cache          0          0          0          0

                   Total          0          0         67       4020

QLD-SBK-3550#show int fa0/7 status

Port      Name               Status       Vlan       Duplex  Speed Type

Fa0/7     RM5+RD30 to HQ Bui connected    trunk      a-full  a-100 10/100BaseTX

  • LAN Switching and Routing
Cisco Employee

Line Protocol Flapping

Hi Dylan,

If the line protocol is down, the line protocol software processes have determined that the line is unusable.

The line protocol Up/Down indicates that while the phsyical line is good - meaning you're seeing that there is a device connected at the other side, but it is not negotiating the link successfully

  • This could be caused by a unidirectional link of some type
  • A hardware/software failure on the switch, or the other side could be err-disabled or shutdown on the other side.
  • This could also be a problem if different protocols are used for trunking(ISL vs dot1q).

Perform the following corrective actions:

  • Replace the cable.
  • Check the local and remote interface for misconfiguration.
  • The local interface Fa0/7 is config as trunk port, please check the config on other end interface
  • check the speed / duplex settings on both end (try to hard core both end or config as auto)



Regards, Aru *** Please rate if the post useful ***
Hall of Fame Super Gold

Line Protocol Flapping

Would be nice if you have a 3750G/E/X or a 2960G/S then things could get mighty interesting if you run TDR with the switch models I've mentioned.

New Member

Line Protocol Flapping

Thanks for the replies

As it is a commercial tower there are many providers and a wide variety  of device s on site so it is difficult  to be 100% certain on the root cause or the offending device but in the end we relocated our radio to a different spot on the tower where there is less devices and as a result the interface is no longer flapping so I suspect that another provider has some damaged or faulty equipment that is leaking onto the tower resulting in the cable interference we were experiencing. very difficult  issue to identify with no errors or real give aways.

New Member

In case others have this, we

In case others have this, we had an issue and it turned out to be a faulty patch cable.  Either cable test your cables if you have this issue or change them out.