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Community Member

High number of Collisions

Hi,during 1 of the troubleshooting i found that there are high number of collisions in my WAN interface.And it caused the IP phone can't get registered in my HQ's CCM.But the phone will back on after 1 day or few hrs later.Is it due to duplex mismatch?

Would much appreciate if you can help me point out the problem.

interface GigabitEthernet0/1

description External LAN/WAN

ip address

ip access-group 130 in

ip inspect myfw out

ip nat outside

ip virtual-reassembly

no ip route-cache cef

no ip route-cache

duplex auto

speed auto

crypto map rtpex

service-policy output policyvoip

GigabitEthernet0/1 is up, line protocol is up

Hardware is MV96340 Ethernet, address is 0018.1891.6b39 (bia 0018.1891.6b39)

Description: External LAN/WAN

Internet address is

MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec,

reliability 255/255, txload 1/255, rxload 5/255

Encapsulation ARPA, loopback not set

Keepalive set (10 sec)

Half-duplex, 10Mb/s, media type is T

output flow-control is XON, input flow-control is XON

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 19:03:56

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

Queueing strategy: Class-based queueing

Output queue: 0/1000/64/0 (size/max total/threshold/drops)

Conversations 0/49/256 (active/max active/max total)

Reserved Conversations 0/0 (allocated/max allocated)

Available Bandwidth 6988 kilobits/sec

5 minute input rate 205000 bits/sec, 6 packets/sec

5 minute output rate 5000 bits/sec, 7 packets/sec

266884 packets input, 168546649 bytes, 0 no buffer

Received 3171 broadcasts, 0 runts, 0 giants, 0 throttles

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

0 watchdog, 0 multicast, 0 pause input

0 input packets with dribble condition detected

371844 packets output, 39383741 bytes, 0 underruns

0 output errors, 214 collisions, 0 interface resets

0 unknown protocol drops

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


Re: High number of Collisions

Hi there!

Your line is set to Half-duplex, 10Mb/s on a Gigabit interface. Auto-negotiation of speed and duplex failed. Try to set manual the duplex and speed on the interface (should be the same on both sides) and then clear the counters and see if the collisions increase again.

Good luck!



Community Member

Re: High number of Collisions


Thanks for the reply.

The other end is connected to a modem which provided by the isp,so i don't know their speed.Probably is not auto because i tried set my WAN int to auto duplex,the collision went up fast and furious.

Is there anything i can do to know the setting of that modem?

Re: High number of Collisions


What kind of connection you have there? As I see your config there is a regular line and it should be no modem there (maybe a switch or something) You can actually see the modem?

If you don't know for sure what is the opposite peer, try "show cdp neighbors detail" and if you are lucky enough to have a Cisco device as peer you can see the duplex and the speed you assume after the interface description.

Otherwise...I don't know.Ask your provider. They should give you all the necessary details related to their connection.



Hall of Fame Super Silver

Re: High number of Collisions

Hello Yuij,

a level of collisions less then 0.1% is considered normal in 10 Mbps half-duplex

Use the show interfaces ethernet command to check the rate

of collisions. The total number of collisions with respect to the

total number of output packets should be around 0.1 percent or


in your case:

214 /371844 < 0.1%

This is normal behaviuor for half duplex.

Collided frames are retrasmitted

How 214 collisions in 19 hours can block a phone registration is difficult to understand.

I would investigate other aspects of the problem : ACLs, the configuration of the dhcp pool (have you got the correct option 150 pointing to tftp servers in the CM cluster).

Hope to help


Community Member

Re: High number of Collisions

Hi,we are now looking at all possible aspects in config,hardware and asp.

The problem sometimes happen but will recovered by itself after 1 day or so!!

My 2851 have 2 interfaces,1 is to the modem provided by ISP which i physically can see it.Another connected to the cisco 2970.

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