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New Member

Collisions when running full duplex

We recently connected a 7500 series router with an FEIP card to a 6506 catalyst switch. This was duplicated in a another location for resilience. Switch port manually set to 100 full, FEIP interface to duplex (it only runs 100, no option for 10)

We saw a high collision and error rate as output below shows. Has anyone an expalanation for this??

FRDC1#sh int f5/1

FastEthernet5/1 is up, line protocol is up

Hardware is cyBus FastEthernet Interface, address is 0005.002c.78a1 (bia 0005.


Description: Connection to FRCBGP01 5/39 BGP Segment

Internet address is

MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec, rely 255/255, load 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 1w3d

Queueing strategy: fifo

Output queue 0/40, 0 drops; input queue 0/75, 0 drops

5 minute input rate 295000 bits/sec, 146 packets/sec

5 minute output rate 131000 bits/sec, 124 packets/sec

216403700 packets input, 205335994 bytes

Received 520578 broadcasts, 0 runts, 0 giants, 0 throttles

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

0 watchdog, 0 multicast

0 input packets with dribble condition detected

208936548 packets output, 21449531 bytes, 0 underruns

2059932 output errors, 3689445 collisions, 0 interface resets

0 babbles, 2059932 late collision, 1628485 deferred

0 lost carrier, 0 no carrier

0 output buffer failures, 0 output buffers swapped out

New Member

Re: Collisions when running full duplex


check both the sides are same config set

including switch and router

steps to test

1. cable

2. distance

3. configuration

New Member

Re: Collisions when running full duplex

yes, we checked this thoroughly. Had same at two sites so unlikely to be bad cable or distance. DIstance is under 50 metres in both cases.

We evenually set to half duplex and it runs better but we need full duplex for throughput.


Re: Collisions when running full duplex

Well the only way to get collisions is to have one side at half duplex , the router looks right so I would check the switch side and make sure it is not set as "AUTO" , this would cause a duplex mismatch . Hardcode switch side to 100/full . The fact that it runns better when you change the router to hald also indicates that maybe the switch is not coded correctly. The only other thing is if the router or switch is giving false readings for some reason when even though it says 100/full it is operating in half duplex mode . It you are absolutely sure both sides are hardcoded to 100/full then I would suspect something like that . Also I would check to see what FEIP you have some will only allow you to run half duplex . Go to and take a look at this , maybe you are running into something like this .

New Member

Re: Collisions when running full duplex

Late collisions = half duplex on one end...


Re: Collisions when running full duplex

Once the switch and router are hard coded to 100 full duplex make sure you reset the switch port and router interface , sometimes this is needed .

New Member

Re: Collisions when running full duplex

This might seem obvious but do reset interface counters to make sure you are not looking at old numbers.

New Member

Re: Collisions when running full duplex

many thanks to you all for the advice. I will schedule a change to make sure duplex and 100 is hard coded and shut no shut the interfaces. THese interfaces are crucial to our branch banking operation so the change will take 3 days to get approval.

Will let you know the outcome.

New Member

Re: Collisions when running full duplex

This may also sound obvious but have you tried to connect the cable to another port on the switch and reconfigure that port.

New Member

Re: Collisions when running full duplex


you must set the port on the catalyst site to 100MB/full duplex .

Use for cat/os the commands :

set port speed slot/port 100

set port duplex slot/port full.

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