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New Member

mismatching interface Speed

Port FastEthernet0/0 on device Router with ifSpeed 10000000

Port FastEthernet4/1 on device 6509Series_Switch with ifSpeed 100000000

Severity report is Medium. Speed on switch isn't specified therefore it is on Auto.

What are the pros & cons to this? Does the mismatch cause extra cpu cycles/processing? Inefficient use of the buffers? etc... Or is it simply just a cosmetic thing that affects absolutely nothing physically or logically?


Re: mismatching interface Speed

a mismatch in speed/duplexing will cause packet loss, retransmission, crc errors, input/output errors, etc...

all of these things make the devices work more to get the same amount of data forwarded through them.

i think it a best practice to statically set network devices speed/duplex such as routers, servers, firewalls, etc.; only using auto/auto on devices such as workstations, printers, etc.

Re: mismatching interface Speed

Hi Friend,

If you have diffrent port speeds and you have interface speed set on auto then both the interface will negotiate the speeds and will agree to the what is the lowest value. If they agree to a mutual value then there will not be any problem as such on the interface.

If you hardcode the different speed on both the interfaces then you might see the problem in which an interface will transmit to its full capacity and the other interface will not be able to accomodate the traffic thus will result in buffer overrun and droping of the legitimate traffic.

If you hadcore one interface and set the other interface on auto negotiation, this will also let to the devices working in half-duplex which will result in a lots of collision and errors on the interface thus impacting the over-all performance of the switchport thus the system.

HTH,Please rate if it does.

-amit singh

New Member

Re: mismatching interface Speed

Thank You for your knowledge. I see no errors other than 2 interface resets.

5 minute input rate 1584000 bits/sec, 268 packets/sec

5 minute output rate 491000 bits/sec, 231 packets/sec

2064821917 packets input, 1342969322637 bytes, 0 no buffer

Received 64554611 broadcasts (35446997 multicast)

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

2460016905 packets output, 473459278600 bytes, 0 underruns

0 output errors, 0 collisions, 2 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