Collisions

[rsrodrigo]

Hello.

I know that collisions are a mechanism used by Ethernet... but I would like to know the best way to monitor this collisions.

Does anyone tell me how to do this?

My router is a Cisco 1005.

Thanks!

[Kevin Dorrell]

The collisions counters are included in the show interface E0 output. Collisions are a phenomenon found only in half-duplex situations like on the 1005. I wish they were not called "collisions" because that makes them sound like errors which they are definitely not. They are a normal part of the half-duplex media access protocol.

They can also be read using SNMP if you have a network management station.

Kevin Dorrell

Luxembourg

[rsrodrigo]

Thanks Kevin.

I know collisions are not an error... but there's not a limit in percent that shows that have some problem in the network?

[Kevin Dorrell]

There is no hard and fast limit. People used to say that a network was becoming noticably congested if it reached 30% load or 20% collisions, and that was generally taken as an upper usability limit.

The counters you need to look out for are LateCollisions and ExcessiveCollisions, both of which are regarded as errors. LateCollision is where another station has started to transmit after 512 bit times. (It is OK for two people to start speaking together - they back off and resolve the collision - but to start speaking well after another has started is considered impolite.) ExcessiveCollision is where the speakers have started at the same time 16 times in a row, and have had to give up.

Hope this helps.

Kevin Dorrell

Luxembourg

[rsrodrigo]

Thank you one more time, Kevin!

I have just one more doubt... LateCollisions and ExcessiveCollisions don't appear in output of show interface command. Where or how can I find this?

[Kevin Dorrell]

Late collisions appear just below the collisions. Excessive collisions do not appear specifically in show int, but are registered in "output errors". They can be read through SNMP on a network management station. They can also be found in show controller E0 as tx_exc_collision_err, as can a lot of other interesting counters. They are so rare that you are unlikely ever to see one.

Hope this helps

Kevin Dorrell

Luxembourg

[rsrodrigo]

Kevin,

Thank you very very much!

Best regards.