Collision domain confusion on a switch

tcp/udp

My understanding is that a connection from a host to one of the ports on a switch is considered a collision domain. Are collisions really possible within that domain? Or is just a name that indicates that a bigger collision domain (as with a hub) has been segmented?
I've always thought of communication through a switch as full-duplex, so I am a bit confused.

mikej412

tcp/udp wrote:

My understanding is that a connection from a host to one of the ports on a switch is considered a collision domain.

That's what you need to know for the CCNA..... since someone could set that 100Mb Full-Duplex NIC in to the host to run at half-duplex.... or the auto configuration could mess up.

But you are right -- when you are running full duplex with the good NICs you shouldn't get any collisions.... but remember that Cisco still counts it as one.

tcp/udp

mikej412 wrote:

tcp/udp wrote:

My understanding is that a connection from a host to one of the ports on a switch is considered a collision domain.

That's what you need to know for the CCNA..... since someone could set that 100Mb Full-Duplex NIC in to the host to run at half-duplex.... or the auto configuration could mess up.

But you are right -- when you are running full duplex with the good NICs you shouldn't get any collisions.... but remember that Cisco still counts it as one.

Thank you so much for your help!

Rearden

It's called that because anything out a port on a switch is collision domain. . .someone could put a 10BaseT hub on the end of a switchport and then collisions could happen.

Collisions wouldn't happen if someone were to attach a switch out a switchport, correct? <- is asking a question here.

dtlokee

If you connect two switches together with a cross-over cable there would be no collisions on the link.

tcp/udp

You can actually configure the duplex on each port of the switch. You can have "auto", "half" and "full", as well as speed. From what I understood, "auto" is not that recommended; however, auto is the default.

Paul Boz

Auto isn't recommended because it often defaults to half, which causes obvious problems. It's best practice to configure every element of your network that's possible, for complete control.

EdTheLad

Fast ethernet will send a 16 bit sequence number(all zero's) as a keepalive, ethernet sends a 1 bit (zero)keepalive.The receiving interface will know whether its neighbor is 100mb or 10mb by the received sequence number length. The duplex capability of the link can be exchanged by modifying the sequence string i.e. setting some of the 16 bits to 1. A port running in Auto mode will modify this sequence number to inform connected devices about its capabilities.A port that is not set as Auto will send a 16 bit all-zero string. If one side is auto and the other full duplex, the auto side will not receive the capabilities from its neighbor so it will default to half duplex for safety.
This 16 bit sequence is not standardized so when connecting different vendors on auto, you may find ports entering the half duplex mode. If you are using a cisco only environment there is no problem to leave the ports as auto as it can provide a safety mechanism for misconfigured ports.

DirtySouth

Just to add a quick bit of info...remember that a switch segments collision domains while a router segments broadcast domains.