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Posted byA+, SEC+6 days ago

Question about collision domains and switches

Reading Odom's book and it says that on a switch, each switch is technically a collision domain, even if a collision could not happen, meaning that it wouldn't happen because there is only one device connected to each collision domain.

At my work, we have VOIP phones setup, and a lot of our setup goes switch>voip phone> workstation, meaning the phone and workstation are connected to the same port and the phone and workstation are basically daisy chained together. How does this work and not cause a collision?

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CCNA R&S9 points·6 days ago·edited 6 days ago

Each port on the switch is its own collision domain, not all the ports on the switch itself. When you refer to each switch being a collision domain, you are referring to a hub. Hubs place all the ports in a single collision domain which is why half duplex is used when hubs are in play.

The ports on the switch and the workstation all support full duplex. This means they can transmit and receive data at the same time, therefore no collisions occur. Additionally, VoIP phones have an internal switch that allows it to connect to the access layer switch and your workstation at the same time. Each of these connections is essentially a separate collision domain (i.e. network to phone and phone to workstation).

A+, SEC+Original Poster2 points·6 days ago

So usually VOIP is a separate VLAN. Would that switchport have to be set to trunk then for that setup?

CCNA R&S8 points·6 days ago

In the old days, yes the port would have to trunk the data vlan and the voice vlan. Nowadays that is not necessary. You can specify both the access (data) vlan and the voice vlan while the port operates in access mode. This can be done with the command switchport voice vlan x where x is the voice vlan number.

CCENT3 points·6 days ago

Reading Odom's book and it says that on a switch, each switch port is technically a collision domain, even if a collision could not happen.

If there is only one device on the segment and it is full-duplex, then a collision cannot happen. The switchport is still a single collision domain, even in this situation.

How does this work and not cause a collision?

Because there is a switch inside the phone connecting the three ports. Your topology is actually

                             +--> phone
                             |
                             v
access switch <---> phone switch
                             ^
                             |
                             +--> workstation

The phone and the workstation are not connected to the same port, they are connected to the same switch.

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