Broadcast Collisions and Domain Collisions

[vekiez]

A hub creates a broadcast domain and a collision domain.
A switch creates a broadcast domain and each connection, a collision domain.


So according to this image I have 7 broadcast domain and 12 collisions domains?

[pieterh]

no not correct.

vlan 1,2,3 each form a separate broadcast domain -> 3 (7:wrong)

each connected switch port forms a collision domain,

and the switch port connected to a hub forms a collision domain with the hub-connected devices -> 12 ( OK)

first a hub does not "create" but "forms" a single broadcast domain AND a single collision domain.

Collission here in a sense like the original ethernet standard as "shared access" medium (CSMA/CD).

a switch port uses ethernet protocol (CSMA/CD), but is effectively not shared,

only sender and switchport detect each other's collision.

a hub formed LAN is a shared network and all ports detect the same collisions.

each isolated (v)lan forms a broadcast domain. broadcast are (by default) not forwarded to another vlan (apart from specific conditions, where broadcasts are flooded to all ports).

[Joseph W. Doherty · Accepted solution]

"A hub creates a broadcast domain and a collision domain."

Yes and no. It depends on what you connect the hub to. One, two, or N number of hubs, interconnected, create both one broadcast and collision domain, as logically there's only one shared link/media.

"A switch creates a broadcast domain and each connection, a collision domain."

Not exactly.

For non-VLAN switches, one or more, interconnected, create one broadcast domain, just like a hub or hubs.

For VLAN switches, one or more, interconnected, sharing the same VLAN, create one broadcast domain for that VLAN, again just like a hub or hubs. Remember a VLAN (virtual LAN), mimics the behavior of physical non-VLAN switches for just that defined VLAN. For example, if you had three switches, physically interconnected such as SW1<>SW2<>SW3, and SW1 and SW2 had VLAN1, SW2 and SW3 had VLAN2 and SW1 and SW3 had VLAN3 (which physically transits through SW2, i.e. no access ports defined to use VLAN3 on it), logically you would have "LANs" SW1<>SW2, SW2<>SW3 and SW1<>SW3. Even though all the switches have multiple VLANs, physically on the switch, the traffic is logically kept separate.

Collision domains, on either kind of switch, would be one per port, if the port is configured in half duplex mode (same as hubs support). If the switch port, though, is configured in full duplex mode, there is no collision domain on that port.