Ethernet – Why Repeaters Are Needed Despite Bridges or Switches

Please ignore the comment above, it was supposed to be an answer but I can't delete it.

I have this evening spent some time in a remote session with Deliberant support, and between us we managed to nail down the issues so it now works. I'm posting this information in case it is of help to anyone else in the future.

1) The Deliberant APC web interface will not respond to tagged frames. This requires the switch ports connected to the APC units to have at least one untagged VLAN assigned, and this VLAN to be used for management of the units. By setting the ports at each end to tagged in VLAN2, untagged in VLAN1, and a PVID of 1 the APCs can be managed from VLAN1 while still allowing VLAN2 traffic.

2) Always check your physical switch ports! After trying to get VLAN2 to work over the bridge unsuccessfully the company electrician handily pointed out that he'd moved some cables, but didn't tell the IT department that he'd unplugged anything. He thought he'd put the cables back in the same place, but when we checked during the support call found that the cables were put in the wrong ports.

The support call wasn't a complete waste of time in that it identified (1) as an issue, but had the IT documentation been updated to reflect the remote switch cable configuration having been changed the VLANs would have been working prior to the support call. As it is we would still have had the issue of not being able to manage the Deliberant APC units given that they drop tagged frames directed to their management interface.

Dan

First of all, a network (collision domain) may be larger than the one in your chart, but its diameter may not be larger than defined by the 5-4-3 rule, ie. no two nodes may be further apart than five (electrical) segments overall, four repeaters, and three mixing (=shared electrical medium and implictly half-duplex) segments. This ensures that a collision propagates reliably across the network within the time slot, avoiding late or undetected collisions.

A "segment" in this context is a single, electrical, non-repeated medium (electrical segment) such as a 10BASE5 cable, a 10BASE-T link, or a 10BASE-FL link. In this sense, a repeater hub is a repeater connecting many electrical segments.

The difference between a "mixing" and a "non-mixing" or link segment is that the latter needs to use a full-duplex capable, point-to-point medium - one that has separate send and receive wiring, like 10BASE-T or 10BASE-FL. The reason for this is that collision detection is faster on such a medium. Coax 10BASE5 and 10BASE2 are mixing segments and the collision detection may be slightly delayed, even with only two nodes attached.

Note that the actual Ethernet communication will still be half-duplex at all times - the 5-4-3 rule is about a repeated network (single collision domain) and full duplex requires switching. If you use switches instead of repeaters there are no such rules to apply. In theory, a switched layer 2 segment could have any size and diameter but there are other practical limits.

Edit: The term layer 1 segment has been removed as it may apply to an electrical segment as well as to a collision domain which are both segments in layer 1 but different things. Only in early, small networks with a single coax cable both were identical.