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Re: CVS commit: src/sys/net

On Tue, Sep 29, 2020 at 05:45:49AM -0000, Michael van Elst wrote:
> mouse%Rodents-Montreal.ORG@localhost (Mouse) writes:
> >>> Maybe "network interface" is not the right abstraction for bridges
> >>> to be instances of, then?  (Though, at least in my experience,
> >>> NetBSD's "bridge" interfaces are more like switches than bridges.)
> >> A switch is a bridge with more than two ports.
> >Then what is my 8-port 10Mb hub?  Switch, bridge, neither?
> Neither, it's a repeater.
> Repeater / Hub -> Layer 1. Incoming signal from one port at a time
> is send out (after some amplification / regeneration) to all other
> ports. Single collision domain.
> Bridge / Switch -> Layer 2. Packets (or often just a few bits to reduce
> latency) are buffered and transmitted to other ports, a forwarding
> database is used to prune destinations. Separates multiple collision
> domains.

See "cut-through switching", where as soon as enough bits of the frame
is received to determine the destination port, it starts transmitting
on the destination port, if it is idle. If the port is busy, it reverts
to the other method, "store and forward". Made a measurable difference
to latency on 100Mbit networks, I don't know that anyone bothers on
modern 10+Gbit gear.

> The terms Hub and Switch came up when the devices got more than two ports,
> but functionally these are not different from the earlier terms.

Yup, I always considered:
- a hub is a multi-port repeater.
- a switch is a multi-port bridge.

> There are hybrid devices. I have a thingie that has some 10Mbit and some
> 100 Mbit ports. Technically it's two repeaters with a two port bridge
> in between. Since repeaters work on layer 1, they cannot translate
> different speeds. A bridge can.

I had one of these - advertised as a "switching hub". 8 ports,
auto-negotiated 10Mbit or 100Mbit half duplex. I guess the auto-neg
somehow switched the ports between the two hubs, and, like you say,
the hubs were bridged together.

Reminiscing... should I mention 10BASE5 and vampire taps?

Paul Ripke
"Great minds discuss ideas, average minds discuss events, small minds
 discuss people."
-- Disputed: Often attributed to Eleanor Roosevelt. 1948.

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