Re: Linux networking Q

From: Ian Northeast (ian_at_house-from-hell.demon.co.uk)
Date: 10/12/03 

Date: Sun, 12 Oct 2003 22:43:15 +0100

Dances With Crows wrote:

> (This explanation is incomplete, and may contain errors. Corrections
> welcome.)
>
> A hub acts as a dumb repeater device. It receives data from one
> interface and broadcasts that data to every interface on the hub. This
> works well when you don't have many machines connected, but the number
> of collisions increases very quickly when you add more machines. A
> 16-port hub with all ports filled, and every machine trying to
> send/receive a lot of data would be wretchedly slow.
>
> A switch has a little more intelligence. It can determine that the
> device plugged into port 1 has MAC address 01:02:03:04:05:06 , and
> transmit packets to port 1 only if the packets' destination MAC matches,

Pretty much correct, but it is possible to have more than one MAC
address on a single port, for instance if it is connected to a bridge or
another switch. Switches recognise this possibility and allow for it.

> or if the device on port 1 is in promiscuous mode.

This isn't true. The switch has no way of knowing a device is in
promiscuous mode and will not replicate all traffic to such interfaces.
This does cause problems with network probes, which would see all the
traffic on hubs and don't on switches. Some expensive managed switches
offer facilities to replicate traffic onto specific ports for the
benefit of probes but the basic "plug and go" cheap ones don't.

> This means no
> collisions, which means your Ethernet is faster.

Collisions are extremely rare but they are possible - broadcasts can
collide. My router has transferred about 62 million packets on its LAN
interface since it was last brought up about 2 months ago and has seen
one collision. So it is certainly true that collisions no longer matter.

Also switches are normally full duplex and hubs are normally half.

> I bought a little 5-port switch for $40 1.5 years ago,

My 16 port job was 64UKP, about $100, last year.

> so I figured
> wired hubs had been driven to extinction by now.

Pretty much. You still see them being sold second hand very cheaply. I
havn't seen them being sold new recently. I keep one around in case I
need to use a network probe (and also as a backup should the switch
fail).

Regards, Ian

Relevant Pages

  • High Collision Rate
    ... The hub is unable to mediate the two speeds. ... hubs operate in ISO layer 1 ... 100Mbps switch. ... >collisions are actually discards and not collisions? ...
    (microsoft.public.windowsxp.network_web)
  • Re: Can a router be used as a switch
    ... do you mean that I could use the router as a switch and only can use as ... And I may expect still lot of collisions then. ... No. Standard SOHO routers include a switch, not a hub. ...
    (microsoft.public.windowsxp.hardware)
  • Network link up and down at boot
    ... when the interface is initialized I get a series of log ... synchronize with the Hub or a Switch. ... switch giving a pulsing red light rather than a green light. ...
    (comp.sys.sun.admin)
  • Re: How to turn switch into hub in embedded linux(netgear dg834g)?
    ... interface. ... laptop to the switch and running erhereal on it to acheave this. ... can I degrade switch into hub? ... , router, etc. I'm also aware of arp ...
    (comp.os.linux.networking)
  • Re: CRC & INPUT Errors
    ... Since this is an ehternet interface which connects to a local area ... should I switch it back to half duplex? ... Late Collisions on the link, which this interface reports as Input ... errors and CRC errors. ...
    (comp.dcom.sys.cisco)