Is an ethernet switch totally transparent?

This must be a stupid question...

I have a Linksys router which has four LAN ports. Only one of these is
connected to, and it goes to a Netgear 16-port switch, which forms our
internal office LAN.

http://www.netgear.co.uk/business/pr...hes/gs116.aspx

All PCs then go to this switch.

And it works, 100% reliably for the LAN traffic... except when I want
to connect to the router's admin (from one of the PCs, which happens
to be on a fixed IP). That works some of the time. It tends to work
just once, and the only way to get it working after that is to power
down the router and the switch.

The LAN is on 192.168.5.x where the router LAN port is on .1 and the
PCs are mostly fixed IP on .100, .101 etc. The router also has DHCP
enabled, for laptops connected by ethernet or wifi, and this starts to
allocate at .51 onwards.

If I connect the PC in question directly to the router, to one of its
unused LAN ports, it works solid.

How can this be? I thought a switch was in effect like a hub i.e.
transparent.

Peter wrote:

> I have a Linksys router which has four LAN ports. Only one of these is
> connected to, and it goes to a Netgear 16-port switch, which forms our
> internal office LAN.
> All PCs then go to this switch.
>
> And it works, 100% reliably for the LAN traffic... except when I want
> to connect to the router's admin (from one of the PCs, which happens
> to be on a fixed IP). That works some of the time. It tends to work
> just once, and the only way to get it working after that is to power
> down the router and the switch.

Ought to be fine like that, can you reliably ping the router from the PC
that can only occasioanlly get to the router's web interface?

Nothing else with a duplicate IP address stashed away in a corner
somewhere? check your PC's ARP cache when it next misbehaves, see if the
MAC address your PC knows for the router's IP address is correct (arp -a
in a 'DOS' window).

On Wed, 02 Nov 2011 16:14:50 +0000, Peter wrote:

> How can this be? I thought a switch was in effect like a hub i.e.
> transparent.

A switch is transparent to tcp/ip, but it will have at least one hardware
(MAC) address, and could even have one per port.

The following is speculation: If the router associates the IP of the
controlling PC with its MAC address, it could notice that you are using a
PC with one MAC address but that the physically connected interface on
the link has a different MAC address, and this might be causing the
issue, because the router might think you're spoofing the IP from a
different MAC.

Rgds

Denis McMahon

Denis McMahon wrote:

> On Wed, 02 Nov 2011 16:14:50 +0000, Peter wrote:
>
>> How can this be? I thought a switch was in effect like a hub i.e.
>> transparent.
>
> A switch is transparent to tcp/ip, but it will have at least one hardware
> (MAC) address, and could even have one per port.

An unmanaged switch has no need for MAC address(es) of its own, so not
likely to be the case.

Andy Burns <(E-Mail Removed)> wrote

>Peter wrote:
>
>> I have a Linksys router which has four LAN ports. Only one of these is
>> connected to, and it goes to a Netgear 16-port switch, which forms our
>> internal office LAN.
>> All PCs then go to this switch.
>>
>> And it works, 100% reliably for the LAN traffic... except when I want
>> to connect to the router's admin (from one of the PCs, which happens
>> to be on a fixed IP). That works some of the time. It tends to work
>> just once, and the only way to get it working after that is to power
>> down the router and the switch.
>
>Ought to be fine like that, can you reliably ping the router from the PC
>that can only occasioanlly get to the router's web interface?

No.

>Nothing else with a duplicate IP address stashed away in a corner
>somewhere? check your PC's ARP cache when it next misbehaves, see if the
>MAC address your PC knows for the router's IP address is correct (arp -a
>in a 'DOS' window).

Not that I know of, but the behaviour does look like a massive loss of
data, because sometimes I get the router's login screen, after a long
delay, and nothing after that.

In fact my original post tried to keep matters simple. Beyond the
Linksys router I have another router and beyond that I have a D-Link
ADSL modem. These have their own asmin logins too, and are on
different subnets. These logins are either as unreliable/dud as the
Linksys one, or they work perfectly (when the PC is plugged directly
into the Linksys rather than into the switch).

I also wondered if some of the 16 switch ports behave differently, but
it is an unmanaged switch with no configuration AFAIK.

I will try the ARP stuff... not sure if I will understand it

The system has been like this for years.

Denis McMahon <(E-Mail Removed)> wrote

>On Wed, 02 Nov 2011 16:14:50 +0000, Peter wrote:
>
>> How can this be? I thought a switch was in effect like a hub i.e.
>> transparent.
>
>A switch is transparent to tcp/ip, but it will have at least one hardware
>(MAC) address, and could even have one per port.
>
>The following is speculation: If the router associates the IP of the
>controlling PC with its MAC address, it could notice that you are using a
>PC with one MAC address but that the physically connected interface on
>the link has a different MAC address, and this might be causing the
>issue, because the router might think you're spoofing the IP from a
>different MAC.
>
>Rgds
>
>Denis McMahon

Interesting... how would the router be associating itself with the
*PC's* MAC # when it may have never seen it, following its own power
cycle, in cases where I *can* make it work via the switch.

Andy Burns <(E-Mail Removed)> wrote

>Denis McMahon wrote:
>
>> On Wed, 02 Nov 2011 16:14:50 +0000, Peter wrote:
>>
>>> How can this be? I thought a switch was in effect like a hub i.e.
>>> transparent.
>>
>> A switch is transparent to tcp/ip, but it will have at least one hardware
>> (MAC) address, and could even have one per port.
>
>An unmanaged switch has no need for MAC address(es) of its own, so not
>likely to be the case.

I was going to put in a very old 10mbits/sec HUB

The LAN speed is hardly relevant, and is way below the limiting factor
which is the ADSL UP link speed.

In fact we used to run on coax ethernet till recently

"Peter" <occassionally-(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> This must be a stupid question...
>
> I have a Linksys router which has four LAN ports. Only one of these is
> connected to, and it goes to a Netgear 16-port switch, which forms our
> internal office LAN.
>
> http://www.netgear.co.uk/business/pr...hes/gs116.aspx
>
> All PCs then go to this switch.
>
> And it works, 100% reliably for the LAN traffic... except when I want
> to connect to the router's admin (from one of the PCs, which happens
> to be on a fixed IP). That works some of the time. It tends to work
> just once, and the only way to get it working after that is to power
> down the router and the switch.
>
> The LAN is on 192.168.5.x where the router LAN port is on .1 and the
> PCs are mostly fixed IP on .100, .101 etc. The router also has DHCP
> enabled, for laptops connected by ethernet or wifi, and this starts to
> allocate at .51 onwards.
>
> If I connect the PC in question directly to the router, to one of its
> unused LAN ports, it works solid.
>
> How can this be? I thought a switch was in effect like a hub i.e.
> transparent.

Duplicate IP address is my guess.

On 02/11/2011 16:14, Peter wrote:
> This must be a stupid question...
>
> I have a Linksys router which has four LAN ports. Only one of these is
> connected to, and it goes to a Netgear 16-port switch, which forms our
> internal office LAN.
>
> http://www.netgear.co.uk/business/pr...hes/gs116.aspx
>
> All PCs then go to this switch.
>
> And it works, 100% reliably for the LAN traffic... except when I want
> to connect to the router's admin (from one of the PCs, which happens
> to be on a fixed IP). That works some of the time. It tends to work
> just once, and the only way to get it working after that is to power
> down the router and the switch.

I've seen this. Turned out we had an interface (Maybe auto, can't
remember) configured for 100 Half Duplex and another for 100 Full
Duplex, and somewhere in the switch while sorting out packets between,
it thoughtfully dismembered some of them and then went deaf....

--
Adrian C

Adrian C <(E-Mail Removed)> wrote

>On 02/11/2011 16:14, Peter wrote:
>> This must be a stupid question...
>>
>> I have a Linksys router which has four LAN ports. Only one of these is
>> connected to, and it goes to a Netgear 16-port switch, which forms our
>> internal office LAN.
>>
>> http://www.netgear.co.uk/business/pr...hes/gs116.aspx
>>
>> All PCs then go to this switch.
>>
>> And it works, 100% reliably for the LAN traffic... except when I want
>> to connect to the router's admin (from one of the PCs, which happens
>> to be on a fixed IP). That works some of the time. It tends to work
>> just once, and the only way to get it working after that is to power
>> down the router and the switch.
>
>I've seen this. Turned out we had an interface (Maybe auto, can't
>remember) configured for 100 Half Duplex and another for 100 Full
>Duplex, and somewhere in the switch while sorting out packets between,
>it thoughtfully dismembered some of them and then went deaf....

AFAICT this switch has no config, so should I just try a different
one? I have an old Linksys 16-port 10/100 switch here.

However I prefer a 1G switch because when we do a tape backup to DDS5,
the network speed is a limiting factor on the tape streaming (perhaps
suprisingly).

Could it be one of the Linksys router ports which is misconfigured?