Re: Linksys LAN -- network fails at a switch
From: Sally Shears (SallyShears_at_gmail.com)
Date: 02/01/05
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Date: Tue, 01 Feb 2005 09:15:32 -0500
In article <1107234043.216587.81830@f14g2000cwb.googlegroups.com>, prg
<rdgentry1@cablelynx.com> wrote:
> Sally Shears wrote:
> > In article <1107144039.741360.206320@c13g2000cwb.googlegroups.com>,
> prg
> > <rdgentry1@cablelynx.com> wrote (...some snips...):
...snip...
> > > ...and craps the spanning tree -- if you're running
> > > spanning tree, that is.
> >
> > If "spanning tree" is running, I don't know it. I have SuSE 8.1 and
> > 9.1, pretty standard setup. Linksys consumer switches (the $50 at Best
> > Buy type) and a WRT54g. These don't provide the spanning tree protocol,
> > do they? (Googling, I can see that Linksys WET54GS5 does do spanning
> > tree.)
>
> This was more just a check -- have noted in the past year that a number
> of commodity switches are offering spanning tree. Doubt that it will
> even engage without another spanning tree switch in the fabric. Guides
> don't speack much about configuring it -- just on or off.
I think my switches are more basic than this. Linksys EZXS55W ver 3 for
example. I don't think there are ANY options.
(Aside... This is so much just an appliance that the model number,
although printed on the label, is SO SMALL you need a magnifying glass
to find it.)
> > > I've had times when MACs began appearing on different sides of a switch
> > > for no apparent reason. This can bring the switch(es) to their knees.
> > > Hard to "see" it without sniffing the wire and the problem source can
> > > be difficult to track down.
> >
> > No loops in the physical wiring. I'm absolutely sure of this.
> >
> > But... what if a host (and its MAC address) are moved from one part of
> > the LAN to another?
> >
> > I move a laptop (Apple OS X) all the time from wireless (with a NAT'd
> > non-routable IP) to wired (with fixed IP) at the same OR different part
> > of the network to outside the building (i.e. yet another IP somewhere
> > else on the internet). Could moving this Mac's MACs be causing the
> > problem?
>
> Well, at the switch level, only MACs and ports are used in the lookup
> tables. This is strictly level 2. If you are moving within the same
> switch group/lan and the ageing timers are long enough, your old
> MAC/port will still be present when you hook up elsewhere -- you
> "announce" your presence to the switch with the very first frame you
> send out.
I can move hosts around on the LAN; they work and no obvious problems.
The "announce" and new routing must be working OK.
By the way, when the failure occurs, it stays failed. Does not recover
after a timeout. Stayed broken for 24 hours once.
> > FWIW, the Mac laptop has two nics, one wired and one wireless.
> > Different MAC addresses. OS X in most configurations automatically
> > changes from one nic to the other as needed. Recently someone opined in
> > alt.os.linux.suse "a host with two nics on the same LAN is trouble."
>
> This is true at the IP level, especially re: arp cache/table of
> MACs/IPs as gateways can get confused when they have MAC/IP pairs
> moving around. It's similar to MACs moving about in a switched LAN.
> Any host/nic setup that produces "gratuitous" arp (announcing arp) can
> exacerbate this. But these will (or at least should if not turned
> off/filtered) return ICMP errors.
>
> If only one nic is active at a time, no problem. Both at the same time
> on a Linux box is usually trouble because Linux stack is designed to
> respond for any host IP out any nic (a sort of failover "feature").
> Don't think Mac stack behaves this way -- it's BSD based, afaik.
Yes, Apple OS X is BSD-based.
> > I
> > wonder if I should start being extremely careful that no laptop ever
> > has ethernet plugged in which it's wireless card is operating. Anyone
> > have comments on two-nics-same-LAN?
>
> Do you gain anything with both nics up simultaneously? Failover and
> bonding are the usual reasons for wanting both up.
If both nics are up, it's not intentional.
> ... The route path to
> the nics is likely different -- true in your case?
Not sure I know. Wired and wireless would be on different parts of the
same LAN, under different switches.
...snip...
> I was sorta hoping that you would find someone complaining about eratic
> networking, suspect misbehaving nic, and locate your problem source.
> Alas, it's never that easy.
Nope. Not sure anybody would notice a hiccup or slowdown.
...snip...
> good luck,
> prg
> email above disabled
Thanks again, prg... I have some things to watch for.
My conclusions:
1. Keep the network simple. I think I'll move some hosts so I can
eliminate some of the switches.
2. Never let wireless laptops connect two-ways to the LAN.
3. Cycle the power from time to time on the switches.
-- Sally
-- Sally Shears (a.k.a. "Molly") SallyShears@gmail.com -or- Sally@Shears.org http://theWorld.com/~sshears
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