From: Jeff K. (jeffbk@xxxxxxxxxxxxx)
Date: Tue May 15 2001 - 23:56:28 GMT-3
Just noticed that I typed 'Like Ron said' instead of Rob. Sorry 'bout that
Rob...
-Jeff
----- Original Message -----
From: "Jeff K." <jeffbk@austin.rr.com>
To: "sanjay" <ccienxtyear@hotmail.com>; "Rob Webber" <rwebber@callisma.com>;
"Rob Rummel" <Rummel@Hawaii.RR.com>; <ccielab@groupstudy.com>
Sent: Tuesday, May 15, 2001 9:28 PM
Subject: Re: Dumb Bridging Question
> Every packet does not need to go through the root bridge. But, yes, you
can
> definitely have a non-optimal switching path if you allow the network to
> pick the root for your vlan. Like Ron said earlier, the root bridge is
> basically a reference point from which all the other switches will select
a
> path in order to prevent bridge loops. In a layer 2 network that is set
up
> as a hub-spoke, for example, then all traffic destined for another switch
> will pass through the root - since your root should be the core switch.
> But, remember, the switch forwards packets based on the CAM table. This
is
> definitely affected by root bridge location, since that determines
> forwarding ports. But, still, a packet doesn't need to hit the root. For
> example. say switch A is root, switch B is linked to switch A, and switch
C
> is linked to switch B (A---B---C). A packet from a host on B needs to go
to
> a host on C. That packet will not touch the root bridge. The root bridge
> will allow B to determine which of its redundant links to A it will use to
> connect to A (and same for C's links to B). The main thing to remember
with
> the root bridge is that it plays a big role in network stability. You
want
> it to be a switch that is as stable as possible since a new root will
cause
> a topology change. You also want it to be a strong box. This gets back
to
> having a good design. When you select the best location for your root,
> change priorities (or us the 'set spantr root' macro) to force this. With
> equal priorities, the lowest MAC address wins, meaning an old router that
is
> bridging would probably become root over your nice, new switch since it
has
> the old, low MAC address... That could easily cause the other switches to
> choose non-optimal paths. It's also a good idea to choose your secondary
> root and lower it's priority as well (use the 'set spantr root secondary'
> macro). A nice layer 2 design is a pair of beefy core switches with a
very
> high-bandwidth link between them, one root for half VLANs and the other
root
> for the remaining. They are each secondary roots for the other. The
access
> / distribution switches are dual homed to the 2 core switches and can run
> nice features like uplinkfast and backbonefast for fast reconvergence
> times...
>
> HTH (sorry if it is too long),
>
> -Jeff
> ----- Original Message -----
> From: "sanjay" <ccienxtyear@hotmail.com>
> To: "Rob Webber" <rwebber@callisma.com>; "Rob Rummel"
> <Rummel@Hawaii.RR.com>; <ccielab@groupstudy.com>
> Sent: Tuesday, May 15, 2001 8:07 PM
> Subject: Re: Dumb Bridging Question
>
>
> > Switching and bridging sometimes result in non optimal routing of
packets
> > because every packet must go through the root bridge of the spanning
tree.
> >
> >
> >
> >
> >
> > ----- Original Message -----
> > From: "Rob Webber" <rwebber@callisma.com>
> > To: "Rob Rummel" <Rummel@Hawaii.rr.com>; <ccielab@groupstudy.com>
> > Sent: Tuesday, May 15, 2001 6:27 AM
> > Subject: RE: Dumb Bridging Question
> >
> >
> > > I don't think this is a dumb question - many people don't fully
> understand
> > > Spanning Tree.
> > >
> > > No, all traffic does not have to flow through the root. It is simply
> used
> > as
> > > a reference point from which all bridges create a network with no
loops.
> > >
> > > The root is the bridge with the lowest number for root priority. On
> every
> > > segment one bridge is the "designated" bridge. This bridge will be
> > > forwarding traffic from that segment. The designated bridge is the one
> > with
> > > the lowest path cost to the root. The path cost is calculated by
adding
> > the
> > > outbound costs on all segments to reach the root. In the case where
two
> or
> > > more bridges have the same path cost to the root, the designated
bridge
> is
> > > the one with the lowest priority.
> > >
> > > You can start drawing some diagrams with one root bridge and many
other
> > > bridges. Create some loops and start seeing which links have to be in
> > > "blocking" mode to prevent loops (and how you can control which ones
are
> > > blocked based on path cost). It will become clear not all traffic HAS
to
> > > pass through the root bridge.
> > >
> > > Good luck - Rob.
> > >
> > > -----Original Message-----
> > > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
> > > Rob Rummel
> > > Sent: Tuesday, May 15, 2001 12:35 AM
> > > To: ccielab@groupstudy.com
> > > Subject: Dumb Bridging Question
> > >
> > >
> > > Cant believe I'm so far along my studies but don't understand a basic
> > > fundamental principle so please bear with me.
> > > When a Root bridge is elected does all the traffic in that STP go THRU
> > > the route bridge or is it just for REFRENCE for STP????
> > >
> > > Any sufficiently advanced technology is indistinguishable from magic.
> > > - Arthur C. Clarke
> > > **Please read:http://www.groupstudy.com/list/posting.html
> > > **Please read:http://www.groupstudy.com/list/posting.html
> > **Please read:http://www.groupstudy.com/list/posting.html
> **Please read:http://www.groupstudy.com/list/posting.html
**Please read:http://www.groupstudy.com/list/posting.html
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