That is why I said the main reason not the only reason. Normally, before
Node A sends any packet, it will do ARP, this will create loop immediately.
Not so normal, you can hard coded the ARP cache.
Jack
""Peter Van Oene"" <[EMAIL PROTECTED]> wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> This is actually not the case. A layer two network that contains a
logical loop with get into trouble with all types of traffic flows. For
example, consider a simple network like the one I'll try and draw below.
>
>
> Node A Node B
> | |
> ---------Ethernet 1---------------------------
> | |
> Bridge A Bridge B
> | |
> --------------Ethernet 2----------------------
>
> Consider a transmission from Node A to Node B and assume that both nodes
were recently powered up and that the bridge tables for the two bridges have
not been built.
>
> Here is a likely scenario. The frame hits Ethernet 1 and both bridge A
and B detect the packet and check their forwarding tables only to realize
they do not have an entry for B. At this time they add Node A to the
forwarding table as residing on Ethernet 1. They then both forward the
frame onto Ethernet 2. Node B also receives the frame and processes it.
However, 2 versions of the frame now exist on or headed two Ethernet 2.
When the bridges forward to Ethernet 2, (1 at a time) each bridge will
process the others frame and decide that A has indeed moved to Ethernet 2
and adjust their forwarding tables appropriately. Assuming B hasn't
responded as yet, they will also process each others frames and still not
finding B in their forwarding table, copy them back onto Ethernet 1.
However, when they do, they will again switch A's address from Ethernet 2 to
1 and likely repeat the process again.
>
> Just think what happens when B responds :)
>
> All in all, this is very unhealthy. The fact that there is no way at
layer 2 in ethernet to detect the age of frames severely hinders loop
mitigation. In fact, there pretty much isn't any loop mitigation at layer 2
in ethernet networks. If you create a looped topology and do not prune the
redundant paths, you will bring a network down in seconds.
>
> Pete
>
>
> *********** REPLY SEPARATOR ***********
>
> On 2/18/2001 at 4:58 PM Jack Yu wrote:
>
> >Actually, the main reason to eliminate multiple paths is because of
> >broadcast at layer 2. Lay 2 devices have to this forward broadcast, and
> >multiple paths to a single destination will cause broadcast storm. Layer
3
> >devices do not have this problem only because they do not forward
broadcast,
> >they either read it or drop it. So if there is a network with no layer 2
> >broadcast, you do not need spanning tree at all. Of course, you can also
> >disable it when you are sure there is no duplicated path exists.
> >
> >Regards,
> >Jack
> >
> >
> >
> >
> >""Groupstudy"" <[EMAIL PROTECTED]> wrote in message
> >[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> >> Spanning Tree's job is to eliminate multiple paths to a single
> >destination.
> >> If it finds more than one path it will put one of them into blocking
mode
> >to
> >> ensure a loop free path. Remember, Spanning Tree runs at layer two
and
> >has
> >> no concept bandwidth. If you need to setup equal cost paths to a
certain
> >> destination, you will need to do it at layers above layer two.
> >>
> >> Pick up a copy of Radia Perlmans 'Interconnections' second edition.
Radia
> >> is the primary authority on the Spanning Tree algorithm, she will
> >enlighten
> >> you. This is also good solid study for deeper networking knowledge.
> >>
> >> ----- Original Message -----
> >> From: AndyD <[EMAIL PROTECTED]>
> >> Newsgroups: groupstudy.cisco
> >> To: <[EMAIL PROTECTED]>
> >> Sent: Sunday, February 18, 2001 8:58 AM
> >> Subject: Equal cost switching
> >>
> >>
> >> > Spanning tree is supposed to choose the one best switched path. But
if
> >> you
> >> > set up two equal cost paths, will it use both? Is there a way to
force
> >it
> >> > to use the bandwidth from both paths?
> >> >
> >> > Thanks!
> >> >
> >> >
> >> > _________________________________
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> >>
> >> _________________________________
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> >>
> >
> >
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>
>
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