At 08:14 AM 2/19/01, AndyD wrote:
>Thanks for all your help. The way I understand it now is that with multiple
>vlans using different root bridges, you can have different vlans splitting
>the bandwidth - some going in one direction, some in the other. But if one
>link goes down, STP will then shift all to the good link. This gives you
>some load balancing and also redundancy.
Yes, and thanks to the people who provided the details. This is often a
good solution. It has been shown to scale to even very large switched networks.
>It looks like you need to go to
>layer 3 switching to do any load balancing other than this. And
>etherchannel is another option for aggregating bandwidth. But someone said
>with etherchannel using 4 full duplex 100 mbp ports will not give 800 mbps
>of throughput? I always thought that in theory that was the case??
It's "statistical" load balancing, according to Cisco. The operation that
determines which link in a Fast EtherChannel to use is quite bizarre, and
does not provide precise load balancing. It provides load sharing. Think of
it like a complex highway system. Adding new highways distributes the load,
but it doesn't usually balance the load very precisely.
The division of traffic across a Fast EtherChannel is based on
source/destination pairs, which is usually not very balanced. There are
usually some big talkers and receivers. The Ethernet Bundling Controller
(EBC) performs an X-OR operation on the last two bits of the source MAC
address and the destination MAC address. This operation yields one of four
results: (0 0), (0 1), (1 0), or (1 1). Each of these values points to a
link in the Fast EtherChannel bundle.
Priscilla
> Since
>the data is transmitted on different wire pairs, if the sender and receiver
>transmit at the same time, why isn't 800 mbps possible????
>
>Thanks again !!
>
>
>
>""Peter Van Oene"" <[EMAIL PROTECTED]> wrote in message
>[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> > Per my other post, STP prevents looping traffic in general, not simply
>broadcasts.
> >
> > Pete
> >
> >
> > *********** REPLY SEPARATOR ***********
> >
> > On 2/19/2001 at 6:50 AM Kenneth wrote:
> >
> > >Jason is right. This will defeat the purpose of Spanning Tree of creating
>a
> > >single path to a destination. The primary reason this was designed was to
> > >prevent broadcast loops.
> > >
> > >If you want to force it to use 2 paths to one destination, use
> > >port-channelling which statically load-balances traffic going out of two
> > >ports. Statically meaning it creates a list of source-destination MAC
> > >address pairs and these pair will communicate from a specific port
> > >configured to be part of the port-channel. This is in contrary to Dynamic
> > >load-balancing where each packet will go out of each port of the
> > >port-channel.
> > >
> > >With this in mind, if 4 ports are configured for 100 Mbps full-duplex
> > >port-channels, this doesn't mean it provides an 800Mbps link.
> > >
> > >
> > >
> > >
> > >"AndyD" <[EMAIL PROTECTED]> wrote in message
> > >96p2uk$rt5$[EMAIL PROTECTED]">news:96p2uk$rt5$[EMAIL PROTECTED]...
> > >> 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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Priscilla Oppenheimer
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