Re: One question about 3550 Port Priority?

From: Prashanth (prashanthcm@yahoo.com)
Date: Fri Jan 03 2003 - 04:13:52 GMT-3

Switch B

if Switch A is the root bridge for all your VLANs
(all ports in forwarding)
and the only way to load-balance is by playing with
the port priority on the downstream switch (B)

say if you have 4 VLANS and Switch A is the Root for
all the 4.... by default one of the trunk ports on
Switch B will be in blocking state...

you can modify the port priority on one one trunk port
for two vlans to be in forwarding state and two in
blocking state....similarly on the other so that each
trunk port carries traffic from two VLANs... when one
fails....one trunk carries all the traffic...

http://www.cisco.com/en/US/products/hw/switches/ps628/products_configuration_guide_chapter09186a008007e8c7.html#xtocid45

-prashanth-

--- Richard Hanks <ccieingroup@hotmail.com> wrote:
> Hi Friends,
>
> I have a question here about the 3550 Port Priority.
> Please refer to the
>
> "Catalyst 3550 Multilayer Switch Software
> configuration Guide.pdf" Chapter
> 10-26 Configuring VLANs (Load Sharing Using STP) and
> Chapter 14-15 configuring
> STP( Configuring the port priority).
>
> My question is if two 3550 connected by two trunk
> links like this one.
>
> Switch A (Root Bridge)
> 0/1| |0/2
> 1/1| |1/2
> Switch B.
>
> I want to use the "spanning-tree vlan 20
> port-priority ..." to make the Switch
> B's 1/1 in forwarding state for vlan 20 but 1/2 in
> the block state. Where
> should I put the "spanning-tree vlan ..
> port-priority" command on? I mean
> whether on Switch A's port 0/1 and 0/2 or Switch B's
> 1/1 and 1/2. I mean
> whether the upstream switch or downstream switch
> directly.
>
> I also refer to the "CCIE Professional Development
> Cisco LAN Switching" Page
> 249: Port/VLAN Priority- "set spantree portvlanpri"
> can be used to load
> balance between back-to-back switches. This
> technique allows network
> administrators to modify port ID values(Port ID=Port
> priority + Port number)
> on upstream switches to influence forwarding and
> blocking decisions on
> neighboring downstream switches."
>
> But I can't find any such description in the 3550
> configuration guide. From
> the example of page 10-26. Though it mentioned where
> to put the commands on,
> but it didn't mention which switch is the root.
>
> Appreciate for any help.
> Rgds,
> Richard
> .
>

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