From: Chris Hugo (chrishugo@xxxxxxxxx)
Date: Wed Jun 19 2002 - 18:56:50 GMT-3
Hi Anthony,
I'll answer a couple of questions. There are a lot of gurus on this mailing lis
t that will be happy enough to fill in the rest :)
Configuring Port Cost
You can configure the port cost of switch ports. Ports with lower port costs ar
e more likely to be chosen to forward frames. Assign lower
numbers to ports attached to faster media (such as full duplex) and higher numb
ers to ports attached to slower media.The possible range
is 1 to 65535. The default differs for different media. Path cost is typically
1000 w LAN speed in megabits per second.
Key Concept: This will set a port cost in General to the port itself for and al
l vlans on that port.
Configuring Port-VLAN Cost
You can configure the port cost for a port on a per-VLAN basis. Ports with lowe
r port-VLAN costs are more likely to be chosen to forward
frames. You should assign lower numbers to ports attached to faster media (such
as full duplex) and higher numbers to ports attached to
slower media. The possible range is 1 to 65535. The default differs for differe
nt media.
Key Concept: This is usually used when you have a two or more uplinks between
two switches and you want traffic to flow over a
specific port rather than another port on your switch. By default spanning tree
will allow only one port to be in the
forwarding state. Now let's say you have 2 uplinks between a switch. You want a
ll traffic for VLANS 6-7 to be switched over port 1
and you want VLANs 8-9 to use port 2. You can set a lower cost on port 1 for VL
ANs 6-7. You would do the same on
port 2 for VLANs 8-9. Now instead of using one port only you can now utilize bo
th ports for traffic.
Now your question might be what if port 1 goes out? Will my VLAN traffic for VL
ANs 6-7 still be switched over port 2? The answer is
yes because of the General port cost that is STILL assigned on port 2. It acts
like a catch-all for all other VLAN traffic. Since
port 1 is down there is no other favored (lower cost) port available and will s
witch the traffic over port 2. Vice-Versa if port 2 goes out.
Another note: You cannot set the Port-VLAN Cost parameter higher than the Gener
ic port cost assigned on a particular interface.
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/sw_5_5/cnfg_gd/span
tree.htm#xtocid1496212 is good link.
I took a couple of excerpts out of there.
hth,
chris hugo
Anthony Pace <anthonypace@fastmail.fm> wrote: Chris,
You sound like you ar definitly "in the know". Can you verify the
following:
I think the SET SPANTREE ROOT is just a "canned" way of pushing a
fairly low prioriy into the switch. I don't think it is as effective a
setting the priority for a VLAN manually because when you do it
manually you can make it really low (100 or 1 or whatever). The ROOT
GUARD funtion can also be used to enforce the root position of the
switch.
I have another question: what is the difference between these two
commands? I know they effect the port priority:
set portvlancost
set port cost
THis is my understanding of how the other commands work and if someone
can confirm/correct me that would be GREAT!
1)On a router doing bridging we set the priority for the "BRIDGE" in
global configuration which determines our stanind in the spanning tree
root election.
2)On a router doing bridging we set the priority for the "PORT" in the
interface configuration. This determines this interfaces standing in
the decision of which port/interface on the ROUTER will be the optimal
path to the ROOT (in case we are not the root and there are redundant
paths and we need to BLOCK one)
3)On a CAT5 switch we use SET SPANTREE ROOT or SET SPANTREE PRIORITY or
SET SPANTREE ROOT which determines our stanind in the spanning tree
root election.
4)On a CAT5 switch we use "set portvlancost" or "set port cost". This
determines this interfaces standing in the decision of which
port/interface on the ROUTER will be the optimal path to the ROOT (in
case we are not the root and there are redundant paths and we need to
BLOCK one)
Is this right or wrong?
Anthony Pace
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