From: Gustavo Novais (gustavo.novais@novabase.pt)
Date: Mon Jun 19 2006 - 08:06:27 ART
Hi group,
I was under the impression until recently that the spanning-tree port
cost of an etherchannel, in this case Gigabit Etherchannel, was the same
as a single gigabit link ( 4 ). On the charts that show the different
default port costs we do not see anything related to an etherchannel.
I also found by testing that the cost of a Portchannel on IOS is 3.
(10Gig is 2, Gig is 4) (see
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/122sx/swcg/s
pantree.htm#wp1020643 )
Nevertheless I cannot find any documentation to support my findings. Can
anyone point me to any link? Or is the cost simply being calculated by
dividing BW/200000000? If it is, things do not add up.
Calculating the Port Cost Using the Long Method
802.1t assigns 32-bit (long) default port cost values to each port using
a formula that is based on the bandwidth of the port. You can also
manually assign port costs between 1-200,000,000. The formula for
obtaining default 32-bit port costs is to divide the bandwidth of the
port by 200,000,000. Table 8-2
<http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/sw_8_5/conf
g_gd/spantree.htm#wp1205114> shows the default port cost values that
are assigned by the switch and the recommended cost values and ranges
for each type of port when you use the long method to calculate port
cost.
Calculating the Port Cost for Aggregate Links
As individual links are added or removed from an aggregate link (port
bundle), the bandwidth of the aggregate link increases or decreases.
These changes in bandwidth lead to recalculation of the default port
cost for the aggregated port. Changes to the default port cost or
changes resulting from links that autonegotiate their bandwidth could
lead to recalculation of the spanning-tree topology which may not be
desirable, especially if the added or removed link is of little
consequence to the bandwidth of the aggregate link (for example, if a
10-Mbps link is removed from a 10-Gbps aggregate link). Because of the
limitations that are presented by automatically recalculating the
topology, 802.1t states that changes in bandwidth will not result in
changes to the cost of the port. The aggregated port will use the same
port cost parameters as a standalone port.
TIA
Gustavo Novais
This archive was generated by hypermail 2.1.4 : Sat Jul 01 2006 - 07:57:33 ART