Owner's Manual

Chapter 18. STP | 521
NETGEAR 8800 User Manual
Figure 32. 802.1D-1998 and 802.1D-2004 Mixed Bridge Topology
If you use the default port path costs, bridge D blocks its port to bridge E, and all traffic
between bridges D and E must traverse all of bridges in the network. Bridge D blocks its port
to bridge E because the path cost to the root bridge is less by going across bridges B and C
(with a combined root cost of 38) compared with going across bridge E (with a root cost of
200,000). In fact, if there were 100 bridges between bridges B, C, and D running the old
802.1D-1998 standard with the default port path costs, bridge D would still use that path
because the path cost is still higher going across bridge E.
As a workaround and to prevent this situation, configure the port path cost to make links with
the same speed use the same path host value. In the example described above, configure
the port path cost for the 802.1D-2004 compliant bridges (bridges A, D, E, and F) to 19.
Note: You cannot configure the port path cost on bridges B and C to
200,000 because the path cost range setting for 802.1D-1998
compliant bridges is 1 to 65,535.
To configure the port path cost, use the following command:
configure stpd <stpd_name> ports cost [auto | <cost>] <port_list>
Bridge Priority
By configuring the STPD bridge priority, you make the bridge more or less likely to become
the root bridge. Unlike the 802.1D-1998 standard, the 802.1D-2004 standard restricts the
bridge priority to a 16-bit number that must be a multiple of 4,096. The new priority range is 0
to 61,440 and is subject to the multiple of 4,096 restriction. The old priority range was 0 to
EX_179
Switch A
Root
bridge
Switch B
Switch C
Switch D
Blocked
IEEE 802.1D-1998
IEEE 802.1D-2004
Switch E
Switch F