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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
another can be filtered from the network. The entry may specified as a source, a destination,
or both (switch must be in IP Routing mode).
Spanning Tree
The IEEE 802.1D Spanning Tree Protocol allows for the blocking of links between switches that form
loops within the network. When multiple links between switches are detected, a primary link is
established. Duplicated links are blocked from use and become standby links. The protocol allows for
the duplicate links to be used in the event of a failure of the primary link. Once the Spanning Tree
Protocol is configured and enabled, primary links are established and duplicated links are blocked
automatically. The reactivation of the blocked links (at the time of a primary link failure) is also
accomplished automatically – without operator intervention.
The DES-3326 STP allows two levels of spanning trees to be configured. The first level constructs a
spanning tree on the links between switches. This is referred to as the Switch or Global level. The
second level is on a port group basis. Groups of ports are configured as being members of a spanning
tree and the algorithm and protocol are applied to the group of ports. This is referred to as the Port or
VLAN level.
On the switch level, STP calculates the Bridge Identifier for each switch and then sets the Root Bridge
and the Designated Bridges.
On the port level, STP sets the Root Port and the Designated Ports.
The following are the user-configurable STP parameters for the switch level:
a
meter Description Default
Value
Bridge Identifier
(Not user-
configurable
except by setting
priority below)
A combination of the User-
set priority and the switch’s
MAC address. The Bridge
Identifier consists of two
parts: a 16-bit priority and
a 48-bit Ethernet MAC
address
32768 + MAC
Priority A relative priority for each
switch – lower numbers give
a higher priority and a
greater chance of a given
switch being elected as the
root bridge
32768
Hello Time The length of time between
broadcasts of the hello
message by the switch
2 seconds
Maximum Age
Timer
Measures the age of a
received BPDU for a port
and ensures that the BPDU
is discarded when its age
exceeds the value of the
maximum age timer.
20 seconds
Forward Delay
Timer
The amount time spent by a
port in the learning and
listenin
g
states waitin
g
for a
15 seconds
30