LevelOne GEP-5070, User Manual

Page: 127 / 318

C
HAPTER
4
| Configuring the Switch
Configuring the Spanning Tree Algorithm
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C
ONFIGURING THE
S
PANNING
T
REE
A
LGORITHM
The Spanning Tree Algorithm (STA) can be used to detect and disable
network loops, and to provide backup links between switches, bridges or
routers. This allows the switch to interact with other bridging devices (that
is, an STA-compliant switch, bridge or router) in your network to ensure
that only one route exists between any two stations on the network, and
provide backup links which automatically take over when a primary link
goes down.
The spanning tree algorithms supported by this switch include these
versions:
◆
STP – Spanning Tree Protocol (IEEE 802.1D)
◆ RSTP – Rapid Spanning Tree Protocol (IEEE 802.1w)
◆ MSTP – Multiple Spanning Tree Protocol (IEEE 802.1s)
STP - STP uses a distributed algorithm to select a bridging device (STP-
compliant switch, bridge or router) that serves as the root of the spanning
tree network. It selects a root port on each bridging device (except for the
root device) which incurs the lowest path cost when forwarding a packet
from that device to the root device. Then it selects a designated bridging
device from each LAN which incurs the lowest path cost when forwarding a
packet from that LAN to the root device. All ports connected to designated
bridging devices are assigned as designated ports. After determining the
lowest cost spanning tree, it enables all root ports and designated ports,
and disables all other ports. Network packets are therefore only forwarded
between root ports and designated ports, eliminating any possible network
loops.
Figure 45: STP Root Ports and Designated Ports
Once a stable network topology has been established, all bridges listen for
Hello BPDUs (Bridge Protocol Data Units) transmitted from the Root Bridge.
If a bridge does not get a Hello BPDU after a predefined interval (Maximum
Age), the bridge assumes that the link to the Root Bridge is down. This
bridge will then initiate negotiations with other bridges to reconfigure the
network to reestablish a valid network topology.
RSTP - RSTP is designed as a general replacement for the slower, legacy
STP. RSTP is also incorporated into MSTP. RSTP achieves must faster
reconfiguration (i.e., around 1 to 3 seconds, compared to 30 seconds or
more for STP) by reducing the number of state changes before active ports
start learning, predefining an alternate route that can be used when a node
or port fails, and retaining the forwarding database for ports insensitive to
changes in the tree structure when reconfiguration occurs.
x
Designated
Root
Designated
Port
Designated
Bridge
x x
x
Root
Port
x