Refine Port Parameters; Rapid Spanning Tree Protocol (Ieee 802.1W) - Symbol ES3000 Manual

5-4 ES3000 Advanced Concept Guide

5.3.2 Refine Port Parameters

When choosing a path from one switch to another, the spanning algorithm will:
• Choose the port marked with the highest priority (lowest number)
• If there is more than one port with the (same) highest priority, choose the port with the
lowest cost
• If there is more than one port with the lowest cost, choose the port with the lowest MAC
number
By default, all ports will have the same priority. By default 10BaseT links have a cost of 2000000,
100BaseT links have a cost of 200000 and Gigabit links have a cost of 20000.
If desired, the port priority and cost parameters can be tuned on the CIST Basic Port Config. page
(Spanning Tree > MSTP Config. > CIST Basic Port Config). The other parameters on this page have no
effect on STP spanning trees. They are used in MSTP spanning trees.

5.4 Rapid Spanning Tree Protocol (IEEE 802.1W)

STP has the disadvantage that it can take a long time, up to fifty seconds, to select a root switch and
to prune the set of all links to a non-looping tree. Under STP, this selection of a root switch and
pruning to a non-looping tree must be duplicated every time a change is made to the network
topology. Since traffic is not forwarded during the reconfiguration, this can lead to unacceptable
interruptions in service. Rapid Spanning Tree Protocol (RSTP) can trim the reconfiguration time to a
second or less.
RSTP acquires and stores more detailed information on the network topology. When the topology
changes, RSTP reconfigures the spanning tree incrementally rather than building the entire spanning
tree from scratch. The combination of the two results in a much faster reconfiguration time.
For example, besides classifying ports as root ports and designated ports, RSTP also classifies ports
as alternate ports and backup ports. Alternate ports provide alternate links to the root switch than
the current root port in use. Backup ports provide a backup link to the destination switches other than
the designated port currently in use. When a given path from the root switch toward the leaves of the
spanning tree fails, RSTP provides that the switch still connected to root propose use of the alternate
port to the switch on the other side of the that port. If that switch accepts, the spanning tree change
is made and forwarding resumes.
RSTP also distinguishes edge ports and point-to-point link ports. Edge ports are ports that connect to
a single end station. Edge ports do not take part in spanning tree reconfiguration, enabling faster