ROUTING FUNCTIONALITY
EMME USER’S GUIDE
Page 1-13
1.5.2 Spanning Tree Algorithm
The Spanning Tree Algorithm (STA) is used by bridges to detect data
loops (duplicate data paths). The bridges will then automatically break the
loop and use the now blocked path as a backup in case the primary path
fails.
When a bridge is powered up, it goes through a series of self tests to
check its internal operation. During this time the bridge is in a standby
condition and does not forward traffic. Also during this standby period,
the bridge sends out special bridge management packets called
configuration Bridge Protocol Data Packets (BPDU). Bridges use the
BPDUs as a way of communicating with each other. The purpose of the
configuration BDPU is to notify other bridges on all of the connected
networks of the current topology. Based on the bridge priority and
address, the other bridges will automatically detect loops and negotiate a
single path. The bridge or bridges involved in this primary data path will
then come on-line and the bridges with lower priority involved in the
backup path(s) will go into a blocking condition.
The other type of BPDU is the topology change BPDU. This BPDU is
made up of four bytes and notifies the other bridges that a change has
taken place. Upon receipt of the topology change BPDU the bridges will
re-arbitrate, or re-span, to form a legal topology.
1.6
ROUTING FUNCTIONALITY
For routing functionality in the EMME, the following is needed:
•
Routing Services software for the EMME module. A license must be
purchased for each module upgrade.
•
8-MB memory upgrade to bring the Dynamic Random Access
Memory (DRAM) up to a total of 12 MB. For information about 8-MB
upgrade kits, refer to the EMME Upgrades section at the end of this
chapter.
