Avaya Media Gateway G250, Administration Manual

Page: 392 / 840

Configuring advanced switching
392 Administration for the Avaya G250 and Avaya G350 Media Gateways
The spanning tree algorithm requires five values to derive the spanning tree topology. These
are:
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A multicast address specifying all bridges on the extended network. This address is
media-dependent and is automatically determined by the software.
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A network-unique identifier for each bridge on the extended network
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A unique identifier for each bridge/LAN interface (a port)
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The relative priority of each port
●
The cost of each port
After these values are assigned, bridges multicast and process the formatted frames (called
Bridge Protocol Data Units, or BPDUs) to derive a single, loop-free topology throughout the
extended network. The bridges exchange BPDU frames quickly, minimizing the time that
service is unavailable between hosts.
Spanning tree per port
Spanning tree can take up to 30 seconds to open traffic on a port. This delay can cause
problems on ports carrying time-sensitive traffic. You can, therefore, enable or disable spanning
tree in the G350 on a per-port basis to minimize this effect.
Rapid Spanning Tree Protocol (RSTP)
The enhanced feature set of the 802.1w standard includes:
●
Bridge Protocol Data Unit (BPDU) type 2
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New port roles: Alternate port, Backup port
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Direct handshaking between adjacent bridges regarding a desired topology change (TC).
This eliminates the need to wait for the timer to expire.
●
Improvement in the time it takes to propagate TC information. Specifically, TC information
does not have to be propagated all the way back to the Root Bridge (and back) to be
changed.
●
Origination of BPDUs on a port-by-port basis
Port roles
At the center of RSTP – specifically as an improvement over STP (802.1d) – are the roles that
are assigned to the ports. There are four port roles:
●
Root port . The port closest to the root bridge
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Designated port . The corresponding port on the remote bridge of the local root port
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Alternate port . An alternate route to the root
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Backup port . An alternate route to the network segment