Intel ER8100STUS - Express 8100 Router Reference Manual page 125

Packet propagation
7 hop limit
Configuration BPDU
messages
Configuration BPDU
message propagation
By enforcing this strict hierarchy, and by only forwarding packets between Root
Ports and Designated Ports, the possibility of bridging loops is removed.
A
Designated Port
Blocking
Root
Root Ports
Designated
Bridge
Designated Port
B
Designated Ports
In the above diagram, packets sent from Bridge B to Bridge A are discarded by
the blocking port on Bridge A. Packets are never forwarded from the blocking
port on Bridge A. Packets can therefore not be sent directly between Bridges A
and B except via the Root Bridge.
In addition to the strict bridging hierarchy imposed by the Spanning Tree Algo-
rithm, a 7 hop limit is introduced. This means that any packets cannot pass more
than 7 bridges. This effectively limits the practical size of the bridged network
when using the Spanning Tree Algorithm.
To establish the optimal paths (during the Learning State), each bridge sends
configuration BPDU packets to its neighboring bridges. These Configuration
BPDU messages contain information about the spanning tree topology. The con-
tents of these packets only changes when the bridged network topology changes
or has not been established.
When the optimal paths within the network have been established, each bridge
continues to send configuration BPDU packets to its neighboring bridges to tell
them that it is still alive.
When a bridged network is in a stable condition, bridges continue to send con-
figuration BPDU packets to its neighboring bridges at regular intervals. Config-
uration BPDU packets are transmitted down the spanning tree from Designated
Ports to Root Ports. If a Configuration BPDU is not received by the Root Port on
a bridge in a predefined time interval (for example because a bridge along the
path has dropped out), the link enters the Listening State to redetermine its opti-
mal path.
Root Port
Designated Port
Port
Root Ports
1062
WAN Bridging Services
Spanning Tree Algorithm
113