Alcatel Carrier Internetworking Solutions Omni Switch/Router - Manual (Page 471)

Configuring Spanning Tree
Page 17-23
Configuring Spanning Tree
Spanning Tree is an algorithm developed to help prevent the occurrence of broadcast storms
in a network. A packet can be broadcast multiple times in a network if the network is physi-
cally configured with loops.
If packets are broadcast to all ports (or flooded) in an attempt to deliver the data, networks
with physical loops will rebroadcast packets repeatedly and cause a network to become
severely congested. This congestion will adversely affect network performance.
Spanning Tree prevents broadcast storms by establishing a loop-free topology throughout the
network. This is done by blocking ports in the physical topology that could result in flooded
traffic being looped.
Both the IEEE and IBM versions of spanning tree are supported in the OmniSwitch/Router.
The IBM Spanning Tree protocol is only supported by IBM Token Ring environments that
make use of functional addresses for the transmission of Bridge Protocol Data Units (BPDUs).
The following are the primary differences between the IEEE 802.1d and IBM Spanning Tree
algorithms:
• The Hello BPDU in IBM Spanning Tree is sent to the bridge functional address,
X’C00000000100’. In the IEEE 802.1d Spanning Tree, it is sent to the Group address
X’800143000000’.
• The Port ID in IBM Spanning Tree consists of a ring identifier and a bridge number. In
802.1d, it consists of a port priority and port number.
• IBM Spanning Tree has no learning process. Therefore, a port can be in one of three
states—blocking, listening, or forwarding.
• IBM Spanning Tree does not support the Topology Change Notification (TCN) protocol.
• When you enable IBM Spanning Tree, the switch automatically sets defaults for the maxi-
mum age, forward delay, and hello time. In the interests of screen consistency, it is possi-
ble to change these defaults with the UI. In IBM Spanning Tree specification, these values
are fixed, and should remain at the set defaults.
• When you enable IBM Spanning Tree, some additional defaults are set:
– All virtual ports attached to the group with a physical port speed of 4 or 16
Mb are set to use Functional Addresses rather than Group Addresses.
– All virtual ports attached to the group with a physical port speed that is not
4 or 16 Mb are set to manual forwarding.
– As other virtual ports are attached to the group, the above two rules are
applied.
Virtual ports in a manual forwarding state do not participate in either the IEEE or IBM
versions of spanning tree. Any IEEE Spanning Tree frame received on a port in a manual
forwarding state is forwarded to all other virtual ports in the same group also in a manual
forwarding state. This is done to prevent loops from occurring in the network topology
that could arise from applying the second default condition automatically.