Chapter 4: Advanced Topics; Layer 2 Switching; Spanning Tree Algorithm - Alcatel OmniStack 8008 User Manual

Chapter 4: Advanced Topics

The Alcatel OmniStack 8008 supports Layer 2 switching and other advanced
®
features, which are described in this chapter.

Layer 2 Switching

When a frame enters a port, its destination MAC address is checked in the address
database to see which port leads to this destination. If the destination address
belongs to the incoming port, the frame is dropped or "filtered" because it is
addressed to the local segment. If the destination address is found on another port,
the frame is forwarded to that port and queued for output. But, if the destination
address is not found in the address database, the frame is sent to one or more
output ports based on the rules for handling tagged or untagged VLAN frames.
If the source MAC address of the frame was not found in the address database, it is
recorded along with the incoming port number where it entered the switch. This
information is then used to make later decisions for frame forwarding.
Switching involves the following steps:
• VLAN Classification
• Learning
• Filtering
• Forwarding
• Aging

Spanning Tree Algorithm

The Spanning Tree Algorithm (that is, the STA configuration algorithm as outlined in
IEEE 802.1D) can be used to detect and disable network loops, and to provide link
backup. This allows the switch to interact with other bridging devices (including
STA-compliant switches, bridges or routers) in your network to ensure that only one
route exists between any two stations on the network. If redundant paths or loops
are detected, one or more ports are put into a blocking state (stopped from
forwarding packets) to eliminate the extra paths. Moreover, if one or more of the
paths in a stable spanning tree topology fail, this algorithm will automatically change
ports from blocking state to forwarding state to reestablish contact with all network
stations.
The STA uses a distributed algorithm to select a bridging device (STA-compliant
switch, bridge or router) that serves as the root of the spanning tree network. It
selects a root port on each bridging device (except for the root device) which incurs
the lowest path cost when forwarding a packet from that device to the root device.
Then it selects a designated bridging device from each LAN which incurs the lowest
path cost when forwarding a packet from that LAN to the root device. All ports
connected to designated bridging devices are assigned as designated ports. After
determining the lowest cost spanning tree, it enables all root ports and designated
ports, and disables all other ports. Network packets are therefore only forwarded
between root ports and designated ports, eliminating any possible network loops.
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