Broadcom NetXtreme BCM57 Series, Руководство пользователя

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Broadcom Gigabit Ethernet Teaming Services NetXtreme User Guide
Broadcom
®
April 2017 • 2CS57XX-CDUM514-R Page 124
After a stable network topology has been established, all bridges listen for hello BPDUs (Bridge Protocol Data
Units) transmitted from the root bridge. If a bridge does not get a hello BPDU after a predefined interval (Max
Age), the bridge assumes that the link to the root bridge is down. This bridge then initiates negotiations with
other bridges to reconfigure the network to re-establish a valid network topology. The process to create a new
topology can take up to 50 seconds. During this time, end-to-end communications are interrupted.
The use of Spanning Tree is not recommended for ports that are connected to end stations, because by
definition, an end station does not create a loop within an Ethernet segment. Additionally, when a teamed
adapter is connected to a port with Spanning Tree enabled, users may experience unexpected connectivity
problems. For example, consider a teamed adapter that has a lost link on one of its physical adapters. If the
physical adapter were to be reconnected (also known as fallback), the intermediate driver would detect that the
link has been reestablished and would begin to pass traffic through the port. Traffic would be lost if the port was
temporarily blocked by the Spanning Tree Protocol.
Topology Change Notice (TCN)
A bridge/switch creates a forwarding table of MAC addresses and port numbers by learning the source MAC
address that received on a particular port. The table is used to forward frames to a specific port rather than
flooding the frame to all ports. The typical maximum aging time of entries in the table is 5 minutes. Only when
a host has been silent for 5 minutes would its entry be removed from the table. It is sometimes beneficial to
reduce the aging time. One example is when a forwarding link goes to blocking and a different link goes from
blocking to forwarding. This change could take up to 50 seconds. At the end of the STP re-calculation a new
path would be available for communications between end stations. However, because the forwarding table
would still have entries based on the old topology, communications may not be reestablished until after 5
minutes when the affected ports entries are removed from the table. Traffic would then be flooded to all ports
and re-learned. In this case it is beneficial to reduce the aging time. This is the purpose of a topology change
notice (TCN) BPDU. The TCN is sent from the affected bridge/switch to the root bridge/switch. As soon as a
bridge/switch detects a topology change (a link going down or a port going to forwarding) it sends a TCN to the
root bridge via its root port. The root bridge then advertises a BPDU with a Topology Change to the entire
network.This causes every bridge to reduce the MAC table aging time to 15 seconds for a specified amount of
time. This allows the switch to re-learn the MAC addresses as soon as STP re-converges.
Topology Change Notice BPDUs are sent when a port that was forwarding changes to blocking or transitions to
forwarding. A TCN BPDU does not initiate an STP recalculation. It only affects the aging time of the forwarding
table entries in the switch.It will not change the topology of the network or create loops. End nodes such as
servers or clients trigger a topology change when they power off and then power back on.
Port Fast/Edge Port
To reduce the effect of TCNs on the network (for example, increasing flooding on switch ports), end nodes that
are powered on/off often should use the Port Fast or Edge Port setting on the switch port they are attached to.
Port Fast or Edge Port is a command that is applied to specific ports and has the following effects:
• Ports coming from link down to link up will be put in the forwarding STP mode instead of going from
listening to learning and then to forwarding. STP is still running on these ports.
• The switch does not generate a Topology Change Notice when the port is going up or down.