Dell Broadcom NetXtreme Family of Adapters - Manual (Page 121)

Broadcom Teaming Services: Broadcom NetXtreme II® Network Adapter User Guide
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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.
Layer 3 Routing/Switching
The switch that the teamed ports are connected to must not be a Layer 3 switch or router. The ports in the team must be in
the same network.
Teaming with Hubs (for troubleshooting purposes only)
Hub Usage in Teaming Network Configurations
SLB Teams
SLB Team Connected to a Single Hub
Generic and Dynamic Trunking (FEC/GEC/IEEE 802.3ad)
SLB teaming can be used with 10/100 hubs, but it is only recommended for troubleshooting purposes, such as connecting a
network analyzer in the event that switch port mirroring is not an option.
Hub Usage in Teaming Network Configurations
Although the use of hubs in network topologies is functional in some situations, it is important to consider the throughput
ramifications when doing so. Network hubs have a maximum of 100 Mbps half-duplex link speed, which severely degrades
performance in either a Gigabit or 100 Mbps switched-network configuration. Hub bandwidth is shared among all connected
devices; as a result, when more devices are connected to the hub, the bandwidth available to any single device connected to
the hub is reduced in direct proportion to the number of devices connected to the hub.
It is not recommended to connect team members to hubs; only switches should be used to connect to teamed ports. An SLB
team, however, can be connected directly to a hub for troubleshooting purposes. Other team types can result in a loss of
connectivity if specific failures occur and should not be used with hubs.