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Port Trunking
Distributed Trunking
Unicast frames are only forwarded by one of the DT switches unless the MAC
address is reachable only through the other DT switch, for example, a host on
DT2 sends or receives frames directly through the DT2 switch.
Forwarding Broadcast, Multicast, and
Unknown Traffic Upstream
When the DT1 switch receives broadcast traffic, multicast traffic, or traffic
with an unknown destination from a server, it broadcasts the traffic across all
the links, including the DT1 link to the DT2 switch. The DT2 switch won’t send
this traffic through the same trunk groups, so the server doesn’t receive the
same traffic that it sent out. The DT2 switch only forwards this traffic to
switches or hosts that are not on its DT links.
Forwarding Unicast Traffic Downstream (to the Server)
In normal trunking, the load balancing is based on the SA/DA MAC address
pair across the links in the trunk (See “Outbound Traffic Distribution Across
Trunked Links” on page 12-37 for more information on SA/DA load balancing).
The last 5 bits of the SA/DA MAC address pair are used in a calculation to
determine the load-balancing across the links. In distributed trunking, this
calculation is extended across the DT switches and its links.
Downstream unicast traffic received by the DT switches is load balanced
across both switches using the DT trunks, based on the SA/DA MAC address
pair. For example, if the DT1 switch and the DT2 switch have the same number
of DT links operational in a trunk group (1 link on DT1 and 1 link on DT2, 2
links on DT1 and 2 links on DT2, etc.), half of the traffic is forwarded by the
DT1 switch and half of the traffic is forwarded by the DT2 switch. This is not
the same as forwarding half of the bandwidth traffic. If DT1 has one link and
DT2 has two links, the SA/DA traffic flow is load balanced with 33% of the
traffic flowing through DT1 and 66% of the traffic flowing through DT2.
Forwarding Broadcast, Multicast, and Unknown Traffic
Downstream (to the Server)
Downstream broadcast traffic, multicast traffic, and traffic with an unknown
destination is sent the same way that unicast traffic is sent, that is, the DT1
switch and DT2 switch forward the traffic proportionately based on the
number of DT links in each switch in a trunk group.
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