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26
Network Scenarios—Preventing Frame Duplication and
Looping
This section discusses preventing frame duplication and frame looping in networks.
Preventing Frame Duplication
The ARE duplication problem can occur only in a configuration with redundant
Fast Ethernet ports in different switches (as in the configuration that follows).
It is assumed that the network consists of a physically connected Ethernet cloud and
a physically connected Token-Ring cloud. The two clouds are then connected by a
number of links that implement translational bridging. See Figure 8. below.
Figure 8. Preventing Frame Duplication
In a pure Token-Ring network, an All Routes Explorer (ARE) frame generated
from Station A traverses all rings only one time based on a Routing Information
Field (RIF) expanded by each source-routed bridge. In this example, both Fast
Ethernet links are in the forwarding state. An ARE frame originating in Ring1 may
go through the Ethernet cloud and come back to Ring 2. Because the RIF is lost
when passing through the Ethernet cloud, LAN segment 1 is no longer included in
the new RIF and the frame will be forwarded by Bridge 1 from Ring 2 back to Ring
1. This results in frame duplication – after visiting Ring 1 again, the frame will be
discarded by the Fast Ethernet ports connected to Ring 1.
This issue can be resolved by making sure that only one Fast Ethernet port can be
in the forwarding state.
CrossFire 8730 Switch Reference Guide, DOC-7047 v. 1.1
Ethernet cloud
LAN segment 3
ARE frame
Redundant
generated from
connections
Station A
to Ethernet
CrossFire 8730 - Link 1
Forwarding
Ring 1
Station A
CrossFire 8730 - Link 2
Forwarding
Ring 2
Bridge 1
Station B
Token-Ring
cloud
Switch Overview
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