Redundant Connections To Fast Ethernet Network; Figure 7. Frame Looping - Enterasys SSIM-H2-02 Installation And User Manual

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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 translational
links are in the forwarding state. An ARE frame originating in Ring1 may go
through the Ethernet cloud and come back to Ring2. 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 Bridge1 from Ring2 back to Ring1.
This results in frame duplication – after visiting Ring1 again, the frame will be
discarded by the switch connected to Ring1.
This issue can be resolved by making sure that only one translational link can be in
the forwarding state.
Preventing Frame Looping
The network presented in Figure 6 was free of the ARE looping issue because the
entire the Ethernet cloud had the same LAN Segment number – each switch had the
same number for its Ethernet ports.
In Figure 7, the switches have different LAN segment numbers for Ethernet ports
(3 and 4). In this configuration, an infinite loop is created in addition to the frame
duplication issue.
Station A sends an ARE frame to Ring1, for example IP ARP with destination
MAC being Broadcast. This frame is converted to a transparent broadcast and sent
on Ethernet link 1. The frame is then received by the switch on link 2 and sent as
an ARE frame to Ring 2 – note that the RIF does not contain LAN Segment 4. The
ARE frame is then forwarded by Bridge 1 and switch link 1 back to the Ethernet,
and so on.

Figure 7. Frame Looping

SSIM-H2-02 Fast Ethernet - Translational Switch SmartStack Interface Module Installation and User Guide Configuration