Cabletron Systems SSIM-H2-02 manual Preventing Frame Looping

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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