Olicom CrossFire 8730, Reference Manual

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CrossFire 8730 Switch Reference Guide, DOC-7047 v. 1.1 Switch Overview
Preventing Frame Looping
The previous network scenario was free of the ARE looping issue because the
entire Ethernet cloud had the same LAN Segment number.
In the next scenario, the Fast Ethernet ports have different LAN segment numbers.
In this configuration, an infinite loop is created in addition to the frame duplication
issue.
Station A sends an ARE frame to Ring1 – ex. 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 Fast Ethernet port 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 link 1 back to the Ethernet, and so
on. See Figure 9. below.
Figure 9. Frame Looping
This loop can be resolved by making sure that only one Fast Ethernet link can be in
the forwarding state or by using the same LAN segment number for all Fast
Ethernet links.
Using two different LAN segment numbers has some interesting advantages. If the
Ethernet cloud becomes physically separated into two parts (see below), both parts
can still communicate via the Token-Ring cloud. If the Ethernet cloud is not split
and the root of the spanning tree is on the Token-Ring side, there might be a loss of
connectivity between Ethernet stations depending on the CRF numbers configured.
Ethernet cloud with different LAN
segment numbers on each CRF
Redundant
connections
to Ethernet
ARE frame
generated from
Station A
LAN segment 4
LAN segment 3
Ring 1
Station A
Token-Ring
cloud
Station B
Bridge 1
Ring 2
CrossFire 8730 - Link 1
Forwarding
CrossFire 8730 - Link 2
Forwarding