3Com SuperStack 4 Configuration Manual page 428

440 A D: 3C XRN
PPENDIX OM
However, in Figure 111, if the interconnect fails, the aggregation is still a single
logical entity at the legacy Switch end, but it is now split over both units within the
Distributed Fabric. The legacy Switch is not aware that the aggregation has split
and will continue to send traffic over both links, resulting in data loss.
Hence the recommendation to use IEEE 802.3ad aggregated links, if possible, as
legacy aggregated links are not resilient to an Fabric Interconnect failure.
Figure 111 How XRN interacts with legacy aggregated links
STP/RSTP STP/RSTP should be used for multihomed links if you are not able to use
aggregated links. Figure 112 shows how STP will prevent a loop occuring on a
multihomed link.
STP/RSTP should always be enabled if your multihomed links are aggregated links.
Figure 110 shows how, on interconnect failure, STP/RSTP will detect the potential
loop caused by the aggregated links splitting and block a path to prevent the loop
occuring.
Figure 112 How XRN interacts with STP/RSTP
Resilient Links In Figure 113, if Switch A within the Distributed Fabric fails, the Switch 3300 will
detect that a link has gone down and will make the standby link to Switch B active
and pass all traffic down the link to Switch B.
When using resilient links in a Distributed Fabric network the resilient links must be
configured at the remote end rather than at the Distributed Fabric. In a unit failure
scenario as described above it would not matter if the resilient links were
configured at the Distributed Fabric end. However, on an interconnect failure it
would matter.