3Com 5500-SI Configuration Manual page 685

How a Failure affects the Distributed Fabric 685
Router
Switch B will continue to do all the routing. As it was routing prior to Switch A's
failure there will be no change of the router identity, that is, the router interface IP
addresses will not change. The router interface MAC addresses may change but
this will have no visible impact on your network. Any MAC address change is
propagated to your network by the issuing of gratuitous ARP messages.
Switch A Recovery
When Switch A recovers and starts to operate again, all links will reconfigure
themselves as they were before the failure, according to the protocols used. The
routing task will once again be shared between Switches A and B, using the same
IP address.
Loss of the Fabric When an interconnect fails between two Switches in the Distributed Fabric,
Interconnect assuming you have STP/RSTP and LACP enabled as recommended in "Important
Considerations and Recommendations" on page 676, your traffic flow should
continue through your network.
Figure 187 XRN Network reaction on Fabric Interconnect failure
Switch 4300
Switch 4200
STP/RSTP enabled
Legacy
Aggregated Link
Switch 4400 units
X
Switch 3300
Resilient Link
802.3ad
Standby Link
Active Link Aggregated Link
XRN Distributed Fabric
Switch A Switch B
Interconnect Failure
In Figure 187, if the interconnect fails, the network will react in the following way:
LACP (IEEE 802.3ad) and Legacy Aggregated Links
The Switch 4400 automatically configured aggregated link (LACP) will reconfigure
itself to create two separate aggregated links.
The Switch 4300 legacy aggregated link will be split between the two Switches in
the Distributed Fabric and will no longer operate and will cause network
disruption.
Legacy aggregated links are not resilient to an interconnect failure. Hence the
3Com recommendation to use IEEE 802.3ad aggregated links (LACP) for maximum
resilience.