Stp/Rstp; Resilient Links - 3Com 5500-SI Configuration Manual

How XRN Interacts with other Features 683

STP/RSTP

STP/RSTP should be used for multihomed links if you are not able to use
aggregated links. Figure 184 shows how STP will prevent a loop occurring on a
multihomed link.
STP/RSTP should always be enabled if your multihomed links are aggregated links.
Figure 182 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
occurring.
Figure 184 How XRN interacts with STP/RSTP

Resilient Links

In Figure 185, 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.
For example, if the resilient links were configured on Switches A and B, if the
interconnect fails, both Switches will detect a failed link to Switch 3300 and both
A and B will activate their links to Switch 3300. So both links in the resilient link
will be passing traffic, potentially causing a loop in the network.
Figure 185 How XRN interacts with Resilient Links
Switch 5500
Resilient Link
Active Link Standby Link
XRN Distributed Fabric
Switch A Switch B