Dell PowerConnect M6220, User Configuration Manual

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Managing a Switch Stack
NSF in the Data Center
Figure 8-14 illustrates a data center scenario, where the stack of two
PowerConnect switches acts as an access switch. The access switch is
connected to two aggregation switches, AS1 and AS2. The stack has a link
from two different units to each aggregation switch, with each pair of links
grouped together in a LAG. The two LAGs and link between AS1 and AS2 are
members of the same VLAN. Spanning tree is enabled on the VLAN. Assume
spanning tree selects AS1 as the root bridge. Assume the LAG to AS1 is the
root port on the stack and the LAG to AS2 is discarding. Unit 1 is the stack
master. If unit 1 fails, the stack removes the Unit 1 link to AS1 from its LAG.
The stack forwards outgoing packets through the Unit 2 link to AS1 during
the failover. During the failover, the stack continues to send BPDUs and LAG
PDUs on its links on Unit 2. The LAGs stay up (with one remaining link in
each), and spanning tree on the aggregation switches does not see a topology
change.
Figure 8-14. Data Center Stack Topology