162 Managing a Switch Stack
NSF in the Data Center
Figure 8-12 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
Management Unit. 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 8-12. Data Center Stack Topology
LAG1
LAG2
Unit 1
Unit 2
AS1
AS2
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