Specifications

132 IBM Flex System p260 and p460 Planning and Implementation Guide
Virtual Link Aggregation Groups (VLAGs)
In many data center environments, downstream switches connect to upstream
devices which consolidate traffic, as shown in Figure 5-2.
Figure 5-2 Typical switching layers with STP versus VLAG
A switch in the access layer might be connected to more than one switch in the
aggregation layer to provide network redundancy. Typically, the Spanning Tree
Protocol is used to prevent broadcast loops, blocking redundant uplink paths.
This setup has the unwanted consequence of reducing the available bandwidth
between the layers by as much as 50%. In addition, STP might be slow to
resolve topology changes that occur during a link failure, which can result in
considerable MAC address flooding.
Using Virtual Link Aggregation Groups (VLAGs), the redundant uplinks remain
active, using all the available bandwidth. Using the VLAG feature, the paired
VLAG peers appear to the downstream device as a single virtual entity for
establishing a multiport trunk. The VLAG-capable switches synchronize their
logical view of the access layer port structure and internally prevent implicit
loops. The VLAG topology also responds more quickly to link failure and does
not result in unnecessary MAC address flooding.
Important: To avoid possible issues when you replace a failed switch module,
do
not use automatic failback for NIC teaming. A newly installed switch
module has no configuration data, and it can cause service disruption.
STP blocks
implicit loops
Aggregation
Layer
Access
Layer
Servers
VLAGs
VLAG
Peers
Links remain
active
ISL