Chapter 11. Virtual Link Aggregation Groups - Lenovo CN4093 Application Manual

Chapter 11. Virtual Link Aggregation Groups

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In many data center environments, downstream servers or switches connect to
upstream devices which consolidate traffic. For example, see
Figure 16. Typical Data Center Switching Layers with STP vs. VLAG
As shown in the example, a switch in the access layer may be connected to more
than one switch in the aggregation layer to provide for network redundancy.
Typically, Spanning Tree Protocol (RSTP, PVRST or MSTP—see
Protocols" on page 151) is used to prevent broadcast loops, blocking redundant
uplink paths. This has the unwanted consequence of reducing the available
bandwidth between the layers by as much as 50%. In addition, STP may be slow to
resolve topology changes that occur during a link failure, and can result in
considerable MAC address flooding.
Using Virtual Link Aggregation Groups (VLAGs), the redundant uplinks remain
active, utilizing all available bandwidth.
Two switches are paired into VLAG peers, and act as a single virtual entity for the
purpose of establishing a multi-port aggregation. Ports from both peers can be
grouped into a VLAG and connected to the same LAG-capable target device. From
the perspective of the target device, the ports connected to the VLAG peers appear
to be a single LAG connecting to a single logical device. The target device uses the
configured Tier ID to identify the VLAG peers as this single logical device. It is
important that you use a unique Tier ID for each VLAG pair you configure. 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 flooding.
VLAGs are also useful in multi-layer environments for both uplink and downlink
redundancy to any regular LAG-capable device. For example:
Aggregation
Layer
STP blocks
VLAGs
implicit loops
Access
Layer
Servers
Figure 16.
ISL
VLAG
Peers
Links remain
active
"Spanning Tree
173