Implementing IBM System Networking 10Gb Ethernet Switches

Chapter 2. IBM System Networking Switch 10Gb Ethernet switch features 79

2.7.2 Virtual Link Aggregation Groups

In Figure 2-13, we show a typical data center design environment with access and

aggregation layers.

Figure 2-13 Spanning Tree Protocoal versus Virtual Link Aggregation Groups

As shown in Figure 2-13, a switch in the access layer may be connected to more than one

switch in the aggregation layer to provide network redundancy. Typically, STP (see 2.1,

“Virtual Local Area Networks” on page 52) is used to prevent forwarding loops by blocking

redundant uplink paths. This configuration 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, and can result in considerable

MAC address flooding. Using Virtual Link Aggregation Groups (VLAGs), the redundant

uplinks remain active, using all available bandwidth.

Using the VLAG feature, switches can be paired as VLAG peers. The peer switches appear

to the connecting 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 flooding.

VLAGs are useful in multilayer environments for both uplink and downlink redundancy to any

regular LAG-capable device. They can also be used in for active-active VRRP connections.

2.7.3 Hot links

For network topologies that require STP to be turned off, Hot Links provides basic link

redundancy with fast recovery.

Hot Links consists of up to 25 triggers. A trigger consists of a pair of Layer 2 interfaces, each

containing an individual port, trunk, or LACP adminkey. One interface is the Master, and the

other is a Backup. Although the Master interface is set to the active state and forwards traffic,

the Backup interface is set to the standby state and blocks traffic until the Master interface

fails. If the Master interface fails, the Backup interface is set to active and forwards traffic.

After the Master interface is restored, it changes to the standby state and blocks traffic until

the Backup interface fails.

Aggregation Layer

Access Layer

...

STP blocks

implicit loops

X

X

X

...

Servers

VLAGs

Links remain

active

VLAG

peers