Cisco Cisco 1700 2600 3600 3700 Series VPN Module White Paper

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Virtual Switch Link

The Cisco Catalyst 6500 Series Virtual Switching System consists of two Cisco Catalyst 6500 chassis. In order to

bond the two chassis together into a single, logical node, special signaling and control information must be

exchanged between the two chassis in a timely manner. To facilitate this information exchange, a dedicated link is

used to transfer both data and control traffic between the peer chassis. This link is referred to as the virtual switch

link (VSL).

The VSL, formed as a Cisco EtherChannel interface, can comprise links ranging from one to eight physical

member ports. These links carry two types of traffic: the Cisco Virtual Switching System control traffic and normal

data traffic.

To make sure that control traffic gets highest priority across the VSL, a special bit is set on all VSL control frames.

This helps ensure that these frames always get priority service from both ingress and egress hardware queues.

From a data-plane perspective, the VSL is used to extend the internal chassis data path to the neighboring

chassis. Data traffic sent on the VSL is load-balanced, using the configured Cisco EtherChannel load-balancing

algorithms.

All frames that are sent across the VSL are encapsulated with a virtual switch header (VSH), which is appended to

the frame by the egress port application-specific integrated circuit (ASIC) and striped off on the other side of the

VSL by the ingress port ASIC. The VSH carries information such as the ingress port index, destination port index,

VLAN, class of service (CoS), etc. The size of the VSH is the same as that of the internal compact header used by

the Cisco Catalyst 6500; it is 32 bytes long. This header is placed after the Ethernet preamble and directly before

the Layer 2 header.

Figure 4. Virtual Switch Header

VSL Initialization

The VSS initialization process must determine which supervisor engine will become the active supervisor for the

VSS. In order to determine the supervisor active and standby roles, the VSL must be initialized and brought online

for control plane communication. Because this determination affects the behavior of each switch, the roles must be

negotiated very early during the chassis bootup cycle. As a result, the system must bring the VSL and its

associated ports online before initializing the rest of the system.

Communication between the two chassis is facilitated with internal messaging that is sent across the VSL.

Because the VSL is implemented as a Cisco EtherChannel interface, it is resilient to single-link failures.