2.1.h [ii] VSS concepts
The VSS incorporates the following key concepts:
● VSS Active and VSS Standby Chassis
● Virtual Switch Link
● Multi-chassis EtherChannel
When you create or restart a VSS, the peer chassis negotiate their roles. One chassis becomes the VSS active chassis, and the other chassis becomes the VSS standby. The VSS active chassis controls the VSS and runs the Layer 2 and Layer 3 control protocols for the switching modules on both chassis. The VSS active chassis also provides management functions for the VSS, such as module online insertion and removal (OIR) and the console interface. The VSS active and VSS standby chassis perform packet forwarding for ingress data traffic on their locally hosted interfaces . However, the VSS standby chassis sends all control traffic to the VSS active chassis for processing.
For the two chassis of the VSS to act as one network element, they need to share control information and data traffic. The virtual switch link (VSL ) is a special link that carries control and data traffic between the two chassis of a VSS. The VSL is implemented as an EtherChannel with up to eight links. The VSL gives control traffic higher priority than data traffic so that control messages are never discarded. Data traffic is load balanced among the VSL links by the EtherChannel load-balancing algorithm.
An EtherChannel (also known as a port channel) is a collection of two or more physical links that combine to form one logical link. Layer 2 protocols operate on the EtherChannel as a single logical entity. A Multi-chassis EtherChannel (MEC) is a port channel that spans the two chassis of a VSS. The access switch views the MEC as a standard port channel.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1872-1879). . Kindle Edition.