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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter 1 Product Overview
Layer 2 Software Features
802.1Q and Layer 2 Protocol Tunneling
802.1Q tunneling is a Q-in-Q technique that expands the VLAN space by retagging the tagged packets
that enter the service provider infrastructure. 802.1Q tunneling allows service providers to assign a
VLAN to each customer without losing the original customer VLAN IDs inside the tunnel. All data
traffic that enters the tunnel is encapsulated with the tunnel VLAN ID. Layer 2 Protocol Tunneling is a
similar technique for all Layer 2 control traffic. 802.1Q tunneling and Layer 2 Protocol Tunneling are
supported on Supervisor Engine V only.
For information on configuring 802.1Q tunneling, see Chapter 18, “Configuring 802.1Q and Layer 2
Protocol Tunneling.”
CDP
The Cisco Discovery Protocol (CDP) is a device-discovery protocol that is both media- and
protocol-independent. CDP is available on all Cisco products, including routers, switches, bridges, and
access servers. Using CDP, a device can advertise its existence to other devices and receive information
about other devices on the same LAN. CDP enables Cisco switches and routers to exchange information,
such as their MAC addresses, IP addresses, and outgoing interfaces. CDP runs over the data-link layer
only, allowing two systems that support different network-layer protocols to learn about each other. Each
device configured for CDP sends periodic messages to a multicast address. Each device advertises at
least one address at which it can receive Simple Network Management Protocol (SNMP) messages.
For information on configuring CDP, see Chapter 19, “Understanding and Configuring CDP.”
EtherChannel Bundles
EtherChannel port bundles allow you to create high-bandwidth connections between two switches by
grouping multiple ports into a single logical transmission path.
For information on configuring EtherChannel, see Chapter 16, “Understanding and Configuring
EtherChannel.”
Jumbo Frames
The jumbo frames feature allows the switch to forward packets as large as 9216 bytes (larger than the
IEEE Ethernet MTU), rather than declare those frames “oversize” and discard them. This feature is
typically used for large data transfers. The jumbo feature can be configured on a per-port basis on
Layer 2 and Layer 3 interfaces and is supported only on non-blocking GB front ports.
For information on Jumbo Frames, see Chapter 4, “Configuring Interfaces.”
MST
IEEE 802.1s Multiple Spanning Tree (MST) allows for multiple spanning tree instances within a single
802.1Q or Inter-Switch Link (ISL) VLAN trunk. MST extends the IEEE 802.1w Rapid Spanning Tree
(RST) algorithm to multiple spanning trees. This extension provides both rapid convergence and load
balancing within a VLAN environment.