Cisco Catalyst X4232 Installation And Configuration Note page 37

For more information about GEC, see the "Configuring the Layer 3 Gigabit Ethernet Ports" section on
page 54.
Spanning Tree Protocol
STP is a bridge protocol that enables a learning bridge to dynamically work around loops in a network
topology by creating a spanning tree. Bridges exchange BPDU messages with other bridges to detect
loops and then remove the loops by shutting down selected bridge interfaces.
STP maintains a network of multiple bridges or switches. When the topology changes, the STP
transparently reconfigures bridges and switches to avoid the creation of loops by placing ports in a
forwarding or blocking state. Each bridge group has a separate instance of the STP.
STP parameters are set for each bridge group. For each spanning tree instance, you configure a set of
global options with a set of port parameters. The port parameter list contains only ports that are
members of a given bridge group. The Layer 3 switch routers support a maximum of 16 bridge groups,
which run their own instance of spanning tree.
Virtual LANs
A VLAN configures switches and routers according to logical rather than physical topologies. Using
VLANs, a network administrator can combine any collection of LAN segments within an internetwork
into an autonomous user group, which appears as a single LAN. VLANs logically segment the network
into different broadcast domains so that packets are switched only between ports within the VLAN.
Typically, a VLAN corresponds to a particular subnet, although not necessarily.
For information about InterVLAN routing, see the "Configuring the Module for InterVLAN Routing"
section on page 44.
IEEE 802.1Q VLAN Encapsulation
802.1Q VLAN encapsulation uses a one-level, packet tagging scheme to multiplex VLANs across a
single physical link, while maintaining strict adherence to the individual VLAN domains. 802.1Q can
have access ports or untagged ports where frames are assigned to VLANs based on a port VLAN
identifier (PVID), or native VLAN for the port. It can also have trunked ports where some frames can
be tagged and others untagged. 802.1Q uses Per VLAN Spanning Tree Plus (PVST+), mapping multiple
spanning trees to the spanning tree of pure 802.1Q switches.
IEEE 802.1Q VLAN encapsulation is supported over the internal and external gigabit router ports. See
Figure 10 on page 41
For more information about 802.1Q on internal VLAN trunks, see the "Configuring 802.1Q Layer 2
VLAN Trunks" section on page 51.
Inter-Switch Link VLAN Encapsulation
Layer 3-switching software also supports Inter-Switch Link (ISL) encapsulation over the external
Gigabit Ethernet ports or if configured as a GEC. The Catalyst 4003 and 4006 Layer 3 Services module
can be deployed in environments with the ISL trunking protocol and can route between external gigabit
ports running ISL and internal gigabit ports running 802.1Q stations.
External ports support ISL, which use one PVST+ over ISL trunks.
78-10164-03
Overview of the Catalyst 4003 and 4006 Layer 3 Services Module Interfaces
as an example on how to set up VLAN encapsulation.
Installation and Configuration Note for the Catalyst 4000 Layer 3 Services Module
37