Spanning Tree Protocol - Cisco NM-8B-U - HW ROUTERS L-M User Manual

16- and 36-Port Ethernet Switch Module for Cisco 2600 Series, Cisco 3600 Series, and Cisco 3700 Series
Feature Overview
Figure 3 shows 802.1x-port-based authentication in a wireless LAN. The 802.1x port is configured as a
multiple-host port that becomes authorized as soon as one client is authenticated. When the port is
authorized, all other hosts indirectly attached to the port are granted access to the network. If the port
becomes unauthorized (reauthentication fails or an EAPOL-logoff message is received), the switch
denies access to the network to all of the attached clients. In this topology, the wireless access point is
responsible for authenticating the clients attached to it, and the wireless access point acts as a client to
the switch.
Figure 3 Wireless LAN Example
Authentication
Cisco router with
server
Ethernet switch
Access point
(RADIUS)
network module
Wireless client

Spanning Tree Protocol

This section describes how to configure the Spanning Tree Protocol (STP) on Ethernet switch network
module systems.
Spanning tree is a Layer 2 link management protocol that provides path redundancy while preventing
undesirable loops in the network. For a Layer 2 Ethernet network to function properly, only one active
path can exist between any two stations. Spanning tree operation is transparent to end stations, which
cannot detect whether they are connected to a single LAN segment or to a switched LAN of multiple
segments.
The Ethernet switch network module uses STP (the IEEE 802.1D bridge protocol) on all VLANs. By
default, a single instance of STP runs on each configured VLAN (provided that you do not manually
disable STP). You can enable and disable STP on a per-VLAN basis.
When you create fault-tolerant internetworks, you must have a loop-free path between all nodes in a
network. The spanning tree algorithm calculates the best loop-free path throughout a switched Layer 2
network. Switches send and receive spanning tree frames at regular intervals. The switches do not
forward these frames, but use the frames to construct a loop-free path.
Multiple active paths between end stations cause loops in the network. If a loop exists in the network, end
stations might receive duplicate messages and switches might learn endstation MAC addresses on multiple
Layer 2 interfaces. These conditions result in an unstable network.
Spanning Tree Protocol defines a tree with a root switch and a loop-free path from the root to all switches
in the Layer 2 network. Spanning tree forces redundant data paths into a standby (blocked) state. If a
network segment in the spanning tree fails and a redundant path exists, the spanning tree algorithm
recalculates the spanning tree topology and activates the standby path.
When two ports on a switch are part of a loop, the spanning tree port priority and port path cost setting
determine which port is put in the forwarding state and which port is put in the blocking state. The
spanning tree port priority value represents the location of an interface in the network topology and how
well located it is to pass traffic. The spanning tree port path cost value represents media speed.
Cisco IOS Release 12.2(2)XT, 12.2(8)T, and 12.2(15)ZJ
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