Cisco Catalyst Blade 3020 Design Manual page 7

Table of Contents

Design Guide

A scenario wherein BPDUs are lost may be caused by unidirectional links, which can cause Layer

2 loops. To prevent this problem, use Loop Guard and UDLD. Loop Guard prevents a port from

forwarding as a result of missed BPDUs, which might cause a Layer 2 loop that could bring down

the network.

UDLD allows devices to monitor the physical configuration of fiberoptic or copper Ethernet cables

and detect when a unidirectional link exists. When a unidirectional link is detected, UDLD shuts

down the affected port and generates an alert. BPDU Guard prevents a port from being active in a

spanning-tree topology as a result of an attack or a misconfigured device connected to the switch

port. The port that sees unexpected BPDUs is automatically disabled and must then be manually

enabled, giving the network administrator full control over port and switch behavior.

The Cisco Catalyst Blade Switch 3020 supports Per VLAN Spanning Tree (PVST) and a maximum

of 128 spanning- tree instances. RPVST+ is a combination of Cisco PVST Plus (PVST+) and

RSTP, provides the flexibility of one spanning-tree instance per VLAN and the fast convergence

benefits of IEEE 802.1w. MST allows the switch to map several VLANs to one spanning-tree

instance, reducing the total number of spanning-tree topologies the switch processor must manage.

A maximum of 16 MST instances is supported. In addition, MST uses IEEE 802.1w for rapid

convergence. MST and RPVST+ create a more predictable and resilient spanning-tree topology,

while providing downward compatibility for integration with devices that use IEEE 802.1d and

PVST+ protocols.

Figure 4 illustrates an example of Spanning Tree Protocol when using two switches in the

crossover configuration. Each blade switch is dual homed to each aggregation switch through a 2-

port Cisco EtherChannel interface. In this figure the blocked links are indicated in red. In this

example, only four of the eight uplinks from each blade switch are used. The network designer can

make those EtherChannel uplinks more robust (up to four 4 ports each), or use them to connect

other devices such as intrusion detection systems (IDSs) or standalone servers.

Figure 4.

Spanning-Tree Example with the HP c-Class Enclosure and Cisco Catalyst Blade Switch 3020s

Page 7 of 28

Table of Contents