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Most complex networks include redundant devices to avoid single points of failure.
Although a redundant topology eliminates some problems, it can introduce other problems.
STP is a Layer 2 link management protocol that provides path redundancy while preventing
undesirable loops in a switched network. It is a standard protocol as defined by IEEE
802.1D.
This section identifies the problems caused by redundant switched-network topologies and
the functionality of STP to prevent these problems.
Building a Redundant Switched Topology
One of the key characteristics of a well-built communications network is that it is resilient.
This means that the network needs to be able to handle a device or link failure. To
accomplish this, you will need to select the best interconnection technologies.
Choosing Interconnection Technologies
A number of technologies are available to interconnect devices in a switched network. The
interconnection technology that you select depends on the amount of traffic the link must
carry. You will likely use a mixture of copper and fiber-optic cabling based on distances,
noise immunity requirements, security, and other business requirements. Figure 2-14
illustrates different connectivity for network devices providing services in the enterprise.
Some of the more common interconnection technologies are as follows:
■ FastEthernet (100-Mbps Ethernet): This LAN specification (IEEE 802.3u) operates
at 100 Mbps over twisted-pair cable. The FastEthernet standard raises the speed of
Ethernet from 10 Mbps to 100 Mbps with only minimal changes to the existing cable
structure. A switch that has ports that function at both 10 Mbps and 100 Mbps can
move frames between ports without Layer 2 protocol translation.
■ Gigabit Ethernet: An extension of the IEEE 802.3 Ethernet standard, Gigabit
Ethernet increases speed tenfold over that of FastEthernet, to 1000 Mbps, or 1 Gbps.
IEEE 802.3z specifies operations over fiber optics, and IEEE 802.3ab specifies
operations over twisted-pair cable.
Improving Performance with Spanning Tree 41
Figure 2-14 Interconnectivity at the User Level
■ 10-Gigabit Ethernet: 10-Gigabit Ethernet was formally ratified as an 802.3 Ethernet
standard (IEEE 802.3ae) in June 2002. This technology is the next step for scaling the
performance and functionality of an enterprise. With the deployment of Gigabit
Ethernet becoming more common, 10-Gigabit Ethernet will become typical for
uplinks.
Departmental Switch Block 1
EtherChannel
10 Mbps 100 Mbps
–100 Mbps 1 Gbps
Copper
Fiber 10 Gbps
1 Gbps
Server Farm
42 Chapter 2: Medium-Sized Switched Network Construction
■ EtherChannel: This feature provides link aggregation of bandwidth over Layer 2
links between two switches. EtherChannel bundles individual Ethernet ports into a
single logical port or link. All interfaces in each EtherChannel bundle must be
configured with similar speed, duplex, and VLAN membership.
Determining Equipment and Cabling Needs
The design of any high-performance network has four objectives: security, availability,
scalability, and manageability. This list describes the equipment and cabling decisions that
you should consider when altering the infrastructure:
■ Replace hubs and legacy switches with new switches at the building access layer.
Select equipment with the appropriate port density at the access layer to support the
current user base while preparing for growth. Some designers begin by planning for
about 30 percent growth. If the budget allows, use modular access switches to
accommodate future expansion. Consider planning for the support of inline power and
QoS if you think you might implement IP telephony in the future.
■ When building the cable plant from the building access layer to the building
distribution layer devices, remember that these links will carry aggregate traffic from
the end nodes at the access layer to the building distribution switches. Ensure that these
links have adequate bandwidth capability. You can use EtherChannel bundles here to
add bandwidth as necessary.
■ At the distribution layer, select switches with adequate performance to handle the load
of the current access layer. In addition, plan some port density for adding trunks later
to support new access layer devices. The devices at this layer should be multilayer
(Layer 2 and Layer 3) switches that support routing between the workgroup VLANs
and network resources. Depending on the size of the network, the building distribution
layer devices can be fixed chassis or modular. Plan for redundancy in the chassis and
in the connections to the access and core layers, as business objectives dictate.
■ The campus backbone equipment must support high-speed data communications
between other distribution modules. Be sure to size the backbone for scalability, and
plan for redundancy.
Cisco has online tools to help designers make the proper selection of devices and uplink
ports based on business and technology needs. Some suggested oversubscription ratios that
you can use to plan bandwidth requirements between key devices on a network with
average traffic flows are as follows:
■ Access to distribution layer links: The oversubscription ratio should be no higher
than 20:1. That is, the link can be 1/20 of the total bandwidth available cumulatively to
all end devices using that link.
Improving Performance with Spanning Tree 43
■ Distribution to core links: The oversubscription ratio should be no higher than 4:1.
■ Between core devices: Little to no oversubscription should be planned here. That is,
the links between core devices should be able to carry traffic at the speed represented
by the aggregate-number bandwidth of all the distribution uplinks into the core.