A network can be perfectly designed and implemented to a specification, but requirements change. New applications are added, traffic patterns change, staff are moved between locations and so on. Nortel's Lee points out that older LANs were often designed with an aggregation layer between the wiring closets serving individual floors and centralised resources, reflecting the use of physically distributed departmental servers and other workgroup infrastructure. It also reduced the number of ports required on the core switches.
The consolidation trend seen over the last few years means that the majority of traffic now flows from desktop PCs to central servers, so removing the aggregation layer will improve performance. This may mean increasing the number of ports on the core switches, but the improvement will be especially noticeable with voice traffic, Lee says.
Hayes says that too often, those deploying an application do not consider the effect it will have on the network, while those responsible for the network do not always understand the effect changes will have on applications. The placement of servers should be optimised in terms of network resources, cost and performance. For example, it may make sense to move an application server closer to the users -- but what effect will that have on communication between the app server and the database? It might be better to move to a thin client architecture, or to rearchitect the entire application, he suggests.
Similarly, the use of spanning tree protocols to handle redundant network links is no longer appropriate, says Lee. Not only does it require the "backup" link to sit idly in reserve, but it also takes between eight and 50 seconds for individual sessions to reconverge on the other link following a failure. That is no great drama for most applications, but it is hopeless for VoIP traffic. Nortel's Split Multilink Trunking (SMLT), an extension of the 80213ad standard, enables simultaneous use of both links and has a reconvergence time of less than one second, he says.
According to Roland Chia, national business manager at Dimension Data, IEEE 802.1d Spanning Tree eliminates network loops in a LAN switching environment but can cause network instability if not configured correctly, for example when a misconfigured switch with highest priority is connected to a production network. "Best practice is to configure the LAN with Layer 3 switching or use Cisco proprietary advanced features such as Spanning Tree Rootguard feature," he says. Hayes says network architecture is about having the right devices in the right places doing the right things for the job, so if you've got a Layer 3 switch at the core of the network, use it as a Layer 3 switch.
Adding VoIP represents a major change. David Paddon, managing director of NSC Enterprise points out that if there is a delay of 30 seconds in transferring a spreadsheet from one place to another, with VoIP its integrity is still intact. That's not true for voice or video, where all packets must arrive in a timely manner.
Think about power outages too -- people expect to be able to use their phone during a blackout. This requires power over Ethernet (PoE) to the handsets, plus backup power to the entire network, Paddon says. People have a "five nines expectation of performance" from a phone system, says Hayes, who also recommends redundant, dual-homed floor switches to ensure high uptime. CSC has installed such a system at its Australian headquarters in Sydney. A high-availability LAN is supported by PoE, UPS and a generator in case of prolonged outages, along with dual links to the data centre using diverse paths and infrastructure. "We see voice as being the most critical application on the network," he says.
Atkinson warns that software configurations need to reflect network changes. One organisation had used frame relay to connect its head office, state offices and branches in a hierarchical arrangement, and updated files were sent to the state offices and then onto the branches. That worked well until it switched to a DSL network with a star topology: each time a state office sent an update to a branch, it went via head office. The new arrangement was "four times as fast, but twice as slow," says Atkinson, but the problem was overcome by having the updates sent directly from head office to each branch.
When adding switches or servers to a network, you should not rely on automatic Ethernet configuration, warns Chia. "Automatic configuration between vendors is not standardised and should always be manually configured to match," he says. Hayes agrees, saying full or half duplex settings should always be explicitly configured to match.
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