Fundamental LAN Planning Guidelines

The Cisco guide recommends a detailed analysis of the following LAN elements:

• LAN/campus topology

• IP addressing plan

• Location of TFTP servers, DNS servers, DHCP servers, firewalls, network address translation (NAT) gateways, and port address translation (PAT) gateways

• Potential location of gateways and call telephony servers

• Protocol implementation including IP routing, Spanning Tree, VTP, IPX, and IBM protocols

• Device analysis including software versions, modules, ports, speeds, and interfaces

• Phone connection methodology (direct or daisy chain)

According to the Cisco guide, the significant LAN topology issues are:

• Available average bandwidth

• Available peak or burst bandwidth

• Resource issues can may affect performance including buffers, memory, CPU, and queue depth

• Network availability

• IP phone port availability

• Desktop/phone QoS between user and switch

• Network scalability with increased traffic, IP subnets, and features

• Back-up power capability

• LAN QoS functionality

• Convergence at Layers 2 and 3

IP addressing issues that should be reviewed are:

• Phone IP addressing plan

• Average user IP subnet size use for the campus

• Number of core routes

• IP route summary plan

• DHCP server plan (fixed and variable addressing)

• DNS naming conventions

Potential considerations with IP addressing include:

• Route scalability with IP phones

• IP subnet space allocation for phones

• DHCP functionality with secondary addressing

• IP subnet overlap

• Duplicate IP addressing

The locations (or potential locations) of servers and gateways are important to ensure that service availability is consistent across the LAN infrastructure and for multiple sites. Gateways and servers in the review should include:

• TFTP servers

• DNS servers

• DHCP servers

• Firewalls

• NAT or PAT gateways

• Call telephony server

• Gateway location

After determining the location of these network elements, the following issues should be analyzed:

• Network service availability

• Gateway support (in conjunction with the IP telephony solution)

• Available bandwidth and scalability

• Service diversity

IP telephony scalability and availability issues will be affected by protocols in the network. The following areas for the protocol implementation analysis are:

• IP routing including protocols, summarization methods, non-broadcast media access (NBMA) configurations, and routing protocol safeguards

• Spanning Tree configuration including domain sizes, root designation, uplink fast, backbone fast, and priorities in relation to default gateways

• HSRP configuration

• VTP and VLAN configuration

• IPX, DLSW, or other required protocol services, including configuration and resource usage

With regard to protocol implementation, the following issues should be reviewed:

• Protocol scalability

• Network availability

• Potential impact on IP telephony performance or availability

All network devices should be reviewed and analyzed to determine whether the network has the desired control plane resources, interface bandwidth, QoS functionality, and power management capabilities. The checklist for this process includes:

• Device (type and product ID)

• Software version(s)

• Quantity deployed

• Modules and redundancy

• Services configured

• User media and bandwidth

• Uplink media and bandwidth

• Switched versus shared media

• Users per uplink and uplink load sharing/redundancy

• Number of VLAN supported

• Subnet size, and devices per subnet

For establishing a network baseline, it is important that the following measurements be made to determine voice quality levels and potential problem areas:

• Device average and peak CPU

• Device average and peak memory

• Peak backplane use

• Average link use (prefer peak hour average for capacity planning)

• Peak link use (prefer 5 minute average or smaller interval)

• Peak queue depth

• Buffer failures

• Average and peak voice call response times (before IP telephony implementation)

Cabling questions that may help determine the readiness of the infrastructure for IP telephony readiness include:

• Does the building wiring conform to EIA/TIA-568A?

• Does your organization comply with National Electric Code for powering and grounding sensitive equipment?

• Does your organization comply with the more rigorous IEEE 1100-1992 standard for recommended practices of grounding and powering sensitive equipment?

• Does the organization have standards for data center and wiring closet power that include circuit distribution, available power validation, redundant power supply circuit diversity, and circuit identification?

• Does the organization use UPS and/or generator power in the data center, wiring closet, phone systems, and internetworking devices?

• Does the organization have processes to SNMP manage or periodically validate and test back-up power?

• Does your business experience frequent lightning strikes? Are there other potential natural disasters?

• Is the wiring to your building above ground?

• Is the wiring in your building above ground?

Network bandwidth consumption is required for each VoIP stream. In any conversation, two such streams are required: one in each direction. The required bandwidth per conversation will be based on several factors, but of primary importance is the codec used to digitize, compress, and convert an analog voice signal into IP format. The two codecs of most interest are G.711 and G.729A. G.711 is the TIA recommended codec to optimize IP telephony QoS because it reduces impairment of the voice signal across the network, but the signal is uncompressed and requires a high amount of bandwidth. To save on network transmission costs, G.729A is used for off-premises traffic because it uses a compression algorithm to reduce bandwidth requirements.

Network performance and capacity planning help to ensure that the network will consistently have available bandwidth for data and VoIP traffic and that the VoIP packets will consistently meet delay and jitter requirements. Cisco recommends the following six-step process for network capacity and performance planning:

1. Determine baseline existing network use and peak load requirements

2. Determine VoIP traffic overhead in required sections of the network based on busy hour estimates, gateway capacities, and/or CallManager capacities

3. Determine minimum bandwidth requirements

4. Determine the required design changes and QoS requirements based on IP telephony design recommendations and bandwidth requirements (overprovide where possible)

5. Validate baseline performance

6. Determine trunking capacity