Some common negative practices that can severely impact network performance, especially when using IP Telephony, include:

• A flat, non-hierarchical network

  For example, cascading small workgroup switches together in a flat non-hierarchical network. This technique quickly results in bottlenecks, because all traffic must flow across the uplinks at a maximum of 10 Gbps, versus traversing switch fabric at speeds of 256 Gbps or greater. The greater the number of small switches or layers, the greater the number of uplinks, and the lower the bandwidth for an individual connection. Under a network of this type, voice performance can quickly degrade to an unacceptable level.

  Avaya recommends that the network segments connected to the Session Manager network interfaces have a netmask of at least 23 bits or more. Issues can arise when the Session Manager is connected to large network segments where ARP traffic on causes kernel buffer issues and continual garbage collection of the ARP cache. For network segments with a netmask of 23 bits or more, you can restrict the number of network devices on that segment to 512, and the kernel ARP cache can handle that with no negative impact on Session Manager.

  For example:

  a netmask of 255.255.255.0 is 24 bit that will allow up to 256 devices in the subnet, and

  a netmask of 255.255.254.0 is 23 bit that will allow up to 512 devices in the subnet.

• Multiple subnets on a VLAN

  A network of this type can have issues with broadcasts, multicasts, and routing protocol updates. This practice can have a significant negative impact on voice performance and complicate troubleshooting.

• A hub-based network

  All hubs must be replaced with switches if they will lie in the path of IP telephony. Hubs are half-duplex by definition and can degrade the performance of real-time communications over IP.

• Too many access lists

  Access lists slow down a router. While access lists are appropriate for voice networks, you must not apply them to unnecessary interfaces. Traffic should be modeled beforehand and access lists applied only to the appropriate interface in the appropriate direction, not to all interfaces in all directions.

Customers must exercise caution when using the following:

• Network Address Translation (NAT)

  IP Telephony cannot work across NAT because if private IP addresses are exchanged in signaling messages, these addresses are not reachable from the public side of the NAT and cannot be used for the media sessions.

• Analog dial-up

  Be careful in using analog dial-up (56 kbps) to connect two locations. Upstream bandwidth can be limited to a maximum of 33.6 kbps and is lesser usually, resulting in insufficient bandwidth to provide quality voice. Some codecs and network parameters provide connections that are acceptable, but consider each connection individually.

• Virtual Private Network (VPN)

  Large delays are inherent in some VPN software products due to encryption, decryption, and additional encapsulation. Some hardware-based products that encrypt at near wire speed can be used. In addition, if the VPN is run over the Internet, sufficient quality for voice cannot be guaranteed unless delay, jitter, and packet loss are contained within the listed parameters.