Routing protocols and convergence

While designing an IP telephony network across a WAN, care should be taken when selecting a routing protocol or a dial-backup solution. Different routing protocols have different convergence times, which is the time that it takes to detect a failure and route around it. While a network is in the process of converging, all voice traffic is lost.

The selection of a routing protocol depends on several factors:

• If a network has a single path to other networks, static routes are sufficient.

• If multiple paths exist, is convergence time an issue? If yes, Enhanced Interior Gateway Routing Protocol (EIGRP) and Open Shortest Path First (OSPF) are appropriate.

• Are open standards-based protocols required? If yes, OSPF and RIP are appropriate, but not EIGRP or IGRP, which are Cisco proprietary.

In general, you must use OSPF when routing protocols. OSPF allows relatively fast convergence and does not rely on proprietary networking equipment.

In many organizations, because of the expense of dedicated WAN circuits, dial-on-demand circuits are provisioned as backup if the primary link fails. The two principal technologies are ISDN (BRI) and analog modem. ISDN dial-up takes approximately 2 s to connect and offers 64 kbps to 128 kbps of bandwidth. Analog modems take approximately 60 s to connect and offer up to 56 kbps of bandwidth. If G.729 is used as the codec, either technology can support IP telephony traffic. If G.711 is used as the codec, only ISDN is appropriate. Also, because of the difference in connection time, ISDN is the preferred dial-on-demand technology for implementing IP telephony.

Multipath routing

Many routing protocols, such as OSPF, install multiple routes for a particular destination into a routing table. Many routers attempt to load-balance across the two paths. There are two methods for load balancing across multiple paths. The first method is per-packet load balancing, where each packet is serviced in a round-robin fashion across the two links. The second method is per-flow load balancing, where all packets in an identified flow (source and destination addresses and ports) take the same path. IP telephony does not operate well over per-packet load-balanced paths. This type of setup often leads to choppy quality voice. In situations with multiple active paths, you must use per-flow load balancing instead of per-packet load balancing.