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342 Avaya Application Solutions IP Telephony Deployment Guide
Multipath routing
Modern routers and Layer 3 switches allow multiple routes for a particular destination to be
installed in the routing table. Depending on the implementation, this can be as high as six
routes. Some implementations require that all routes that are inserted in the routing table have
the same metric, while others allow unequal metric routing. In cases where the metric for all
installed routes are the same, the router will load balance traffic evenly across each path. When
the metric for multiple routes vary, the traffic is load balanced in proportion to the metric (in other
words, if one path is “twice as good” as another, two-thirds of the traffic travels down the good
path, and one-third of the traffic selects the other one). Asymmetric routing is suboptimal for
voice, so route-caching (described earlier) should be considered in this environment.
In addition to using all (up to 6) active paths and optimally using available bandwidth, multipath
routing greatly improves convergence time. As soon as a router detects a path failure, it remove
it from the routing table, and sends all traffic over the remaining links. If this is a physical link
failure, the detection time is nearly instantaneous. Therefore, Avaya recommends the use of
multipath routing, where available, across multiple links to a particular location.
Dial backup
One cost-effective technique for installing backup WAN links is to use dial backup. This can be
done using either ISDN-BRI or analog lines. ISDN lines typically take 2 seconds to connect,
while 56-k analog modems take approximately 1 minute. While this strategy is effective for data
traffic, it is less effective for voice. First, the bandwidth may have been greatly reduced. If this is
the case, the number of voice channels that can be supported might have been reduced
proportionally. Also, if QoS is not properly applied to the backup interface, high packet loss and
jitter can adversely affect voice quality. Finally, the time that is required to establish the new link
can be up to 1 minute, which disrupts active calls. However, providing that these considerations
are taken into account, proper QoS is applied, and a compressed codec is chosen, dial backup
can be an effective solution for two to four users.