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Cisco IP Telephony Network Design Guide

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Chapter 3 Cisco CallManager Clusters

Intercluster Communication

In Figure 3-7 the dotted lines represent the H.323 intercluster links, which are

configured in pairs to provide redundancy in the event of loss of IP connectivity

to any member of the cluster. If desired, you could configure these links as a full

mesh. However, Cisco recommends limiting intercluster configuration to two

peers. In the majority of situations, this is sufficient to provide adequate

resiliency. For deployments where a gatekeeper is used, Cisco recommends a

single H.323 connection per cluster. You can implement redundancy by using a

Cisco CallManager redundancy group assigned to the gatekeeper.

Unlike earlier releases of Cisco CallManager, release 3.0(5) does not require the

use of an MTP to allow supplementary services for H.323 devices.

Cisco CallManager 3.0(5) uses the “empty capabilities set” of H.323v2 to

facilitate the opening and closing of logical channels between H.323 devices such

as Cisco CallManager clusters and Cisco IOS gateways running

Cisco IOS Release 12.0(7)T or greater.

Clusters for Multisite WAN with Distributed Call Processing

Where clusters are interconnected over a WAN, there is a pinch point for

congestion between clusters, and the network should be engineered to

accommodate the required volume of voice traffic. In such cases a method of

providing call admission control is required. Because clusters are interconnected

using H.323, a Cisco IOS gateway can be added to facilitate this gatekeeper

function. Each cluster can be designated as a zone with a maximum configured

bandwidth for voice calls.

When using a gatekeeper, Cisco CallManager requests 128 kbps of bandwidth per

G.711 inter-cluster call and 20 kbps of bandwidth per G.729a intercluster call. In

general, Cisco recommends configuring a single codec for calls that traverse the

WAN because this greatly simplifies the provisioning of bandwidth.

Table 3-4 and Table 3-5 give recommendations for bandwidth configuration for

intercluster calls.

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Cisco WS-X4604-GWY - VoIP Gateway Especificaciones Pagina 87