Cisco Cisco Customer Voice Portal 8.0(1) Design Guide

Minimize round trips between the VXML server and the ICM script

When control is passed from a running VXML server application back to the ICM script, a significant 
WAN delay will be incurred.

Once a VXML Server application starts executing, the best practice is to minimize the number of trips 
back to the ICM script. Each round trip between the VXML Server and the ICM script incurs delay due 
to establishing two new TCP connections and HTTP retrieval of several VXML documents, including 
the VXML server root document.

Decrease the size of the VMXL server root document.

On the VXML server, in your specific gateway adapter's plugin.xml file:

Change:

<setting name="vxml_error_handling">default</setting>

To:

<setting name="vxml_error_handling">minimal</setting>

As an example, the location of the plugin.xml file for the CISCO DTMF 1 GW adapter is:

Cisco\CVP\VXMLServer\gateways\cisco_dtmf_01\6.0.1\plugin.xml

When a gateway first receives a call, the gateway signals the Unified CVP Call Server (Call Server) using 
H.323 in order to hand off the call control responsibilities. To establish this initial call, a short media 
stream is established between the gateway and the Call Server. The media stream is only in one direction, 
from the gateway to the Call Server. Because this media stream is not accounted for by Unified CM’s 
locations-based call admission control, Cisco recommends that the media stream be blocked from 
traversing bandwidth-constrained links to avoid oversubscribing the priority queue. This precaution is 
needed only for H.323 deployments; SIP deployments do not have this consideration.

The following IOS configuration will avoid the unnecessary connection attempts and avoid ICMP 
destination unreachable responses on the network due to failure to establish an RTP connection with the 
Voice Browser. The symptom that the customer is having will be a high CPU spike as shown in the "show 
proc cpu" output, and the process called "IP Input" can get over 10% CPU utilization without this 
workaround.

access-list 100 deny udp host 10.0.0.1 host 10.10.0.100 range 16384 65535

access-list 100 permit ip any any

interface serial0/0

 ip access-group 100 out

In the preceding example, 10.0.0.1 is the voice gateway’s H.323-bound IP address and 10.10.0.100 is the 
Call Server. If there are multiple Call Servers, add one ACL entry for each. The interface serial0/0 is the 
WAN interface connecting to the central site that is hosting the Call Server.

The preceding workaround will also avoid sending ICMP “unreachable” messages that occur due to the 
failure to establish the RTP connection.