/mcpQuality of Service for Voice over IP Resource Reservation Protocol 27 QoSVoIP.mif The third possibility, available in Cisco IOS releases later than 12.1(5)T, is to configure the gateways to proceed with the call even if RSVP reservations fail. This option, however, does not provide a major improvement over earlier Cisco IOS release functionality. The only benefit it provides is that, in case of a successful RSVP reservation, the call does not proceed until the reservation is established. As mentioned earlier, a call can fail admission control if at least one of the two RSVP reservations needed for the call fails. For each RSVP reservation, admission control is performed on all interfaces where you have enabled RSVP by using the ip rsvp bandwidth interface configuration command. You can configure two values with the ip rsvp bandwidth command: the maximum total reserved bandwidth and the maximum bandwidth per reservation. The maximum total bandwidth is limited by default to no more than 75 percent of the total bandwidth of the interface. You can modify that limit with the max-reserved-bandwidth interface configuration command. Exceptions to the maximum total bandwidth limitation are Frame Relay and ATM PVCs. For Frame Relay PVCs, the maximum reservable bandwidth is the minimum CIR, or, if not configured, half of the CIR. For ATM PVCs, the maximum reservable bandwidth is 75 percent of the configured available bit rate output minimum cell rate, near real-time VBR output-SCR, or real-time VBR average rate, whichever is configured. The total bandwidth available for RSVP reservations may be lower if you have reserved bandwidth using CBWFQ or LLQ through MQC. A bandwidth manager makes sure that the interface or the PVC bandwidth is not oversubscribed during the router operation. Note This check is not performed during router configuration. You should configure the maximum bandwidth per reservation to be no lower than what the codec requires plus all other protocol overhead except the Layer 2 protocol overhead. Table 6 shows the lowest values that you can use for different codecs. Remember that these values do not account for the bandwidth savings introduced by cRTP or voice activity detection (VAD). The actual voice stream may use less bandwidth, but the system will use the worst-case bandwidth. This example shows an implementation of call rerouting on the gateway. Calls to seven-digit numbers starting with digit 3 try two voice gateways first. Calls are routed through the PSTN via voice port 1/1:23 if the VoIP calls fails due to CAC or any other reason. Configuration Example 12: Call Rerouting on the Gateway (continued) Table 6 Bandwidth Reserved by RSVP per VoIP Call Codec Reserved Bandwidth per VoIP Call (kbps) G711alaw 80 G711ulaw 80 G723ar53 22 G723ar63 23 G723r53 22 G723r63 23 G726r16 32 G726r24 40 G726r32 48 G728 32