Enterprise QoS Solution Reference Network Design Guide

Site-to-Site V3PN QoS Considerations
6-10
Enterprise QoS Solution Reference Network Design Guide
Chapter 6 IPSec VPN QoS Design
As shown in Table 6-2, the percentage of LLQ required on 64-, 128-, and 256-kbps links for a single
encrypted G.729 call exceeds the recommended 33 percent LLQ limit. Enterprises considering such
deployments must recognize the impact on data traffic traversing such links when encrypted voice calls
were made—specifically, data applications would slow down significantly. If that is considered an
acceptable trade-off, not much can be said or done. Otherwise, it is recommended to increase bandwidth
to the point that encrypted VoIP calls can be made and still fall within the 33 percent bandwidth
recommendation for priority queuing.
When planning the bandwidth required for a branch office, consider the number of concurrent calls
traversing the IPSec VPN that the branch is expected to make during peak call periods. This varies based
on the job function of the employees located at a branch. For example, an office of software engineers
would be expected to make fewer calls than an office of telemarketers. A typical active call ratio may be
one active call for every six people (1:6), but this could range from 1:4 or 1:10, depending on the job
function of the employees. Given the 512-kbps link from Table 6-2 as an example, with a target of 3
G.729 calls, this link theoretically could support a branch office of between 12 and 30 people. As with
all other topologies, call admission control must be administered properly to correspond to the QoS
policies deployed within the network infrastructure.
Note Although VoIP has been discussed as a primary example of bandwidth provisioning considerations when
deploying QoS over IPSec VPNs, it is important to recognize that VoIP is not the only application that
might require such considerations; this exercise needs to be performed for any application that is being
encrypted.
Logical Topologies
Table 6-2 G.729 Calls by Link Speeds (FRF.12 Is Enabled on Link Speeds £ 768 kbps Only)
Maximum Number of
LLQ Bandwidth
Line Rate (kbps) G.729 Calls LLQ Bandwidth (kbps) (Percentage)
64 (FRF.12) 1 (58 kbps) 58 91%
128 (FRF.12) 1 (58 kbps) 58 46%
256 (FRF.12) 2 (58 kbps) 116 46%
512 (FRF.12) 3 (58 kbps) 174 34%
768 (FRF.12) 4 (58 kbps) 232 31%
1024 6 (56 kbps) 336 33%
1536 9 (56 kbps) 504 33%
2048 12 (56 kbps) 672 33%
Unlike VoIP, however, other applications—such as video and data—have varying packet rates and sizes.
Therefore, crisp provisioning formulas might not apply. Moderate traffic analysis and best guesses,
along with trial-and-error tuning, are usually the only options for factoring bandwidth provisioning
increases for non-VoIP applications.
Similar to private WANs, but unlike fully meshed MPLS VPNs, IPSec VPNs typically are deployed in a
(logical) hub-and-spoke topology.
Version 3.3