Enterprise QoS Solution Reference Network Design Guide

Chapter 3 WAN Aggregator QoS Design
Version 3.3
WAN Edge Link-Specific QoS Design
Two options exist for carrying voice traffic over slow-speed ATM PVCs: either Multilink PPP over ATM
(MLPoATM), in conjunction with MLP LFI, or ATM PVC bundling. ATM PVC bundling is a legacy
technique that has drawbacks such as inefficient bandwidth utilization and classification limitations (IP
precedence versus DSCP). But sometimes service providers make ATM PVC bundles economically
attractive to enterprise customers, so both approaches are discussed.
Slow-Speed (£ 768 kbps) ATM Links: MLPoATM
Recommendation: Use MLP LFI. Tune the ATM PVC Tx-ring to 3. cRTP can be used only in Cisco
IOS Release 12.2(2)T or later.
Serialization delays on slow-speed ATM links, as shown in Figure 3-14, necessitate a fragmentation and
interleaving mechanism. The most common ATM adaptation layers (such as AAL5) do not have
sequence numbers in the cell headers and, thus, require cells to arrive in the correct order. This
requirement makes interleaving a problem that cannot be solved at these ATM adaptation layers and thus
must be solved at a higher layer.
Figure 3-14 Slow-Speed MLPoATM Links
WAN Aggregator
MLPoATM Link
768 kbps
ATM
Cloud
Branch Router
A solution to this problem is to run MLPoATM and let MLP LFI handle any necessary fragmentation
and interleaving so that such operations are completely transparent to the lower ATM layer. As far as the
ATM layer is concerned, all cells arrive in the same order they were sent.
MLPoATM functionality is enabled through the use of virtual-access interfaces. Virtual-access
interfaces are built on demand from virtual-template interfaces and inherit their configuration properties
from the virtual templates they are built from. Thus, the IP address, service-policy statement, and LFI
parameters all are configured on the virtual template, as shown in Example 3-22.
cRTP is supported only on ATM PVCs (through MLPoATM), as of Cisco IOS Release 12.2(2)T.
Additionally, as discussed previously in this chapter, it is recommended that the value of the final output
buffer, the Tx-ring, be tuned on slow-speed ATM PVCs to a value of three particles to minimize
serialization delay.
Example 3-22 Slow-Speed (£ 768 kbps) MLPoATM QoS Design Example
!
interface ATM4/0
bandwidth 768
no ip address
no atm ilmi-keepalive
!
interface ATM4/0.60 point-to-point
pvc BRANCH#60 0/60
vbr-nrt 768 768 ! ATM PVC definition
tx-ring-limit 3 ! Per-PVC Tx-ring is tuned to 3 particles
protocol ppp Virtual-Template60 ! PVC is bound to the Virtual-Template
!
interface Virtual-Template60
bandwidth 768
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