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4-22
Cisco AVVID Network Infrastructure Enterprise Quality of Service Design
956467
Chapter 4 QoS in an AVVID-Enabled Wide-Area Network
QoS Recommendations for WAN Aggregation Routers
A new ATM to Frame Relay Service Interworking standard, FRF.8.1, supports MLP over ATM and
Frame Relay Service Interworking, but it can be years before all switches are updated to the new
standard.
Optimizing Fragment Sizes
When enabling MLP over ATM the fragment size should be optimized such that it fits into an integral
number of cells. Otherwise, the bandwidth required could potentially double due to cell padding. For
example, if a fragment size of 49 bytes is configured, then this fragment would require 2 cells to transmit
(as ATM cells have 48 byte payloads). This would generate 57 bytes of overhead (2 cell headers plus 47
bytes of cell padding), which is more than double the fragment itself.
Tip
Currently, IOS does not automatically optimize the fragment sizes (which are indirectly defined by the
MLP fragment-delay parameter). Arriving at such optimal sizes requires a fair bit of math, which is well
documented in the Multilink PPP over Frame Relay and ATM white paper (internal).
Table 4-4
provides a summary of the optimal fragment-delay parameters for MLP over ATM.
Table 4-4
MLP over ATM Optimal Fragment-Delay Values
Sample ATM-to-Frame Relay Configuration
An ATM-to-Frame Relay configuration is shown below, in two parts:
·
Example 4-14
shows the Central Site WAN Aggregator ATM configuration.The Central Site WAN
Aggregator ATM configuration is identical to the configuration required for end-to-end low-speed
ATM connections implementing MLP LFI.
·
Example 4-15
shows the Remote-Branch Frame Relay Configuration.
Example 4-14 Central Site WAN Aggregator ATM Configuration
ip cef
For distributed platforms, use ip cef distributed.
!
...
!
interface ATM4/0
no ip address
no atm ilmi-keepalive
!
PVC Speed
Optimal Fragment Size
ATM Cells
(Rounded Up)
ppp multilink fragment-delay
value
56 kbps
84 bytes
2
12 ms
64 kbps
80 Bytes
2
10 ms
128 kbps
176 Bytes
4
11 ms
256 kbps
320 Bytes
7
10 ms
512 kbps
640 Bytes
14
10 ms
768 kbps
960 Bytes
21
10 ms