C H A P T E R 3 1-Port Oc-12 Atm Line Card Configuration - Cisco 10000 Series Configuration Manual

Chapter 3 1-Port OC-12 ATM Line Card Configuration
The number of SAR priority levels and the service categories supported at each priority level vary from
line card to line card. For example, the 1-port OC-12/STM-1 line card supports the four levels of priority
and the service categories listed in
The ATM line cards support a maximum number of VCs per priority. That VC limit depends on the VC
limit of the SAR (SAR limit) and the number of priority levels configured.
determine the VC limit per priority level for the 1-port OC-12/STM-1 line card.
Table 3-4
ATM Line Card
1-Port OC-12/
STM-1
Configuring more channels or VCs than there are available priority locations can cause random channels
or VCs to get stuck in the SAR. This occurs when an active channel tries to reschedule itself, but no
priority locations are available. Therefore, the channel cannot find a place to reschedule itself, which
results in a lost event for the channel, and the channel becomes stuck in the SAR.
On the PRE2, when a VC becomes stuck in the SAR, the PRE2 scheduler stops forwarding traffic on
only the VC that is stuck in the SAR; the other VCs still carry traffic. On the PRE3, the PRE3 scheduler
stops forwarding traffic on all the VCs configured on that ATM line card.
Shaped UBRs on the OC-12 ATM Line Card
On an OC-12 ATM line card, when you configure UBR PVCs with a shaped value (UBR-PCR) and the
shaped value is greater than one-half of the line rate (for example, 299,520 Kbps), the following
limitations apply:
•
•
•
OL-8834-04
Maximum Number of VCs per Priority
SAR Priority Levels
0 = CBR VCs
1 = VBR-rt VCs
2 = VBR-nrt VCs
3 = UBR VCs
The number of VCs the OC-12 line card supports is up to one-half of the VC scaling limit of 16,384
VCs. Cisco IOS software counts each UBR-PVC above 299,520 as two VCs. Therefore, the active
VC count must be maintained at the following:
16,384 > (number of VCs at 299,520 and above * 2) + (number of VCs below 299,520)
At any time, if more VCs are active than the allowed number above, the SAR on the line card leaks
buffers, which results in a reduced buffer pool for active VCs and the SAR might fail if enough
buffers are lost. To recover the lost buffers, reboot the system.
The router allows you to enter shaping values between 299,520 and 599,040, which the SAR does
not support. The SAR performs shaping in the range of 599,040 and 299,520 to 299,538. If you
configure a shaping value between 299,528 and 399,032, the shape rate the SAR returns is unclear.
If you initially set a shaping rate of 599,040 and then change to another rate, or you initially
configure a shape rate and change to a rate of 599,040, the router accepts the command and the show
commands display the new rate. However, the SAR does not perform shaping correctly until the next
reload.
Table 3-4.
VC Rate Maximum Number of VCs per Priority
Full line rate SAR limit / 2 / number of priority levels
With 4 priority system:
65,536 / 2 / 4 = 8192 VCs per priority level
Half line rate SAR limit / number of priority levels
and below
4 priority system:
65,536 / 4 = 16,384 VCs per priority level
Cisco 10000 Series Router Line Card Configuaration Guide
Line Card VC Limitations
Table 3-4 describes how to
3-3