Cisco 10000 - Manual (Page 62)

2-2
Cisco 10000 Series Router Software Configuration Guide
OL-2226-23
Chapter 2 Scalability and Performance
Line Card VC Limitations
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
Table 2-2 , but the 4-port OC-3 line card supports only two levels of
priority and the service categories listed in the table.
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.
Table 2-2 describes how to
determine the VC limit per priority level per port for the specified line cards.
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.
For example, suppose a 1-port OC-12 line card at full line rate is configured for four levels of priority
and a 4-port OC-3 line card at half line rate is configured for two levels of priority. By calculating the
maximum number of VCs as described in
Table 2-2 , you can configure 8192 VCs per priority level for
the 1-port OC-12 and 8192 VCs per priority level per port for the 4-port OC-3—a total of 16,384 VCs
per priority level per port. If the number of VCs you configure exceeds the VC limit, the VCs get stuck
in the SAR.
Table 2-2 Maximum Number of VCs per Priority
ATM Line Card SAR Priority Levels VC Rate Maximum Number of VCs per Priority
1-Port OC-12/
STM-1 0 = CBR VCs
1 = VBR-rt VCs
2 = VBR-nrt VCs
3 = UBR VCs
Full line rate SAR limit / 2 / number of priority levels
4 priority system:
65,536 / 2 / 4 = 8192 VCs per priority level
Half line rate
and below SAR limit / number of priority levels
4 priority system:
65,536 / 4 = 16,384 VCs per priority level
4-Port OC-3 0 = CBR, VBR-nrt VCs
1 = UBR VCs
Half line rate
and below SAR limit / number of PHYs / number of
priority levels
2 priority system:
65,536 / 4 / 2 = 8192 VCs per priority level
per port
8-Port E3/DS3 0 = CBR VCs
0 = VBR-nrt VCs
1 = UBR VCs
Half line rate
and below SAR limit / number of PHYs / number of
priority levels
2 priority system:
65,536 / 8 / 2 = 4096 VCs per priority level
per port