Troubleshooting guide

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ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-01
Chapter 11 Troubleshooting Layer 3 Network Connections
Troubleshooting Route Processor Route Table and Utilization Problems
• The Layer 3 enabled ATM switch router route processor does the selective discard.
• Packets with high precedence are queued in “headroom” and processed before low precedence
packets. These high precedence packets are typically routing protocol packets.
Some of the information does not apply to CEF based forwarding which the Layer 3 enabled ATM switch
router uses. However, you can use this information to see what is being dropped by the Fast Ethernet
interfaces and the route processor.
Note SPD is enabled by default on the switch router.
Follow these steps to troubleshoot SPD:
Step 1 Use the show ip spd command to confirm the SPD settings.
Switch# show ip spd
Current mode: normal.
Queue min/max thresholds: 8/9, Headroom: 1024
IP normal queue: 0, priority queue: 0.
SPD special drop mode: none
Switch#
Step 2 Check the Queue min/max thresholds field. This determines when the lower-priority packets are
discarded. Typically, lower-priority packets are discarded when the input queue size hits min-threshold.
When the max-threshold is reached all lower-priority packets are dropped. For all the switch routers, the
min/max queue thresholds are almost the same, if there are more than 75 packets in the input queue and
all lower-priority packets will be discarded.
Step 3 Check the Headroom field. This indicates how many high-precedence packets will be enqueued over the
normal input hold queue limit. This is to reserve room for incoming high precedence packets.
Since the switch router is a nonblocking switch, the lower-priority packets will actually be dropped by
the route processor or the switch fabric, but the counters will be shown on the interfaces in Step 4.
Step 4 Use the show epc spd command to check the SPD on the interfaces.
Switch# show epc spd
INPUT-INT TOT-DROPS PRIORITY-RCVD PRIORITY-DROPS NO-BUFS
FastEthernet3/0/0 0 7813353 0 0
FastEthernet3/0/1 0 7773376 0 0
FastEthernet3/0/2 0 7773593 0 0
FastEthernet3/0/3 0 7773568 0 0
FastEthernet3/0/4 0 7773593 0 0
FastEthernet3/0/5 0 7812594 0 0
FastEthernet3/0/6 0 7773593 0 0
FastEthernet3/0/7 0 7773569 0 0
FastEthernet3/0/8 0 7773592 0 0
FastEthernet3/0/9 0 7773592 0 0
FastEthernet3/0/10 0 7812705 0 0
FastEthernet3/0/11 0 7773567 0 0
FastEthernet3/0/12 0 7812538 0 0
FastEthernet3/0/13 0 7773642 0 0
FastEthernet3/0/14 0 7773591 0 0
FastEthernet3/0/15 0 7812666 0 0
ATM0 0 45177 0 0
Ethernet0 0 0 0 0
High precedence packets are Layer 2 and Layer3 control protocol traffic carried on Stream ID 35. Lower
priority packets are carried on Stream ID 36, which would be used for traffic where there is no entry in
the CAM table.