Troubleshoot Input Drops in Cisco IOS XR

Introduction

This document describes how to troubleshoot input drops on the interface on Cisco IOS XR routers. It covers ASR 9000 series routers, CRS series routers, and GSR 12000 series routers.

Background Information

In Cisco IOS, an input drop was due to the interface input queue getting full. Too many packets were punted to the CPU for process switching and it was not able to handle them fast enough. The input queue built up until full, causing drops.

On the ASR 9000, the Network Processor (NP) of the line card realizes that the router misses a load info and increments an NP drop counter, which is uploaded to the interface input drop counter.

Problem: Increment in the Input Drop

Unknown Destination MAC Address or dot1q VLAN

When a Cisco IOS router sends Ethernet keepalives on its GigabitEthernet interface, those keepalives increment the input drops on the XR router because they do not have the destination MAC address of the XR router.

On the ASR 9000, the Input drop invalid DMAC and Input drop other counters in the controller stats are not incremented. To recognize these drops on the ASR 9000, find the NP handling the interface with the input drops:

show controllers np ports all location 0/<x>/CPU0
show contr np counters np<y> location 0/<x>/CPU0

For packets received with a dot1q VLAN not configured on a subinterface of the ASR 9000, the Input drop unknown 802.1Q counter is not incremented in show controllers gigabitEthernet 0/0/0/30 stats.

Packets Dropped Due to Unrecognized Upper-Level Protocol

This category of drops is described in the Cisco Support Community article "ASR9000/XR: Drops for Unrecognized Upper-Level Protocol Error".

NP Drops on the ASR 9000

Drop counters in the NP of the ASR 9000 are reported as input drops when they apply to a packet received on an interface and dropped. This does not happen for Packet Switch Engine (PSE) drops on the CRS and the XR 12000 — they are not counted as input drops.

If input drops appear on an ASR 9000 and do not match one of the reasons above, run the commands shown previously to find the NP handling the interface, then check its counters. For example, IPV4_PLU_DROP_PKT in the NP counters means the CEF/PLU entry says the packet must be dropped — for instance, there is no default route and unreachables are disabled, so packets not matching a more specific route hit a drop entry in the default CEF handler.

If a drop counter explains the input drops but the counter name is not self-explanatory, refer to "ASR9000/XR: Troubleshoot Packet Drops and Understanding NP Drop Counters". Note that first-generation (Trident) line cards use certain counter names, while new-generation (Typhoon) line cards have new counter names; find the similar counter name based on the description.

Netio

Collect a show netio idb <interface> to see the interface input drop and the netio node drop counters, which helps isolate where in the data path the packets are being dropped.

Products Covered

• ASR 9000 Series Aggregation Services Routers
• IOS XR Software

原文链接：https://www.cisco.com/c/en/us/support/docs/ios-nx-os-software/ios-xr-software/213967-troubleshoot-input-drops-in-ios-xr.html