Introduction
This document explains how to troubleshoot high CPU utilization due to the IP input process.Note: This document does not provide strategies to prevent different types of attacks.
Prerequisites
Requirements
Cisco recommends that you read Troubleshooting High CPU Utilization on Cisco Routers before you proceed with this document.Components Used
This document is not restricted to specific software and hardware versions.The information presented in this document was created from devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If you are working in a live network, ensure that you understand the potential impact of any command before using it.
Conventions
Refer to Cisco Technical Tips Conventions for more information on document conventions.IP Input
The Cisco IOS® software process called IP input takes care of process-switching IP packets. If the IP input process uses unusually high CPU resources, the router is process-switching a lot of IP traffic. Check these issues:-
Interrupt switching is disabled on an interface (or
interfaces) that has (have) a lot of traffic
Interrupt switching refers to the use of switching algorithms other than process switching. Examples include fast switching, optimum switching, Cisco Express Forwarding switching, and so on (refer to Performance Tuning Basics for details). Examine the output of the show interfaces switching command to see which interface is burdened with traffic. You can check the show ip interface command to see which switching method(s) are used on each interface. Re-enable interrupt switching on that interface. Remember that regular fast switching is configured on output interfaces: if fast switching is configured on an interface, packets that go out of that interface are fast-switched. Cisco Express Forwarding switching is configured on input interfaces. To create Forwarding Information Base (FIB) and adjacency table entries on a particular interface, configure Cisco Express Forwarding switching on all interfaces that route to that interface.
-
Fast switching on the same interface is
disabled
If an interface has a lot of secondary addresses or subinterfaces and there is a lot of traffic sourced from the interface and destined for an address on that same interface, then all of those packets are process-switched. In this situation, you should enable ip route-cache same-interface on the interface. When Cisco Express Forwarding switching is used, you do not need to enable Cisco Express Forwarding switching on the same interface separately.
-
Fast switching on an interface providing policy routing is
disabled
If a route-map has been configured on an interface, and a lot of traffic is handled by the route-map, then the router process-switches this traffic. In this situation, you should enable ip route-cache policy on the interface. Check the restrictions mentioned in the "Enabling Fast-Switched Policy-Based Routing" section of Configuring Policy-Based Routing.
-
Traffic that cannot be interrupt-switched
arrives
This can be any of the listed types of traffic. Click on linked items for more information.
-
Packets for which there is no entry yet in the switching cache
Even if fast, optimum, or Cisco Express Forwarding switching (CEF) is configured, a packet for which there is no match in the fast-switching cache or FIB and adjacency tables is processed. An entry is then created in the appropriate cache or table, and all subsequent packets that match the same criteria are fast, optimum, or CEF-switched. In normal circumstances, these processed packets do not cause high CPU utilization. However, if there is a device in the network which 1) generates packets at an extremely high rate for devices reachable through the router, and 2) uses different source or destination IP addresses, there is not a match for these packets in the switching cache or table, so they are processed by the IP Input process (if NetFlow switching is configured, source and destination TCP ports are checked against entries in the NetFlow cache as well). This source device can be a non-functional device or, more likely, a device attempting an attack.
(*) Only with glean adjacencies. Refer to Cisco Express Forwarding for more information about Cisco Express Forwarding adjacencies.
-
Packets destined for the router
These are examples of packets destined for the router:
-
Routing updates that arrive at an extremely high rate. If the
router receives an enormous amount of routing updates that have to be
processed, this task might overload the CPU. Normally, this cannot happen in a
stable network. The way you can gather more information depends on the routing
protocol you have configured. However, you can start to check the output of the
show
ip route summary
command periodically. Values that change
rapidly are a sign of an unstable network. Frequent routing table changes mean
increased routing protocol processing, which results in increased CPU
utilization. For further information on how to troubleshoot this issue, refer
to the
Troubleshooting
TCP/IP section of the Internetwork Troubleshooting Guide.
-
Any other kind of traffic destined for the router. Check who is
logged on to the router and user actions. If someone is logged on and issues
commands that produce long output, the high CPU utilization by the "IP input"
process is followed by a much higher CPU utilization by the
Virtual
Exec process.
-
Spoof attack. To identify the problem, issue the
show
ip traffic
command to check the amount of IP traffic. If
there is a problem, the number of received packets with a local destination is
significant. Next, examine the output of the show
interfaces and
show
interfaces switching
commands to check which interface the
packets are coming in. Once you have identified the receiving interface, turn
on
ip
accounting
on the outgoing interface and see if there is a
pattern. If there is an attack, the source address is almost always different,
but the destination address is the same. An access list can be configured to
solve the issue temporarily (preferably on the device closest to the source of
the packets), but the real solution is to track down the source device and stop
the attack.
-
Routing updates that arrive at an extremely high rate. If the
router receives an enormous amount of routing updates that have to be
processed, this task might overload the CPU. Normally, this cannot happen in a
stable network. The way you can gather more information depends on the routing
protocol you have configured. However, you can start to check the output of the
show
ip route summary
command periodically. Values that change
rapidly are a sign of an unstable network. Frequent routing table changes mean
increased routing protocol processing, which results in increased CPU
utilization. For further information on how to troubleshoot this issue, refer
to the
Troubleshooting
TCP/IP section of the Internetwork Troubleshooting Guide.
-
Broadcast traffic
Check the number of broadcast packets in the show interfaces command output. If you compare the amount of broadcasts to the total amount of packets that were received on the interface, you can gain an idea of whether there is an overhead of broadcasts. If there is a LAN with several switches connected to the router, then this can indicate a problem with Spanning Tree.
-
IP packets with options
-
Packets that require protocol translation
-
Multilink Point-to-Point Protocol (supported in Cisco Express
Forwarding switching)
-
Compressed traffic
If there is no Compression Service Adapter (CSA) in the router, compressed packets must be process-switched.
-
Encrypted traffic
If there is no Encryption Service Adapter (ESA) in the router, encrypted packets must be process-switched.
-
Packets that go through serial interfaces with X.25 encapsulation
In the X.25 protocol suite, flow control is implemented on the second Open System Interconnection (OSI) layer.
-
Packets for which there is no entry yet in the switching cache
-
A lot of packets, that arrive at an extremely high rate, for a
destination in a directly attached subnet, for which there is no entry in the
Address Resolution Protocol (ARP) table. This should not happen with TCP
traffic because of the windowing mechanism, but can happen with User Datagram
Protocol (UDP) traffic. To identify the problem, repeat the actions suggested
in order to track down a spoof attack.
-
A lot of multicast traffic goes through the router. Unfortunately,
there is no easy way to examine the amount of multicast traffic. The
show
ip traffic
command only shows summary information.
However, if you have configured multicast routing on the router, you can enable
fast-switching of multicast packets with the
ip
mroute-cache
interface configuration command
(fast-switching of multicast packets is off by default).
-
Router is oversubscribed. If the router is over-used and cannot
handle this amount of traffic, try to distribute the load among other routers
or purchase a high-end router.
-
IP Network Address Translation (NAT) is configured on the router, and
lots of Domain Name System (DNS) packets go through the router. UDP or TCP
packets with source or destination port 53 (DNS) are always punted to process
level by NAT.
-
There are other packet types that are punted to
processing.
-
There is fragmentation of IP Datagram. There is a small increase in
CPU and memory overhead due to fragment of an IP datagram. Refer to
Resolve
IP Fragmentation, MTU, MSS, and PMTUD Issues with GRE and IPSEC for more
information on how to troubleshoot this
issue.
