Cisco ASR 1000 Series Configuration Guide

an invalid option, it sends an Internet Control Message Protocol (ICMP) parameter problem message to the
source of the packet and discards the packet.
IP provides a provision known as source routing that allows the source IP host to specify a route through the
IP network. Source routing is specified as an option in the IP header. If source routing is specified, the software
forwards the packet according to the specified source route. IP source routing is employed when you want to
force a packet to take a certain route through the network. The default is to perform source routing. IP source
routing is rarely used for legitimate purposes in networks. Some older IP implementations do not process
source-routed packets properly, and it may be possible to crash devices running these implementations by
sending them datagrams with source routing options. Disable IP source routing whenever possible. Disabling
IP source routing will cause a Cisco router to never forward an IP packet that carries a source routing option.
ICMP Overview
Originally created for the TCP/IP suite in RFC 792, the Internet Control Message Protocol (ICMP) was
designed to report a small set of error conditions. ICMP can also report a wide variety of error conditions and
provide feedback and testing capabilities. Each message uses a common format and is sent and received by
using the same protocol rules.
ICMP enables IP to perform addressing, datagram packaging, and routing by allowing encapsulated messages
to be sent and received between IP devices. These messages are encapsulated in IP datagrams just like any
other IP message. When the message is generated, the original IP header is encapsulated in the ICMP message
and these two pieces are encapsulated within a new IP header to be returned as an error report to the sending
device.
ICMP messages are sent in several situations: when a datagram cannot reach its destination, when the gateway
does not have the buffering capacity to forward a datagram, and when the gateway can direct the host to send
traffic on a shorter route. To avoid the infinite regress of messages about messages, no ICMP messages are
sent about ICMP messages.
ICMP does not make IP reliable or ensure the delivery of datagrams or the return of a control message. Some
datagrams may be dropped without any report of their loss. The higher-level protocols that use IP must
implement their own reliability procedures if reliable communication is required.
ICMP Unreachable Error Messages
Type 3 error messages are sent when a message cannot be delivered completely to the application at a destination
host. Six codes contained in the ICMP header describe the unreachable condition as follows:
• 0—Network unreachable
• 1—Host unreachable
• 2—Protocol unreachable
• 3—Port unreachable
• 4—Fragmentation needed and the “don’t fragment? (DF) bit is set
• 5—Source route failed
software can suppress the generation of ICMP unreachable destination error messages, which is called
rate-limiting. The default is no unreachable messages more often than once every half second. Separate
IP Application Services Configuration Guide, Cisco IOS XE Release 3S (Cisco ASR 1000)
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Configuring IP Services
ICMP Overview

Cisco ASR 1000 Series Specifications

General

Series	ASR 1000
Category	Network Router
Operating System	Cisco IOS XE
Memory	Up to 64 GB
Interfaces	10 Gigabit Ethernet
Power Supply	AC or DC options
Dimensions	Varies by model
Encryption	IPsec, SSL
Storage	Varies by model
Operating Temperature	32°F to 104°F (0°C to 40°C)
Humidity	5% to 95% non-condensing
MTBF	Varies by model