No IP Unreachable

Overview

The "no ip unreachable" feature in networking devices is a configuration used to enhance network security and efficiency by disabling the generation of Internet Control Message Protocol (ICMP Internet Control Message Protocol (ICMP) is a fundamental protocol used in networking to relay error messages and operational information.) unreachable messages. Normally, these messages are sent by routers and other network devices in response to packets that cannot be delivered to their intended destination for various reasons.

When the "no ip unreachable" command is enabled, the network device stops sending these ICMP unreachable messages.

Supported ICMP Unreachable Codes

Here are the codes used in ICMPv6 Unreachable.

Table 81. ICMP Unreachable Codes

Code	Message	Description
0	Destination network unreachable
1	Destination host unreachable
2	Destination protocol unreachable
3	Destination port unreachable	The destination network is not reachable from the current router.
4	Fragmentation needed and DF flag set	The specific destination host within a reachable network is not accessible.
5	Source Route Failed	The protocol specified in the packet is not supported by the destination.
6	Destination Network Unknown	The destination port is not open or not listening on the destination device.
7	Destination Host Unknown	NA
8	Source Host Isolated	NA
9	Network Administratively Prohibited	NA
10	Network Administratively Prohibited	NA
11	Network Unreachable for TOS	NA
12	Host Unreachable for TOS	NA
13	Communication Administratively Prohibited	NA
14	Host Precedence Violation	NA
15	Precedence Cutoff in Effect	NA

Supported ICMPv6 Unreachable Codes

Here are the codes used in ICMPv6 Unreachable.

Table 82. ICMPv6 Unreachable Codes

Codes	Description
0	No route to destination
1	Communication with destination administratively prohibited
2	Beyond scope of source address
3	Address unreachable
4	Port unreachable
5	Source address failed ingress/egress policy
6	Reject route to destination

Feature Characteristics

The "no ip unreachable" feature is used to prevent a device from sending ICMP unreachable messages. These messages are typically generated when a router cannot forward a packet because the destination is unreachable. Disabling these messages can enhance network performance and security.

Benefits

The advantages of utilizing a No IP Unreachables:

• Enhanced Security
• Performance Optimization
• Simplified Troubleshooting.