CCNA 2 Router and Routing Basics Module 8 TCP/IP Suite Error and Control Messages

CCNA 2 Module 8 Introduction In this module we shall introduce the following subject matter: Describe ICMP ICMP Message Format ICMP Message Types and Causes

Overview of error messages TCP/IP and ICMP IP is known as a best effort delivery system, it has no means of reporting errors which is why it needs ICMP.

ICMP Main Features Of ICMP: Error reporting protocol for IP Reports Errors back to source address Dose not fix problems only reports them Can only report to the source address Encapsulated in to Datagrams (same as IP) Subject to delivery failures (same as IP) ICMP Messages therefore do not generate their own ICMP messages if delivery fails

Unreachable Networks Basic Conditions need to be met for network connection to be successful:- Correct TCP/IP Configuration at both ends Correct IP address and Subnet masks Correct default gateway Routers IP configuration Correct routing protocols Routers interfaces up and running If any of these conditions are not met then an ICMP unreachable network message will be generated.

ICMP Echo Request What is a ping? The ping command is issued to generate an ICMP Echo Request Used to check connectivity with another device Destination device replies with Echo Response

Long Routes and ICMP Example RIP RIP routing protocol uses hop counts as its primary metric as it is a distance vector routing protocol. An ICMP message would be generated if RIP was in use and a hop count exceeded the maximum of 15 hops. This is known as reaching its Time To Live (TTL) As a router processes a datagram it decreases the TTL value by one. When TTL = 0 the packet is discarded

ICMP Message Types ICMP Message format ICMP messages have 3 common fields, these include the Type, Code and Checksum field.

ICMP Message Types What each filed does? Type Field:- Indicates the type of ICMP message being sent. Code Field:- Gives further info specific to the ICMP message. Checksum Field:- Used to verify the integrity of the ICMP data.

Miscellaneous ICMP Messages Parameter Problem ICMP Message:- May be generated due to some kind of error in the datagram header This error does not give info about the state of the destination or the network but still stops the datagram from being delivered This error would be indicated with 12 in the type field of the ICMP datagram

TCP/IP Control Messages All implementations of IP must have some kind of control message capabilities which is done by ICMP IP Does not guarantee delivery and has no means to tell hosts when this occurs IP has no way of providing information or control messages to hosts Control messages are different to error messages as they may be used to notify hosts of network congestion or different routes on a network

ICMP Redirect Message Main Features: Control message used to notify host of different gateway. Generated by a router only. Router notifies host of different available gateway. Used when 2 gateways may be on a LAN Router has to be able to send redirect messages, by default Cisco routers do. The command “no ip redirects” will stop a router from sending ICMP redirect messages.

Clock Syncronisation Main Features: Software may require time syncronisation ICMP timestamp messages combat this problem Allows local host to ask for current time from remote host using ICMP Timestamp request (type 13) Remote host uses ICMP Timestamp reply (type 14) Code is always set to 0 as there is never any additional parameters Estimates time based on Universal Time (UT) Better way of synchronising clocks may be to use the network time protocol (NTP)

Info Request Messages Main Features: Obsolete as DHCP and BootP are now commonly used Used to request information and reply to information requests Example: May be used by a host to request a network number such as IP. Type 15 indicates info request message Type 16 indicates info reply message The command “no ip redirects” will stop a router from sending ICMP redirect messages.

Router Discovery Message Main Features: Host can learn about available gateways to other networks Host send a router solicitation message to begin the process using a multicast address of as the destination May also be a broadcast message in case a router does not accept multicast messages When a router receives the message it will advertise its available gateway

ICMP Source Quench Main Features: Used to tell a device to slow down the transmission speed of data that it is sending. May be used when bottle necks occur, for example on a WAN link with too much congestion Used to reduce the amount of data lost Warning! Source quench messages will in turn generate network congestion.

Good luck on the test! End of Presentation