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CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 81

Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 81 is the Cisco approved textbook to use alongside version 3.1 of the Cisco Networking Academy Program CCNA 1 and CCNA 2 web-based courses. The topics covered provide you with the necessary knowledge to begin your preparation for the CCNA certification exam (640-801, or 640-821 and 640-811) and to enter the field of network administration. | 1102.book Page 769 Tuesday May 20 2003 2 53 PM ICMP 769 Table 17-2 Cisco ping Return Codes Continued Code Meaning Possible Cause s C An ICMP source quench message was received. A device along the path possibly the destination might be receiving too much traffic check input queues. An ICMP time exceeded message was received. A routing loop might have occurred. Detecting Excessively Long Routes Several problem situations can occur in network communication. In one situation a datagram travels in a circle never reaching its destination. In another situation no path exists between the source and the destination that conforms to the limitations of the routing protocol. An example of the first situation arises when two routers continually route a datagram back and forth. In this scenario each router operates with the understanding that the other router should be the next hop to the destination. One potential cause of this problem is faulty routing information. Routing protocols can also have limits on the distance that a packet is allowed to travel. For example RIP has a hop limit of 15 which simply means that the packet is allowed to pass through only 15 routers. In each of these cases an excessively long route exists. Whether the actual path includes too many hops or a circular routing path exists the packet eventually will reach the end of its life. When a packet reaches the end of its life it has reached its Time-To-Live TTL . The TTL value typically matches the maximum hop count defined by the routing protocol. In each datagram a TTL value is defined. As each router processes the datagram it decreases the TTL value by one. When the TTL value of the datagram reaches 0 the packet is discarded. ICMP uses a time exceeded message to notify the source device that the TTL has been exceeded. ICMP Message Format Echo Messages As with any type of packet ICMP messages have special formats. Each ICMP message type shown in Table 17-3 has its own unique characteristics. However .