Ccna routing and Switching Complete Study Guide

Buffer full/source quench    If a router’s memory buffer for receiving incoming datagrams is 

full, it will use ICMP to send out this message alert until the congestion abates.

Hops/time exceeded    Each IP datagram is allotted a certain number of routers, called 

hops, to pass through. If it reaches its limit of hops before arriving at its destination, 

the last router to receive that datagram deletes it. The executioner router then uses 

ICMP to send an obituary message, informing the sending machine of the demise of its 

datagram.

Ping    Packet Internet Groper (Ping) uses ICMP echo request and reply messages to check 

the physical and logical connectivity of machines on an internetwork.

Traceroute    Using ICMP time-outs, Traceroute is used to discover the path a packet takes 

as it traverses an internetwork.

Traceroute is usually just called trace. Microsoft Windows uses tracert to 

allow you to verify address configurations in your internetwork.

The following data is from a network analyzer catching an ICMP echo request:

Flags:         0x00

 Status:        0x00

 Packet Length: 78

 Timestamp:     14:04:25.967000 12/20/03

Ethernet Header

 Destination: 00:a0:24:6e:0f:a8

---

TCP/IP and the DoD Model 

113

 Source:      00:80:c7:a8:f0:3d

 Ether-Type:  08-00 IP

IP Header - Internet Protocol Datagram

 Version:             4

 Header Length:       5

 Precedence:          0

 Type of Service:     %000

 Unused:              %00

 Total Length:        60

 Identifier:          56325

 Fragmentation Flags: %000

 Fragment Offset:     0

 Time To Live:        32