Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 82 is the Cisco approved textbook to use alongside version 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. | Page 779 Tuesday May 20 2003 2 53 PM TCP IP Suite Control Messages 779 However this support is not always the case. If a router-solicitation message is sent to a router that does not support the discovery process the solicitation will go unanswered. Figure 17-15 Router-Discovery Message 0 8 16 31 Type 9 Code 0 Checksum Number of Addresses Address Entry Size Lifetime Router Address 1 Preference Level 1 Router Address 2 Preference Level 2 When a router that supports the discovery process receives the router-solicitation message a router advertisement is sent in return. Table 17-7 provides an explanation for each field of the frame format. Table 17-7 Router Frame Format ICMP Field Description Type 9. Type 0. Checksum The 16-bit 1s complement of the 1s complement sum of the ICMP message starting with the ICMP type. For computing the checksum the checksum field should be 0. Number of Addresses The number of router addresses advertised in this message. Address Entry Size The number of 32-bit words of information per each router address 2 in the version of protocol described here . Lifetime The maximum number of seconds that the router addresses are considered valid. Router Addresses The sending router IP address es . The number of the address interface from which this message is sent. Preference Level The preference of each router address. The number address as a default router address relative to other router addresses on the same subnet. A signed twos-complement value higher values mean more preferable. Page 780 Tuesday May 20 2003 2 53 PM 780 Chapter 17 TCP IP Error and Control Messages Router-Solicitation Message A host generates an ICMP router-solicitation message in response to a missing default gateway. This message is sent using multicast. This message is the first step in the routerdiscovery process. A local router responds with a router advertisement identifying the default gateway for the local host. Figure 17-16 identifies the frame format
