Trong khi mỗi cây chỉ có một cây cầu gốc, là một trong cầu nối cho mỗi LAN, trở thành cầu nối tất cả các host và cầu về việc sử dụng mạng LAN để đạt được mục gốc. Cây cầu được chỉ định là lựa chọn bằng cách xác định cầu nối trong mạng LAN có chi phí đường đi thấp nhất để cây cầu gốc. Vì vậy, sử dụng Hình 15-2 là một ví dụ | This document was created by an unregistered ChmMagic please go to http to register it. Thanks . Record Route Option The purpose of this option is to ask the routers along the way between source and destination to store the IP addresses of the outgoing interfaces they use to forward the packet. Because of limited space in the header only nine addresses at most can be stored and even fewer if the header contains other options . Therefore the packet arrives with the first nine addresses stored in the option the receiver has no way of knowing what routers were used after that. Since this option makes the header and therefore the IP packet grow along the way and since other options may be present in the header the sender is supposed to reserve the space that will be used to store the addresses. If the reserved space becomes full before the packet gets to its destination the additional addresses are not added to the list even if the maximum size of an IP header would permit it. No errors ICMP messages are generated when there is no room to store a new address. For obvious reasons the sender is supposed to reserve an amount of space that is a multiple of 4 bytes the size of an IP address . 40-3 4 9 where 40 is the maximum size of the IP options 3 is the size of the options header and 4 is the size of an IPv4 address. The value of length is not an exact multiple of 4 because the option header type length and pointer is 3 bytes long. This means that the 32-bit IP addresses are inconveniently split across 32-bit word boundaries. Figure 18-7 shows how the IP header portion dedicated to the option changes hop by hop. As each router fills its address it also updates the pointer field to indicate the end of the data in the option. The offsets at the bottom of the figure start from 1 so that you can compare them to the value of the pointer field. Simbo PDF Merge anq Split nnreSig req Vereion - bA www mbobqvcom This document was created by an unregistered .
