tailieunhanh - The Complete IS-IS Routing Protocol- P24

The Complete IS-IS Routing Protocol- P24: IS-IS has always been my favourite Interior Gateway Protocol. Its elegant simplicity, its well-structured data formats, its flexibility and easy extensibility are all appealing – IS-IS epitomizes link-state routing. Whether for this reason or others, IS-IS is the IGP of choice in some of the world’s largest networks. | IS-IS Application Level Fragmentation 235 in the Hello message. Actually it has to occur several times in the Hello because a single Padding TLV can only hold and therefore pad up to 255 bytes. So there may be up to five full-sized Padding TLVs necessary to make the frame big enough. The following tcpdump output shows several occurrences of the Padding TLV 8. Tcpdump output 00 58 OSI IS-IS length 1497 L1 Lan IIH hlen 27 v 1 pdu-v 1 sys-id-len 6 0 max-area 3 0 Point-to-point Adjacency State TLV 24 0 length 1 Adjacency State Up Protocols supported TLV 129 length 2 NLPID s IPv4 IPv6 IPv4 Interface address es TLV 132 length 4 IPv4 interface address Area address es TLV 1 length 4 Area address 3 Restart Signaling TLV 211 length 3 Restart Request bit clear Restart Acknowledgement bit clear Remaining holding time 0s Checksum TLV 12 length 2 checksum 0xadbb correct 236 9. Fragmentation Padding TLV 8 length 255 Padding TLV 8 length 255 Padding TLV 8 length 255 Padding TLV 8 length 255 Padding TLV 8 length 255 Padding TLV 8 length 168 There is no mechanism in the Hello protocol to support more information than fits in a single packet. There is no concept of distributing for instance certain capability codes over several Hello messages. In IS-IS each preceding Hello message entirely supersedes the previous one. There is simply no support for multi-part Hello messages. That gives also the upper boundary of 1492 bytes that each neighbour may advertise. Luckily IS-IS today utilizes only 5 per cent of that space. In Hello messages there is no need to support multi-packet messages and therefore in the application IS-IS there is no hook for multi-part Hello messages specified. Sequence Number Packets SNPs Sequence Number Packets SNPs have two flavours complete CSNP and partial PSNP and three purposes 1. Acknowledging receipt of a link-state packet a job for PSNP 2. Requesting a more recent version of LSPs due to detection of a mismatch of some