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The Complete IS-IS Routing Protocol - Part 14

The Complete IS-IS Routing Protocol- P14: 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. | MTU Check 117 Bytes TLV Type TLV Length Padding Data 1 1 1-255 Figure 5.5. The Padding TLV 8 is used to bloat IIHs up to at least 1492 bytes Tcpdump output 20 16 37.411690 OSI IS-IS length 1492 L1 Lan IIH hlen 27 v 1 pdu-v 1 sys-id-len 6 0 max-area 3 0 source-id 1921.6800.1008 holding time 120s Flags L1 L2 lan-id 1921.6800.1008.02 Priority 64 PDU length 1492 IS Neighbor s TLV 6 length 6 SNPA 0090.692b.0e52 Protocols supported TLV 129 length 1 NLPID s IPv4 0xcc IPv4 Interface address es TLV 132 length 4 IPv4 interface address 193.83.223.236 Area address es TLV 1 length 4 Area address length 3 49.0001 Restart Signaling TLV 211 length 3 Flags none Remaining holding time 0s 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 150 If a router exchanges these bloated Hello PDUs in both directions then it can be sure that the underlying media sufficiently supports the maximum packet sizes necessary for IS-IS. IOS and JUNOS do have different styles of how and when they do implement adjacency checks. IOS pads each and every Hello that it transmits on the wire. On large WAN Hub Routers that terminate a lot of circuits - for example on a Router running Frame relay or ATM circuits - periodic emission of large packets can be a burden to the control plane processor. If you know that your underlying link supports at least 1492 bytes sized packets then you can turn off the artificial bloating of Hello PDUs using the no hello padding router configuration command. 118 5. Neighbour Discovery and Handshaking IOS configuration The no hello padding command turns off MTU check against the underlying media. router isis no hello padding JUNOS encompasses a technique called smart padding where the router transmits padded Hellos only at the beginning of the Adjacency Bring up. After both ends of a router have completed the handshake procedure JUNOS automatically omits the Padding TLVs in the Hello .