TAILIEUCHUNG - The Complete IS-IS Routing Protocol- Part 15

The Complete IS-IS Routing Protocol- P15: 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. | Figure . Two reported unidirectional LSP advertisements make other routers think that there is a single bi-directional advertisement 128 5. Neighbour Discovery and Handshaking TLV Type 2 40 TLV Length 1 5 11 15 Adjacency State Extended Local Circuit-ID Neighbour System-ID Neighbour Extended Local Circuit-ID Bytes 1 1 1 4 6 4 Optional Figure . The second part of the Adjacency State TLV is optional The 3-way Handshake on Point-to-point Circuits In LAN environments the IS Neighbour TLV 6 does convey the information elements needed for performing the 3-way handshaking function. Unfortunately this specific TLV is tailored to LAN environments only. Recall that the information elements to transport the Hello I have seen you message is the SNPA a MAC address. MAC addresses are typical to broadcast circuits such as Ethernet however the typical WAN OSI-RM Layer 2 protocols like PPP Cisco-HDLC Frame-Relay or ATM RFC 1483-SNAP do not have the notion of MAC addresses. All of those WAN protocols are optimized for point-to-point environments where MAC addressing is not used or necessary. Typically the WAN protocols just need to frame a packet and transmit it to the remote end. Addressing is not needed because there are just two speakers on the circuit the remote router and the local router. Fortunately there is an extension to the base ISO 10589 specification RFC 3373 that specifies an optional TLV that carries adjacency states and a few other information elements in a special TLV. The Adjacency State TLV 240 is discussed in the next section. Adjacency State TLV 240 The main purpose of transporting adjacency states is to find out if the Hello message that a router has received was sent in response to receipt of a previous Hello or is just any Hello sent by the remote router. If a router detects that the Hello received was sent in response to a previous Hello message sent it is safe to assume the routers are on a working bi-directional circuit. This excludes

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