TAILIEUCHUNG - The Complete IS-IS Routing Protocol- Part 16

The Complete IS-IS Routing Protocol- P16: 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. | Neighbour Liveliness Detection 137 interface JUNOS displays the Hello timers in millisecond resolution using the show isis interface detail operational level command. JUNOS command output hannes@Frankfurt show isis interface detail IS-IS interface database so-0 1 Index 67 State 0x6 Circuit id 0x1 Circuit type 3 LSP interval 100 ms CSNP interval 5 s Level Adjacencies Priority Metric Hello s Hold s Designated Router 1 1 64 10 1 2 1 64 10 1 The JUNOS interface multiplier is hard coded meaning it cannot be changed to a value of 3. A hold-timer of 1 second therefore results in a Hello interval of 333 ms which is the lowest Hello interval possible on JUNOS. Relying on Hellos puts an upper boundary of 1 second to the detection time following a link-failure on the routing protocols. But by tracking an interface state routers can detect the liveliness state much more quickly. Interface Tracking The chipsets that drive modern router interfaces report link errors such as a loss of signal to the routing sub-system within a few milliseconds. For high-speed detection therefore optical interfaces are the best choice. However there are still similar problems as illustrated in Figure . If there are active elements in the middle of the transmission chain then local errors are not propagated downstream and the receiving router does not detect that the light went out. SONET SDH offers a true advantage over other physical media like Ethernet which do not propagate local errors to downstream Network Elements. Many Protocols like Frame Relay and ATM also include their own Local Management Interface LMI protocol which performs link-layer keep-alive checking and so on. Unfortunately there is still no LMI-like protocol for Ethernet. Bi-directional fault detection attempts make a neutral liveliness-checking protocol available. Bi-directional Fault Detection BFD BFD is defined in draft-katz-ward-bfd-01 and its encoding rules are documented in .

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