The Complete IS-IS Routing Protocol- P6

The Complete IS-IS Routing Protocol- P6:IS-IS has always been my favourite Interior Gateway Protocol. Its elegant simplicity, itswell-structured data formats, its flexibility and easy extensibility are all appealing – IS-ISepitomizes link-state routing. Whether for this reason or others, IS-IS is the IGP of choicein some of the world’s largest networks. Thus, if one is at all interested in routing, it is wellworth the time and effort to learn IS-IS.
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The Complete IS-IS Routing Protocol- P6 Neighbour Liveliness Detection 137 } interface lo0.0; }} JUNOS displays the Hello timers in millisecond resolution using the show isisinterface detail operational level command.JUNOS command outputhannes@Frankfurt> show isis interface detailIS-IS interface database:so-0/1/2.0 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 0.333 1 2 1 64 10 0.333 1 The JUNOS interface multiplier is hard coded (meaning it cannot be changed) to avalue of 3. A hold-timer of 1 second therefore results in a Hello interval of 333 ms, whichis the lowest Hello interval possible on JUNOS. Relying on Hellos puts an upper boundary of 1 second to the detection time followinga link-failure on the routing protocols. But by tracking an interface state, routers candetect the liveliness state much more quickly.5.6.2 Interface TrackingThe chipsets that drive modern router interfaces report link errors, such as a loss ofsignal, 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 prob-lems, as illustrated in Figure 5.10. If there are active elements in the middle of the trans-mission chain, then local errors are not propagated downstream and the receiving routerdoes not detect that the light went out. SONET/SDH offers a true advantage over other physical media like Ethernet, whichdo not propagate local errors to downstream Network Elements. Many Protocols like Frame Relay and ATM also include their own Local ManagementInterface (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 detec-tion attempts make a neutral liveliness-checking protocol available.5.6.3 Bi-directional Fault Detection (BFD)BFD is defined in draft-katz-ward-bfd-01, and its encoding rules are documented indraft-katz-ward-bfd-v4v6-1hop-00. BFD is an answer to the following problems:• Link-Layer neutral high frequency keep-alive protocol• Offload high frequency keep-alive processing from the IGP Layer138 5. Neighbour Discovery and Handshaking• Support sub-second timers on behalf of protocols that cannot• Negotiate timers dynamically The BFD protocol, unlike many other protocols, includes no auto-neighbour discov-ery. It has client software instead, typical of the IP routing protocols, and based on thedetected IGP neighbours. The IGP asks the BFD module to set up a BFD session to theLink IP addresses of the provided neighbours. BFD is (at time of writing this book) only available for JUNOS. The first release withsupport for BFD is JUNOS 6.1 onwards. The configuration of BFD is a property of theinterface {} stanza inside the protocols isis {} branch.JUNOS configurationUnder the bfd-liveness-detection stanza you can configure the minimum transmitinterval plus the detection-time multiplier.protocols { isis { interface so-1/2/0.0 { bfd-liveness-detection { minimum-interval 100; multiplier 5; } } [ … ] interface lo0.0; }} In this example the router emits Hello packets at a rate of once every 100 ms. If theneighbour does not receive BFD control packets for 500 ms, this router can declare theorigniator dead and move to an interface down state in the FSM. BFD runs on top of IP UDP port 3784 and 3785. Port 3784 is used for control packetsand 3785 is used for Echo Mode traffic. The JUNOS implementation just supports con-trol packets for liveliness detection. Echo Mode is envisioned for the future: the plan isthat forwarding plane software can generate that traffic and the control plane is onlyneeded for parameter setup. The following Tcpdump output shows the parameters that are conveyed using the24-bytes fixed length packet.Tcpdump outputThe BFD protocol runs on top of UDP port 3784 and 3785. It is meant as a high frequencykeep-alive mechanism which augments routing protocols that do not have sub-secondtimer support. Neighbour Liveliness Detection 139 09:32:30.884968 IP 172.16.223.236.3784 > 172.16.223.235.3784: BFDv0, length: 24 Control, Flags: [I Hear You], Diagnostic: Control Detection Time Expired (0x01) Detection Timer Multiplier: 5 (500 ms Detection time), BFD Length: 24 My Discriminator: 0x00000001, Your Discriminator: 0x00000002 Desired min Tx Interval: 100 ms Required min Rx Interval: 200 ms Required min Echo Interval: 0 ms Session state transactions are provided using the Flag contents. The Desired/Required Timer Fields are used for negotiating a common timer that both peers canaccept. The pair of discriminators is necessary to multiplex several sessions between apair of hosts. After BFD has been enabled on both sides, one can verify if a BFD-capable neighbourhas been found on the other end and if the BFD session is Up. The show bfd ses-sion command displays the session state.JUNOS command outputUsing the show bfd session command you can display the current state and details ofthe active BFD sessions.hannes@Frankfurt> show bfd session extensive TransmitAddress State Interface Detect Time Interval ...