The Complete IS-IS Routing Protocol- P16 pdf

} interface lo0.0; } } 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-0/1/2.0 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 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. 5.6.2 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 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 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 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 in draft-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 Layer Neighbour Liveliness Detection 137 138 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 the detected IGP neighbours. The IGP asks the BFD module to set up a BFD session to the Link IP addresses of the provided neighbours. BFD is (at time of writing this book) only available for JUNOS. The first release with support for BFD is JUNOS 6.1 onwards. The configuration of BFD is a property of the interface {} stanza inside the protocols isis {} branch. JUNOS configuration Under the bfd-liveness-detection stanza you can configure the minimum transmit interval 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 the neighbour does not receive BFD control packets for 500 ms, this router can declare the origniator 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 packets and 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 is that forwarding plane software can generate that traffic and the control plane is only needed for parameter setup. The following Tcpdump output shows the parameters that are conveyed using the 24-bytes fixed length packet. Tcpdump output The BFD protocol runs on top of UDP port 3784 and 3785. It is meant as a high frequency keep-alive mechanism which augments routing protocols that do not have sub-second timer support. 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 can accept. The pair of discriminators is necessary to multiplex several sessions between a pair of hosts. After BFD has been enabled on both sides, one can verify if a BFD-capable neighbour has 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 output Using the show bfd session command you can display the current state and details of the active BFD sessions. hannes@Frankfurt> show bfd session extensive Transmit Address State Interface Detect Time Interval Multiplier 172.16.223.236 Up so-0/1/2.0 1.000 0.100 5 Client ISIS L1, TX interval 0.100, RX interval 0.100, multiplier 5 Client ISIS L2, TX interval 0.100, RX interval 0.100, multiplier 5 Session up time 12:34:22, previous down time 00:00:06 Local diagnostic CtlExpire, remote diagnostic None Remote heard, hears us Min async interval 0.100, min slow interval 1.000 Adaptive async tx interval 0.100, rx interval 0.200 Local min tx interval 0.100, min rx interval 0.100, multiplier 5 Remote min tx interval 0.100, min rx interval 0.100, multiplier 5 Local discriminator 1, remote discriminator 2 Echo mode disabled/inactive 1 sessions, 2 clients Cumulative transmit rate 10.0 pps, cumulative receive rate 5.0 pps BFD is likely to become the dominant keep-alive protocol due its open implemen- tation. It is expected to even be the protocol of choice between routers and servers. For server applications like voice-over IP or financial applications there are open-source BFD implementations for hosts available. Neighbour Liveliness Detection 139 5.7 Summary IS-IS adjacency processing has changed over the years. It started out with simple 2-way finite state machines and, due to the underlying problems of not detecting half-broken links, it quickly evolved to a 3-way FSM. It is remarkable that the defects of the under- lying protocol have been solved with just the addition of an optional Adjacency State TLV. Reliably detecting that a neighbour is Up or Down is not enough for today’s ser- vice provider environments. On the one hand the implementation has to be slow enough to protect the network from flapping adjacencies that are propagated through the network – on the other hand there is a need for quick keep-alive detection mechanisms. Due to the rise of Ethernet as popular core-facing interface technology, an LMI-like protocol like Bi-directional Fault Detection (BFD) had to be designed. The application of BFD to serve as an IGP detection protocol is just the start. It is expected that BFD will be used for other network protocols or other environments like application keep-alive detection for mission-critical servers. 140 5. Neighbour Discovery and Handshaking 6 Generating, Flooding and Ageing LSPs 141 Unlike distance vector protocols, such as RIP, link-state routing protocols, such as OSPF and IS-IS, don’t tell only their neighbours about the topology of the network. Link-state protocols distribute both their IP reachability and topological view far beyond their adja- cent neighbours, ultimately flooding this information to all routers in an area. To keep the reachability information in the network current, link-state protocols need to have a basic set of functions available that can be used to originate, distribute and finally revoke, or time-out topology information. In IS-IS-speak, that piece of topology information is encoded in a link-state protocol data unit (LSP). This chapter covers these functions and the surrounding network events th at cause the IS-IS protocol to generate, flood and finally age LSPs. Link-state routing protocols such as IS-IS follow a paradigm that can best be described as distributed databases with local computation, which is quite different to the way other common routing protocols like RIP and BGP work. Distributed databases are discussed in more detail in the following section. 6.1 Distributed Databases Before explaining how a distributed database works, first consider what a localized data- base looks like and how routing protocols use it. Localized databases mean that every router has its own local view of the network and does not know the exact topology of the network as a whole. This is like a tourist in a foreign city having no clue about what the overall topology of the city (the street layout) looks like. All the tourist has is a local view of the places and streets that are next to the tourist’s immediate location. This makes it very difficult to find the best path to a landmark or museum, and in the worst case situation, the tourist has to try out several paths, being careful not to circle around the same locations. Localized databases work the same way. In contrast, a distributed database approach works differently: here all of the routers share common information about what the network looks like. If the tourist in the example has got lost, a distributed database map would give them a more complete map of the best way to get to a particular destination in the city (or in this case, the network). How does IS-IS compute the map of the network? If each router just contributes its local view to its neighbours, and if that information can be shared among all the routers in a network, then each will ultimately have a global map of the network. Link-state rout- ing protocols, such as IS-IS, work like a jigsaw puzzle, as shown in Figure 6.1. Each router in the figure represents one piece of the puzzle, and if all of the puzzle pieces are present, then each router can start to put the puzzle together to acquire an understanding of what the big picture looks like. The collection of puzzle pieces is called the link-state database. IS-IS has a number of techniques called flooding and synchronizing to get a complete map of the network. In this chapter, you will learn about the individual puzzle pieces, which are called link-state protocol data units (LSPs), and how IS-IS distributes the information they contain. IS-IS, by default, tries to acquire two maps from its neighbours and therefore maintains two databases to store topology-related information. Information for the first map, which typically represents the topology inside the close collection of routers, called a point- of-presence (POP), is stored in the Level 1 (L1) database. Sticking with the lost tourist example, think of this as just a local map that guides you around the next few streets. The second map, which typically represents the backbone structure of the network, is stored 142 6. Generating, Flooding and Ageing LSPs Pennsauken Frankfurt London Washington NewYork Paris FIGURE 6.1. A distributed link-state database is like a jigsaw puzzle Distributed Databases 143 in the Level 2 (L2) database. This would best compare to a nationwide map where all the freeways and highways are shown. You can take a quick look into both of these link-state databases to find out exactly which puzzle pieces the database holds by issuing a show isis database command on both the IOS and JUNOS software platforms. IOS command output The contents of the IS-IS link-state database can be displayed using the show isis database command: Amsterdam# show isis database [… ] IS-IS Level-1 Link State Database: LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL New-York.00-00 0x00002fac 0xC24F 60128 1/0/0 [… ] IS-IS Level-2 Link State Database: LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL LINX-gw.00-00 0x00000128 0xB8EF 36163 0/0/1 LINX-gw.01-00 0x00000128 0x455A 42001 0/0/0 VIX-gw.00-00 0x00000123 0xEFC1 5023 0/0/0 [… ] The output shows you one “puzzle piece” of information in each line. Each LSP is uniquely identified by its LSP-ID. The exact format of the LSP-ID will be discussed later in this chapter. All that is important for now is that it says something about the router that originated that LSP. The sequence number is a kind of version field that tells receivers which information is more recent. The checksum enables to check at the receiver if the LSP has been corrupted during its long way through the network. Finally, hold-time and the ATT/P/OL gives some information about the validity of that information and how long it will be valid, and just like a passport it has an expiration date. JUNOS command output In the JUNOS software, you can display the IS-IS database using the show isis data- base command. Watch for an inconsistency between the LSPs being sent and received, as this is a problem indication: hannes@New-York> show isis database IS-IS level 1 link-state database: LSP ID Sequence Checksum Lifetime Attributes New-York.00-00 0x2fac 0xc24f 62063 L1 Attached [… ] 4 LSPs IS-IS level 2 link-state database: LSP ID Sequence Checksum Lifetime Attributes LINX-gw.00-00 0x128 0xb8ef 36063 L1 L2 Overload LINX-gw.01-00 0x128 0x455a 41901 L1 L2 VIX-gw.00-00 0x123 0xefc1 4922 L1 L2 [… ] 12 LSPs The JUNOS software output contains similar information to the IOS software output. The only difference is the little bit more detailed breakdown of the so-called attribute typeblock, which will be discussed later in this chapter. As far as the attribute typeblock is concerned, the JUNOS output is more verbose than the IOS equivalent. Based on the information in the two link-state databases for L1 and L2, each router in an IS-IS network computes the topology of the network independently of every other router. This principle of independent router operation is called local computation. This is the topic of the next discussion. 6.2 Local Computation Routing protocols, such as RIP or BGP, compute the best path through a network in a dis- tributed fashion. That is, no single RIP or BGP router knows what the other routers know about the route, and this is a real limitation. For instance, each time a RIP router passes on a route to its neighbour, the route gets a worse value. This “worseness” is indicted in a metric field, which represents the hop-count (number of routers) that a router is away from the router attached to the source sub-net. In Figure 6.2 the sub-net 192.168.1/24 is directly connected to RIP Router #1. Router #1 reports the sub-net to its neighbours with a hop count of 1. Router #2 learns the sub-net with a metric of 1 and reports it further to the right side of the figure after incrementing the metric field by one. The routing update therefore arrives at Router #3 with a metric of 2. But Router #1 has no idea what value this route has on Router #3. RIP routing illustrates how a distributed computation scheme works. IS-IS utilizes a totally different way of calculating routing information. Before the route calculation takes place, all IS-IS routers distribute the information about the local views of the routers to each other. Intermediate routers along the way must not change these views (represented in the LSPs). After this distribution (flooding), a common route- calculation scheme, which in IS-IS is called the shortest path first (SPF) algorithm, is applied. Note that each router computes the routes independently from every other router. 144 6. Generating, Flooding and Ageing LSPs 192.168.1/24 … RIP Router 1 RIP Router 2 RIP Router 3 RIP Router 4 192.168.1/24 (1) 192.168.1/24 (2) 192.168.1/24 (3) … 192.168.1/24 (4) … … FIGURE 6.2. RIP calculates the metric in a distributed fashion Local Computation 145 You can watch the result of the SPF calculation by issuing a show ip route isis com- mand on IOS platforms and a show isis route command on Juniper Network routers. In IOS, you cannot see the entire results of the SPF calculation – all you can see are the results that make it into the main routing table. That excludes redundant routers that happen to be in both IS-IS levels. The IS-IS learned routes that are active in the routing table can be displayed using the show ip route isis command. IOS command output Amsterdam#show ip route isis [… ] i L1 192.168.0.55 [115/10] via 172.16.144.2, POS3/0 i L1 192.168.0.57 [115/10] via 172.16.144.2, POS4/0 i L2 192.168.1.122 [115/10] via 172.16.177.18, GigabitEthernet3/0 [… ] The output tells us basically what routes (second column) did get installed in the local routing and forwarding tables. Each line contains information about a single route. The first column shows the level. The numbers in the brackets after the route give informa- tion about the weight or, as it is called in Cisco IOS speak, the administrative distance of the routing protocol that inserted this route into the routing table (in this case, IS-IS). After the “via” statement the IP address of a locally connected router appears (the next- hop). Finally the end of each line gives the physical interface through which the next-hop can be reached and this is how packets to this destination will leave the router. In the JUNOS software, you can display both the immediate results from the SPF calcu- lation as well as the routes installed in the routing table. The SPF results are displayed using the show isis route command. The IS-IS learned routes that are active in the main routing table can be displayed using the show route protocol isis command. JUNOS software command output hannes@Pennsauken> show isis route IS-IS routing table Current version: L1: 0 L2: 485 Prefix L Version Metric Type Interface Via 172.16.44.248/30 2 485 61770 int so-3/0/0.0 London 192.168.49.5/32 2 485 67850 int so-3/0/0.0 London 192.168.49.67/32 2 485 67860 int so-3/0/0.0 London 192.168.52.177/32 2 485 127850 int so-3/0/0.0 Frankfurt 192.168.54.164/32 2 485 128510 int so-3/0/0.0 Frankfurt 172.16.176.0/24 2 485 121770 int so-3/0/0.0 Frankfurt 172.16.176.32/30 2 485 127830 int so-3/0/0.0 New-York 172.16.176.60/30 2 485 123790 int so-3/0/0.0 New-York [… ] The format of the output is one route entry per line. The first column contains the route and the second column contains information about the level where this route did result from. The version field is just an internal number that tells you how the SPF run number based upon this route was calculated. The version field is typically not interesting in troubleshooting networks. The metric tells the distance relative to the local router of the prefix. Next is an indication whether the route is internal or external. Typically all the routes are internal unless routes have been injected from other routing protocols into the IS- IS database. Finally, the interface where the traffic leaves the router is displayed, plus the forwarding router’s name. JUNOS software command output hannes@Pennsauken> show route protocol isis inet.0: 118243 destinations, 246129 routes (118243 active, 0 holddown, 0 hidden) + = Active Route, - = Last Active, * = Both 172.16.44.248/30 *[IS-IS/18] 4d 12:57:11, metric 41550, tag 2 > to 172.16.5.93 via so-3/2/0.0 192.168.49.5/32 *[IS-IS/18] 2d 07:26:54, metric 67850, tag 2 > to 172.16.5.93 via so-2/3/0.0 192.168.49.67/32 *[IS-IS/18] 1d 20:01:28, metric 67860, tag 2 > to 172.16.5.93 via so-7/0/0.0 [… ] In the show route protocol isis output we can see a subset of routes that got displayed in show isis routes. Those are the routes that competed for installation in the routing table with other routing protocols that may have had similar information; however, the routes in this table are the ones that have won. In JUNOS the level of routes is displayed in the tag field – a tag 2 means that this is a Level 2 route. The number in the brackets is a similar value to the administrative distance for IOS platforms, called sim- ply the route preference. The to and via keywords indicate the next-hop and the outgoing interface. The universal transport vehicle to build the IS-IS database map is called a link-state protocol data unit or LSP for short, which is another OSI-speak word for link-state packet. In the following sections, you will see what information an LSP contains, how the LSP gets distributed, and how LSPs get throttled when the network is busy. 6.3 LSPs and Revision Control The information element that transports IS-IS-related information to populate all the routers’ link-state databases is called the link-state PDU or LSP. Each router in an IS-IS network generates at least one LSP that describes, as the name implies, the current state of the links to other routers. Actually, an LSP conveys more than just the state of the links or circuits on the router. Routers use LSPs as a kind of envelope to get different types of information elements such as IP routes, checksums and even router names across to other routers. LSPs need to be accurate and up-to-date. If, for example, a link between a pair of routers goes down, both routers must immediately tell the other routers in the network 146 6. Generating, Flooding and Ageing LSPs . calcu- lation as well as the routes installed in the routing table. The SPF results are displayed using the show isis route command. The IS-IS learned routes that are active in the main routing table can. route into the routing table (in this case, IS-IS) . After the “via” statement the IP address of a locally connected router appears (the next- hop). Finally the end of each line gives the physical. column shows the level. The numbers in the brackets after the route give informa- tion about the weight or, as it is called in Cisco IOS speak, the administrative distance of the routing protocol