1. IS-IS Suman Pandey As a part of EECS702: Topics in Computer Systems - Future Internet (Spring 2008)

2. Outline Basics Link-state routing fundamentals CLNS (OSI model) Overview of IS-IS IS-IS history Integrated IS-IS recent enhancements of IS- IS Future

3. What type of protocol is IS-IS? Exterior Gateway protocol (EGP) BGP Interior Gateway Protocol (IGP) Distance vector routing Routing Information Protocol (RIP) Interior Gateway Routing Protocol (IGRP) Link State Routing Open Shortest Path First (OSPF) Intermediate system to intermediate system (IS-IS)

4. Link-state protocol fundamentals

5. About link-state protocols Building Link State Packets (LSP PDU) Learning about the Neighbors Measuring Line Cost Distributing or flooding the Link State Packets everywhere All routers exchange all LSPs via a reliable flooding mechanism All routers store all LSPs in a link-state database (LSPDB) All routers should have exactly the same LSPDB but different RIB (Routing Table) Computing the New Routes using SPF Each router executes Dijkstra’s Shortest Path First algorithm (SPF) The topology is calculated as a ShortestPath Tree (SPT), with itself as root Each router computes a different SPT From the SPT the RIBs are calculated

6. Building Link State Packets (LSP PDU) A LSP PDU Seq. Age B4 C2 E7 Hello Pack Ack

7. Distributing or flooding the Link State Packets everywhere A LSP PDU Seq. Age B4 C2 E7

8. All routers have same view Computing Routes using SPF

9. Brief overview of IS-IS

10. About IS-IS IS-IS was defined in 1992 in the ISO/IEC recommendation 10589 IS IS “OSI speak” for router Supports 2-level hierarchy : level 1 (areas) and level 2 (backbone) IS-IS is the Intermediate System to Intermediate System intra-domain routing protocol developed by Digital Equipment Corporation as part of DECnet Phase V Easy to extend for other protocols mainly IPv6 The latest extension of IS-IS supports MPLS (Multi Protocol Label Switching) traffic Engineering features The purpose of IS-IS was to make possible the routing of datagram using the ISO-developed OSI protocol stack called CLNS ( Connection Less Network Services).

11. CLNS (OSI network layer) OSI CLNS (Connection Less Network Service) is a network layer service CLNS service is provided using CLNP protocol. In OSI Routers are ISs, and hosts are End Systems (ESs). There is no ARP, ICMP, IDRP for CLNS, but there is ES-IS protocol which provides similar kind of services. ISs discover ESs by listening to ESHs, and ISs send ISHs to ESs. ES doesn't have any information about routing On the internet the CLNS service is provided by IP.

12. CLNS (OSI network layer) cont….. Table 1 Comparing IP and OSI Services Basic connectionless service: IP (RFC 791) CLNS (ISO 8473) Neighbor greeting and error Reports to source about packet delivery: ICMP (RFC 792) ARP (RFC 826), IRDP (RFC 1256) CLNP (ISO 8743) ES-IS (ISO 9542) Routing: Integrated IS-IS (RFC 1195) Participants are routers and hosts IS-IS (ISO 10589) Participants are ISs and ESs IP autonomous systemISO routing domain Interior Gateway Protocol (IGP)Intradomain Routing Protocol Exterior Gateway Protocol (EGP) Border Gateway Protocol (BGP) for IP (RFC 1105) Static IP routes Interdomain Routing Protocol (IDRP) ISO IDRP (proposal) Static CLNS routes

13. Integrated or Dual IS-IS IETF developed specification of Integrated IS-IS (RFC 1195) To integrate IS-IS to IP and OSI both IS-IS allows the interconnection of dual (IP and OSI) routing domains with other dual domains, with IP-only domains, and with OSI-only domains.

14. IS-IS Routing Operation at a glance Routers running IS-IS will send hello packets out all IS-IS-enabled interfaces to discover neighbors and establish adjacencies. Routers may build a link-state packet (LSP) Generally, routers flood LSPs to all adjacent neighbors except the neighbor from which they received the same LSP. All routers will construct their link-state database from these LSPs. A shortest-path tree (SPT) is calculated by each IS, and from this SPT the routing table is built.

15. IS-IS Routing Operation at a glance cont…. The receive process is the entry point for all data, including user data, error reports, routing information, and control packets. It passes user data and error reports to the forward process and passes routing information and control packets (hellos, LSPs, and sequence number packets) to the update process. The update process generates local link information that is flooded to adjacent routers; in addition, the update process receives, processes, and forwards link information received from adjacent routers. This process manages the Level 1 and Level 2 link-state databases and floods Level 1 and Level 2 LSPs throughout an area. Each LSP that resides in the link-state database has a remaining lifetime, a checksum, and a sequence number. The decision process runs shortest-path-first (SPF) algorithm on the link-state database, and creates the forwarding database. It computes next-hop information and computes sets of equal-cost paths, creating an adjacency set that is used for load balancing. On a Cisco router, IS- IS supports load balancing over and up to six equal-cost paths. The forward process gets its input from the receive process and uses the forwarding database to forward data packets toward their destination. It also redirects load sharing and generates error reports. IS-IS Data-Flow Diagram

16. IS-IS topology concepts Area Backbone L1 Router L2 Router L1/L2 Router

17. Backbone in IS-IS vs. OSPF IS-IS backboneOSPF backbone

18. Area Design in IS-IS vs OSPF OSPFIS-IS The border is inside routers (ABRs) Each link belongs to one area Each IS-IS router belong to exactly one area IS-IS is more flexible when extending the backbone

19. Hierarchy of IS-IS : L1, L2 Routers L1 Routers Intra-area routing Neighbors only in the same area L1 has information only about their own area Can not know the routers outside the area L1 routers looks at the attached-bit in level 1 LSPs to find the closest L1/L2 router in the area Use the closest L1/L2 to exit the area

20. Hierarchy of IS-IS : L1, L2 Routers L2 Routers Inter-area routing May have neighbors in other area Know the level 2 topology Know which routers are reachable via each level 2 router Can exchange data packets and routing information directly with external routers located outside of its own area L1/L2 router must maintain both a level 1 and level 2 link state database separately and will calculate SPF tree for L1 and L2 topology.

21. CLNS addressing

22. OSI network layer addressing is implemented with NSAP (network service access point) address NSAP address will identify any system in the OSI network Various NSAP address format are used in various system, as different protocol may use different representation of NSAP. NSAP encoding and format are specified by ISO 8348/Ad2. ISO 8348/Ad2 uses the concept of hierarchical addressing domains. The global domain is the highest level. This global domain is subdivided into sub-domains, and each sub-domain is associated with an addressing authority that has a unique plan for constructing NSAP addresses.

23. NSAPs and addressing The initial domain part (IDP) consists of a 1- byte authority and format identifier (AFI) and a variable-length initial domain identifier (IDI) domain specific part (DSP) is a string of digits identifying a particular transport implementation of a specified AFI authority

24. NSAPs and addressing cont.. Everything to the left of the system ID can be thought of as the area address of a network node. System IS is ES or IS identifier in the area The NSEL (NSAP selector) is like a TCP port number: It indicates the transport layer. NSEP router is referred as NET, as the value of NSAP for router will be zero (no transport layer)

25. NSAP vs. IP Addressing In general, there will be a single NSAP address for the entire router, whereas with IP there will be one IP address per interface. It is possible to configure multiple NETs on a router, but no router is ever in more than one area. Configuring multiple NETs causes the areas to merge into a common area, leaking the Level 1 databases into each other. The only reasons to have multiple NETs are for splitting, merging, or renumbering areas; this method should only be used in periods of transition. Cisco Systems limits the number of configurable NETs to three per router.

26. NSAPs and Addressing cont… All ISs and ESs in a routing domain must have system IDs of the same length. All routers in an area must have the same area address. All Level 2 routers must have a unique system ID domain-wide, and all Level 1 routers must have a unique system ID area-wide. There are several techniques for creating unique system IDs: Start numbering 1, 2, 3, 4, and so on. Use Media Access Control (MAC) addresses. Convert and use the loopback IP address: 192.168.11.1 --> 192.168.011.001--> 1921.6801.1001.

27. Some predefined AFI Authority and Format Identifier AFI =49 Addresses starting with value 49 are considered as local addresses (as network 10.0.0.0/8 in IP). These addresses are routed by IS-IS routing protocol. However, there should not be advertised to other CLNS networks. With AFI 49 the IDI value is null and IDP length is 2 digits. AFI=39 Data Country Code. In this case the address assignment is done per country and a local authority will assign IDI codes. AFI=47 International Code Designator. The IDI consist of 4 digits identifying a particular organization. IDP length is 6 digits. Note that in most cases the IDI has to be registered to an OSI/CLNS authority

28. ISIS routing logic L1 router: for a destination address, compare the area ID to this area. If not equal, pass to nearest L1/L2 router. If equal, use L1 database to route by system ID L1/L2 router: for a destination address, compare the area ID to this area. If not equal, use L2 database to route by area ID If equal, use L1 database to route by system ID

29. IS-IS packets and flooding

30. IS-IS packet types IS-IS Hello Packets Used for maintaining adjacencies Sends hello packet every 10 sec, dead interval time is 30 sec ESH (End System Hello) ISH (Intermediate System Hello) IIH (Intermediate System to Intermediate System Hello) Link State Packets Called LSPs Contains all information about one router, such as connected IP prefixes, area addresses, etc One LSP per router CSNP (Complete sequence number packet) CSNPs contain a list of all LSPs from the current database inform other routers of LSPs that may be outdated or missing from their own database Synchronize and ensure that all routers have same information PSNP (partial sequence number packet) requesting and confirming (ack) the link state information.

31. Reliable Flooding Reliable Flooding Point-to-point networks Reliable Flooding on Broadcast Network Concept of pseudonode Concept of DIS

32. Reliable Flooding Point-to- point networks Point-to-point networks (connect a single pair of routers) Once the adjacency is established, both IS send CSNP Missing LSPs are sent by both ISs if not present in the received CSNP(Complete Sequence Number Packet) Missing LSPs may be requested through PSNP (Partial Sequence Number Packet) LSPs hold for a Remaining Lifetime Periodic refresh : ISs resend their LSP at least every 15 minutes

33. Reliable Flooding on Broadcast Network Broadcast networks (LAN) On LANs only, there’s a Designated Router (DIS) DIS is same as designated router in OSPF The DIS creates a pseudonode (a virtual node), and all the routers on a LAN, including the DIS, form an adjacency with the pseudonode DIS has two tasks : Creating and updating the pseudonode LSP Conducting the flooding over the LAN Flooding over the LAN means that the DIS sends periodic complete sequence number protocol data units (CSNPs) (default setting of 10 seconds) summarizing the following information: LSP ID Sequence Number Checksum Remaining Lifetime DIS is elected for each LAN DIS election is based on interface priority When priority is tied, the highest SNPA (MAC address) will be a DIS Or router with the higher system ID (for frame relay)

34. IS-IS commands Router#conf t Router(config)#router isis : enable IS-IS Router(config-router)#net [network entity title] ex) Router(config-router)#net 49.0401.2101.8302.8174.00 Router(config-router)#is-type [level-1/level-1-2/level-2-only] Router(config-router)#exit Router(config)#interface [interface type] [interface number] Router(config-if)#ip router isis Router(config-if)#isis circuit-type [level-1/level-1-2/level-2- only]: need to repeat for all interfaces that need to run IS-IS Router#show ip route isis : check routing table

35. Security and Authentication Security LSP authentication Interface password

36. OSPF Vs IS-IS OSPF has traffic tuning features that are especially suitable to enterprise networks while ISIS has stability features especially suitable to ISP infrastructure. IS-IS can support more routers in an area than OSPF IS-IS is neutral regarding the type of network addresses for which it can route. OSPF, on the other hand, was designed for IPv4. Thus IS-IS was easily adapted to support IPv6 The logical view is that OSPF creates something of a spider web or star topology of many areas all attached directly to Area Zero and IS-IS by contrast creates a logical topology of a backbone of Level 2 routers with branches of Level 1-2 and Level 1 routers forming the individual areas. NBMA networks, such as Frame Relay, Asynchronous Transfer Mode (ATM), and X.25, can connect multiple devices but have no broadcast capability. All the other routers attached to the network will not receive a packet sent by a router. Special consideration should be taken when configuring IS-IS over NBMA networks. IS-IS considers these media to be just like any other broadcast media such as Ethernet or Token Ring. In general, it is better to configure point-to-point networks on WAN interfaces and subinterfaces. Unlike OSPF, no configuration is necessary to tell IS-IS what the network type is. The DIS election is preemptive (unlike OSPF). If a new router boots on the LAN with a higher interface priority, the new router becomes the DIS. It purges the old pseudonode LSP and floods a new set of LSPs.

37. Thank You for listening Suman Pandey