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Introduction to IS-IS

Introduction to IS-IS. NANOG 20, Washington DC October 22-24. Abe Martey amartey@cisco.com. Topics. 1. IS-IS Overview 2. IS-IS Protocol Concepts 3. CLNP Addressing 4. IS-IS Database 5. MPLS TE Extensions for IS-IS 6. ISP Network Design Considerations 7. Configuration Examples

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Introduction to IS-IS

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  1. Introduction to IS-IS NANOG 20, Washington DC October 22-24 Abe Martey amartey@cisco.com

  2. Topics 1. IS-IS Overview 2. IS-IS Protocol Concepts 3. CLNP Addressing 4. IS-IS Database 5. MPLS TE Extensions for IS-IS 6. ISP Network Design Considerations 7. Configuration Examples 8. Basic Troubleshooting 9. Further Reading

  3. 1. IS-IS Overview Terminology and Acronyms Intermediate system (IS)- Router Designated Intermediate System (DIS) - Designated Router Pseudonode - Broadcast link emulated as virtual node by DIS End System (ES) - Network Host or workstation Network Service Access Point (NSAP) - Network Layer Address Subnetwork Point of attachment (SNPA) - Datalink interface Packet data Unit (PDU) - Analogous to IPPacket Link State PDU (LSP) - Routing information packet

  4. IS-IS Overview • The Intermediate Systems to Intermediate System Routing Protocol (IS-IS) was originally designed to route the ISO Connectionless Network Protocol (CLNP) . (ISO10589 or RFC 1142) • Adapted for routing IP in addition to CLNP (RFC1195) as Integrated or Dual IS-IS • IS-IS is a Link State Protocol similar to the Open Shortest Path First (OSPF). OSPF supports only IP

  5. IS-IS Overview • IS-IS is an Interior Gateway Protocol (IGP) - for Intra-domain (AS) routing • BGP provides inter-domain routing for IP networks • ISO-IGRP is a Cisco proprietary routing protocol that can be used between CLNP domains

  6. IS-IS Overview • 3 network layer protocols play together to deliver the ISO defined Connectionless Network Service • - CLNP • - IS-IS • - ES- IS - End System to Intermediate System Protocol • All 3 protocols independently go over layer 2

  7. IS-IS Overview • CLNP is the ISO equivalent of IP for datagram delivery services (ISO 8473, RFC 994) • ES-IS is designed for routing between network hosts and routers (ISO9542, RFC 995). • IS-IS for layer 3 routing between routers. (ISO 10589/RFC 1142). Integrated IS-IS (RFC 1195) works within the ISO CNLS framework even when used for routing only IP.

  8. IS-IS Overview • End System Hellos (ESH) from Hosts and Intermediate System Hellos (ISH) from Routers used for ES-IS neighbor discovery • Intermediate System to Intermediate Systems Hellos (IIH) are used for establishing IS-IS layer3 adjacencies • ES-IS is somehow tied into IS-IS layer 3 adjacency discovery. ES-IS enabled automatically when IS-IS is configured on Ciscos

  9. 2. IS-IS Protocol Concepts • Nodes • Links • Areas and Adjacencies • Level-1 and level-2 routing

  10. IS-IS Protocol ConceptsNetwork Nodes • Hosts • Level-1 Routers • Level-2 Routers • Level-1 and Level-2 Pseudonodes on broadcast links only

  11. IS-IS Protocol ConceptsNetwork Nodes DIS DIS PSN • Broadcast link represented as virtual node, referred to as Pseudonode (PSN) • PSN role played by the Designated Router (DIS) • DIS election is preemptive, based on interface priority with highest MAC address being tie breaker • IS-IS has only one DIS. DIS/PSN functionality supports database synchronization between routers on a broadcast type link

  12. IS-IS Protocol ConceptsNetwork/Link Types • Point-to-Point Links (PoS, ptp FR, ptp ATM) • Broadcast Links (Multi-point/multi-access, ie Ethernet, mpt FR, mpt ATM or Cisco DPT) • Non-broadcast multi-access treated as broadcast or point-to-point by configuration

  13. IS-IS Protocol Concepts Areas Area 49.001 L1 Level-1 Area L1L2 Level-2 Backbone Area 49.003 Area 49.0002 Level-1 Area Level-1 Area L1L2 L1L2 L1 L1

  14. IS-IS Protocol ConceptsIS-IS Adjacencies • IIH advertised for detecting neighbor and forming network layer adjacencies • 3 types of IIHs: • Point-to-point • LAN Level-1 • LAN Level-2

  15. IS-IS Protocol ConceptsPoint-to Adjacencies Bytes Intra-domain Routing Protocol Discriminator 1 Length Indicator 1 Version/Protocol ID Extension • Pt-to-pt IIH used to establish level-1 or Level-2 pt-to-point adjacency • Only two way communication required on pt-to-pt links by ISO 10589 • 3-way reliable process recently proposed for standardization in the IETF. Introduces pt-to-pt adjacency state TLV (Type 240) 1 ID Length 1 R R R PDU Type 1 Version 1 Reserved 1 Maximum Area Addresses 1 Reserved (6 bits) Circuit Type 1 Source ID ID Length Holding Time 2 2 PDU Length Local Circuit ID 1 Variable Length TLV Fields

  16. IS-IS Protocol Concepts LAN (Broadcast link) Adjacencies bytes 1 1 1 1 1 1 1 1 1 ID Length 2 2 1 ID Length + 1 Variable Holding Time = Hello Interval x Hello Multiplier Priority - Interface Priority for DIS arbitration LAN ID - System ID of DIS + PSN number Intradomain Routing Protocol Descr. • LAN Level-1 and LAN Level-2 IIH multicast independently to AllL1ISs (01-80-C2-00-00-14)and AllL2ISs (01-80-C2-00-00-14) • 3-way reliable adjacency formation can be enforced due to IS neighbors TLV (Type 2) appended to LAN IIHs Lenth Indicator Version/Protocol ID Extension ID Length R R R PDU Type Version Reserved Maximum Area Addresses Reserved/Circuit type Source ID Holding Time PDU Length R Priority Lan ID TYPE LENGTH FIELDS

  17. IS-IS Protocol ConceptsHierarchical Routing Backbone Area 49.0002 Area 49.001 • IS-IS supports 2-level routing hierarchy • Routing domain is carved into areas. Routing in an area is level-1. Routing between areas is level-2 • All ISO 10589/RFC1195 areas are stubs Level-1 Routing Level-1 Routing Level-2 Routing

  18. IS-IS Protocol ConceptsIS-IS Packet Types • IS-IS Hello Packets (IIH) • Level 1 LAN IS-IS Hello • Level 2 LAN IS-IS Hello • Point-to-point Hello • Link State Packets (LSP) • Level 1 and Level 2 • Complete Sequence Number packets (CSNP) • Level 1 and Level 2 • Partial Sequence Number Packets (PSNP) • Level 1 and Level 2

  19. 3. CLNS Addressing • NSAP Format • AFI Values • Requirements and Caveats • Examples • Globally unique NSAPs

  20. CLNS AddressingNSAP Format Area ID Sys ID NSEL AFI Variable length Area address System ID NSEL • NSAP format has 3 main components • Area ID • System ID • N-Selector (NSEL) - value is 0x00 on a router • NSAP of a router is also called a NET 1 byte 1 - 12 bytes 6 bytes 1 byte

  21. CLNS AddressingAFI Values Address Domain AFI Value 37 39 47 49 • X.121 - Int’l plan for public data networks • ISO DCC - Data country code • IS0 6523 ICD - Telex • Local - For local use within network domain only X.121 ISO DCC ISO 6523 Local

  22. CLNS AddressingRequirements and Caveats • At least one NSAP is required per node • All routers in the same area must have a common Area ID • Each node in an area must have a unique System ID • All level 2 routers in a domain must have unique System IDs relative to each other • All systems belonging to a given domain must have System IDs of the same length in their NSAP addresses

  23. CLNS AddressingRequirements and Caveats • Multiple NSAPs allowed on Cisco routers for merging, splitting or renumbering • All NSAPs on the same router must have the same system ID. • The maximum size of an NSAP is 20 bytes • Minimum of 8 bytes allowed on Ciscos. • 1 byte for area, 6 bytes for system ID and 1 byte for N-selector. • AFI prefix recommended to make minimum of 9 bytes

  24. CLNS AddressingNSAP Examples Example 1 47.0001.aaaa.bbbb.cccc.00 Area = 47.0001, SysID = aaaa.bbbb.cccc, NSel = 00 Example 2 39.0f01.0002.0000.0c00.1111.00 Area = 39.0f01.0002, SysID = 0000.0c00.1111, NSel = 00 Example 3. 49.0002.0000.0000.0007.00 Area = 49.0002, SysID = 0000.0000.0007, Nsel = 00

  25. CLNS AddressingHow do most ISP define System IDs? Router A# Interface Loopback 0 IP address 192.168.3.25 Router isis Net 49.0001.1921.6800.3025.00 IP Address conversion process: 192.168.3.25 -> 192.168.003.025 | V 1921.6800.3025 | V 49.001.1921.6800.3025.00

  26. CLNS AddressingGlobally Unique NSAPs • AFI 47 (ISO 6523 ICD) is allocated via national sponsoring authority of the International Registration Authority (RA), usually a national standards body • NIST - allocated IDI 0005 and 0006 • BSI subsidiary IOTA allocated 0124 for assignment of ATM End Systems Addresses • AFI 39 also administered through national institutions • IDI 0840 allocated to ANSI NIST - United States National Institute of Standards BSI - British Standards Institute IOTA -Identifiers for Organizations for Telecommunications Addressing

  27. CLNS AddressingGlobally Unique NSAPs 47 0124 XXXXX YYYYYYYYYYYYYY AABBCCDDEEFF 00 NSEL SYS ID AFI IDI ORG ID ORG ASSIGNED Globally unique NSAPs make sense for interconnected telecommunications systems such as ATM switches, SONET/SDH ADMs that use CLNS based applications for management. For ISP networks, the local AFI (49) would just be fine! Why?

  28. 4. IS-IS Link State Database • Link State Packets • Sequence Number Packets • More on IS- IS Metrics • Flooding • Database Parameters, Flags and Timers • SPF Algorithm

  29. IS-IS LS DatabaseLink State Packets LSP: RTB.00-00 LSP: RTC.00-00 RTC RTB. RTA LSP: RTA.00-00 RTD. RTE LSP: RTE.00-00 LSP: RTD.00-00

  30. IS-IS LS DatabaseIS-IS Packet Format IS-IS Packets are made of the following: • A Fixed Header • Contains generic packet information and other specific information about the packet • Type, Length, Value (TLV) Fields • TLVs are blocks of specific routing-related information in IS-IS packets

  31. No. of Octets Intra-domain Routing Protocol Discriminator 1 Length Indicator 1 Version/Protocol ID Extension 1 ID Length 1 R R R PDU Type 1 Version 1 1 Reserved Maximum Area Addresses 1 Packet-Specific Header Fields TLV Fields IS-IS LS DatabaseGeneric Packet Format

  32. IS-IS LS DatabaseLSP Format Octets 1 1 1 1 1 1 1 1 2 2 ID Length + 2 4 2 1 Variable Intradomain Routing Proto Descriminator Lenth Indicator Version/Protocol ID Extension ID Length R R R PDU Type Version Reserved Maximum Area Addresses PDU Length Remaining Lifetime LSP ID Sequence Number Checksum P ATT LSPDBOL IS Type TYPE LENGTH VALUE FIELDS

  33. IS-IS LS DatabaseLS Packet Identifier (LSPID) 00c0.0040.abcd.02-01 • Sys ID - System ID of IS or DIS in case of PSN LSP. • PSN ID - Zero for Non-PSN LSP and Non-zero for PSN LSP • LSP Number - Fragmentation number Sys ID PSN ID LSPNo.

  34. Complete Sequence Number Packets (CSNPs) • Describes all known LSPs in the LS Database • Standard IS-IS packet format • Fixed Header and • TLV Fields • Each TLV includes the following info about each known LSP: • LSP ID • Remaining Lifetime • Sequence number • Checksum

  35. Partial Sequence Number Packets (PSNPs) • PSNPs are similarly constructed as CSNPs but have summaries of only specific LSPs • PSNPs are used in 2 ways: • To acknowledge receipt of LSP on pt-to-pt links • To request transmission of the latest version of an LSP

  36. TLV Name Type Origin Area Address ISO 10589 1 2 Intermediate System Neighbors ISO 10589 End System Neighbors 3 ISO 10589 Authentication information 10 ISO 10589 IP Internal Reachability Information 128 RFC 1195 Protocols Supported 129 RFC 1195 IP Interface Address 132 RFC 1195 Level-1 TLVs

  37. TLV Name Type Origin Area Address ISO 10589 1 2 ISO 10589 Intermediate System Neighbors Partition Designated Level-2 IS 4 ISO 10589 Prefix Neighbors 5 ISO 10589 10 ISO 10589 Authentication information 128 RFC 1195 IP Internal Reachability Information 129 RFC 1195 Protocols Supported 130 RFC 1195 IP External Reachability Information 131 Inter-domain Routing Protocol Information RFC 1195 132 RFC 1195 IP Interface Address Level-2 TLVs

  38. New TLVs TLV Name Type Comments Extended IS Reachability Information 22 TE Extension. Replaces TLV 2 134 TE Extension Router-Id TE extension used in place of TLV 128 or 130 Extended IP Reachability Information 135 For dynamic distribution of Hostname to NET mapping via LSP Flooding Dynamic Hostname Information 137 Point-to-Point Adjacency State 240 Reliable Pt-to-pt adjacency formation

  39. Old IS-IS Metrics • ISO 10589 specifies 4 metric types • Default - Supported by all routers. • Delay - measures transit delay • Expense - Measures the monetary cost of link utilization • Error - measures error probability • Default metric type must be supported by all implementations • Other metric types specified for QoS routing. Not supported in Cisco IOS and most other vendor implementations

  40. Old IS-IS Metrics • Metric Information is carried in the following TLVs: • IS Neighbors • ES Neighbors • Prefix Neighbors • IP Internal Reachability • IP External Reachability • Extended IP Reachability • Extended IS Reachability

  41. Old IS-IS Metrics Byte(s) 1 Default Metric (6 bits) 0 I/E S 1 I/E Delay Metric • Maximum metric per interface is 63 • Maximum path metric is 1023 • There is no automatic assignment based on interface bandwidth • Cisco uses default of 10 on all interfaces regardless of bandwidth Expense Metric 1 S I/E I/E Error Metric S 1 4 IP Address Subnet Mask 4

  42. Flooding and LSDB synchronization • Acknowledgement-based reliable mechanism on pt-to-point links • Flooding on broadcast links depends on periodic advertisements of CSNPs for reliable flooding and synchronization

  43. SRM and SSN Flags • Send Routing Message flag (SRMflag) and Send Sequence Number flag (SSNflag) are used by the Update Process to control delivery and acknowledgement of LSPs SRMFlag - If set for a LSP on a specific link, indicates the LSP should be flooded on that link SSNFlag - If set for a LSP on a link, means send a PSNP over that link to acknowledge receipt of LSP .

  44. Flooding on Pt-to-pt links RTA RTB RTC Interface 1 Interface 4 Interface 2 Interface 3 LSP RTA.00-00 SEQ#100 LSP RTA.00-00 SEQ#100 PSNP RTA.00-00 SEQ#100 PSNP RTA.00-00 SEQ#100

  45. Flooding on Broadcast Links RTX RTA (DIS) RTB RTC RTX LSP RTX.00-00 SEQ#1 RTA PSNP RTX.00-00 SEQ#1 LSP RTX.00-00 SEQ#1 X RTA CSNP RTA.00 RTB.00 RTC.00 RTX.00 RTA PSNP RTX.00-00 SEQ#1 RTB PSNP RTX.00-00 SEQ#1 RTC RTA LSP RTX.00-00 SEQ#1

  46. IS-IS Database Timers Timer Default Value Cisco IOS Command Maxage 1200s isis max-lsp-interval isis refresh-interval 900s LSP Refresh Interval isis lsp-interval LSP Transmission Interval 33ms LSP Retransmit Interval 5s isis retransmit-interval isis csnp-interval CSNP Interval 10s

  47. SPF Algorithm • Shortest Path First Algorithm (SPF), also known as Dijkstra (named after Dutch Mathematician) is used by link state protocols to obtain best paths through network • Works by building shortest path tree from calculating router (root) to all destinations in the network • SPF Process runs over level-1 and level-2 databases separately

  48. SPF Algorithm • In default mode, SPF process runs no frequent than every 5s • Full SPF is run when topology changes • When leaf elements such as IP prefixes change, routing table is adjusted with Partial Route Calculation (PRC) • PRC evaluates only routes that changed hence less CPU intensive and relatively fast

  49. SPF Algorithm • Duration of SPF depends on many factors such as: • Number of nodes • Number of links • Number of IP prefixes • Degree of mesh (especially for NBMA) • Speed of Route Processor

  50. 5. IS-IS Extensions for MPLS TE • Wide Metrics • Extended IS Reachability Information • Extended IP Reachability Information • Sub TLVs

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