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Reference: IS-IS vs OSPF

Reference: IS-IS vs OSPF 28
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Published Date:19-07-2017
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Reference: IS-IS vs OSPF Shivkumar Kalyanaraman 1IS-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 (1990)  IS-IS is a Link State Protocol similar to the Open Shortest Path First (OSPF). OSPF supports only IP  IS-IS competed neck-to-neck with OSPF.  OSPF deployed in large enterprise networks  IS-IS deployed in several large ISPs Shivkumar Kalyanaraman 2IS-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 Shivkumar Kalyanaraman 3CLNS Addressing NSAP Format Area ID Sys ID NSEL Variable length Area address AFI System ID NSEL 1 byte 1 - 12 bytes 6 bytes 1 byte  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 Shivkumar Kalyanaraman 4CLNS Addressing Requirements 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 Shivkumar Kalyanaraman 5IS-IS Terminology 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 IP Packet Link State PDU (LSP) - Routing information packet Level 1 and Level 2 – Area 0 and lower areas Shivkumar Kalyanaraman 6IS-IS Protocol Concepts: Network Nodes  Hosts  Level-1 Routers  Level-2 Routers  Level-1 and Level-2 Pseudonodes on broadcast links only Shivkumar Kalyanaraman 7IS-IS Protocol Concepts: Network 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 Shivkumar Kalyanaraman 8IS-IS Protocol Concepts: Areas Area 49.001 L1 Level-1 Area L1L2 Level-2 Backbone Area 49.003 Area 49.0002 Level-1 Level-1 L1L2 L1L2 Area Area L1 L1 Shivkumar Kalyanaraman 9IS-IS Protocol Concepts: Hierarchical Routing Backbone Area 49.0002 Area 49.001 Level-1 Level-1 Routing Level-2 Routing Routing  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 Shivkumar Kalyanaraman 10IS-IS Protocol Concepts: IS-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 Shivkumar Kalyanaraman 11IS-IS LS Database: IS-IS Packet Format 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 Shivkumar Kalyanaraman 12IS-IS LS Database: Generic Packet Format No. of Octets Intra-domain Routing Protocol Discriminator 1 Length Indicator 1 Version/Protocol ID Extension 1 1 ID Length R R R 1 PDU Type Version 1 1 Reserved 1 Maximum Area Addresses Packet-Specific Header Fields TLV Fields Shivkumar Kalyanaraman 13IS-IS LS Database: LSP Format Octets Intradomain Routing Proto Descriminator 1 1 Lenth Indicator 1 Version/Protocol ID Extension 1 ID Length 1 R R R PDU Type 1 Version 1 Reserved 1 Maximum Area Addresses 2 PDU Length 2 Remaining Lifetime ID Length + 2 LSP ID 4 2 Sequence Number 1 Checksum Variable P ATT LSPDBOL IS Type TYPE LENGTH VALUE FIELDS Shivkumar Kalyanaraman 14Level-1 TLVs 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 Shivkumar Kalyanaraman 15Level-2 TLVs 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 Shivkumar Kalyanaraman 16High-level Comparison w/ OSPF  Protocols are recognizably similar in function and mechanism (common heritage)  Link state algorithms  Two level hierarchies  Designated Router on LANs  Widely deployed (ISPs vs enterprises)  Multiple interoperable implementations  OSPF more “optimized” by design (and therefore significantly more complex)  IS-IS not designed from the start as an IP routing protocol (and is therefore a bit clunky in places) Shivkumar Kalyanaraman 17Detailed comparison points  Encapsulation  OSPF runs on top of IP= Relies on IP fragmentation for large LSAs  IS-IS runs directly over L2 (next to IP) = fragmentation done by IS-IS  Media support  Both protocols support LANs and point-to-point links in similar ways  IS-IS supports NBMA in a manner similar to OSPF pt- mpt model: as a set of point-to-point links  OSPF NBMA mode is configuration-heavy and risky (all routers must be able to reach DR; bad news if VC fails) Shivkumar Kalyanaraman 18Comparison: Packet Encoding  OSPF is “efficiently” encoded  Positional fields, 32-bit alignment  Only LSAs are extensible (not Hellos, etc.)  Unrecognized types not flooded. Opaque-LSAs recently introduced.  IS-IS is mostly Type-Length-Value (TLV) encoded  No particular alignment  Extensible from the start (unknown types ignored but still flooded)  All packet types are extensible  Nested TLVs provide structure for more granular extension Shivkumar Kalyanaraman 19Comparison: Area Architecture  Both protocols support two-level hierarchy of areas  OSPF area boundaries fall within a router  Interfaces bound to areas  Router may be in many areas  Router must calculate SPF per area  IS-IS area boundaries fall on links  Router is in only one area, plus perhaps the L2 backbone (area)  Biased toward large areas, area migration  Little or no multilevel deployment (large flat areas work so far) Shivkumar Kalyanaraman 20