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Reference: IS-IS vs OSPF

Reference: IS-IS vs OSPF 28
Reference: ISIS vs OSPF Shivkumar Kalyanaraman 1ISIS Overview  The Intermediate Systems to Intermediate System Routing Protocol (ISIS) 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 ISIS (1990)  ISIS is a Link State Protocol similar to the Open Shortest Path First (OSPF). OSPF supports only IP  ISIS competed necktoneck with OSPF.  OSPF deployed in large enterprise networks  ISIS deployed in several large ISPs Shivkumar Kalyanaraman 2ISIS Overview  3 network layer protocols play together to deliver the ISO defined Connectionless Network Service  CLNP  ISIS  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  NSelector (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 5ISIS 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 6ISIS Protocol Concepts: Network Nodes  Hosts  Level1 Routers  Level2 Routers  Level1 and Level2 Pseudonodes on broadcast links only Shivkumar Kalyanaraman 7ISIS 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  ISIS has only one DIS. DIS/PSN functionality supports database synchronization between routers on a broadcast type link Shivkumar Kalyanaraman 8ISIS Protocol Concepts: Areas Area 49.001 L1 Level1 Area L1L2 Level2 Backbone Area 49.003 Area 49.0002 Level1 Level1 L1L2 L1L2 Area Area L1 L1 Shivkumar Kalyanaraman 9ISIS Protocol Concepts: Hierarchical Routing Backbone Area 49.0002 Area 49.001 Level1 Level1 Routing Level2 Routing Routing  ISIS supports 2level routing hierarchy  Routing domain is carved into areas. Routing in an area is level1. Routing between areas is level2  All ISO 10589/RFC1195 areas are stubs Shivkumar Kalyanaraman 10ISIS Protocol Concepts: ISIS Packet Types  ISIS Hello Packets (IIH)  Level 1 LAN ISIS Hello  Level 2 LAN ISIS Hello  Pointtopoint 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 11ISIS LS Database: ISIS 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 routingrelated information in ISIS packets Shivkumar Kalyanaraman 12ISIS LS Database: Generic Packet Format No. of Octets Intradomain 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 PacketSpecific Header Fields TLV Fields Shivkumar Kalyanaraman 13ISIS 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 14Level1 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 15Level2 TLVs TLV Name Type Origin Area Address ISO 10589 1 2 ISO 10589 Intermediate System Neighbors Partition Designated Level2 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 Interdomain Routing Protocol Information RFC 1195 132 RFC 1195 IP Interface Address Shivkumar Kalyanaraman 16Highlevel 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)  ISIS 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  ISIS runs directly over L2 (next to IP) = fragmentation done by ISIS  Media support  Both protocols support LANs and pointtopoint links in similar ways  ISIS supports NBMA in a manner similar to OSPF pt mpt model: as a set of pointtopoint links  OSPF NBMA mode is configurationheavy 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, 32bit alignment  Only LSAs are extensible (not Hellos, etc.)  Unrecognized types not flooded. OpaqueLSAs recently introduced.  ISIS is mostly TypeLengthValue (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 twolevel 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  ISIS 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 20Comparison: Database Granularity  OSPF database node is an LSAdvertisement LSAs are mostly numerous and small (one external per LSA, one summary per LSA) Network and Router LSAs can become large LSAs grouped into LSUpdates during flooding LSUpdates are built individually at each hop Small changes can yield small packets (but Router, Network LSAs can be large) Shivkumar Kalyanaraman 21Comparison: Database Granularity  ISIS database node is an LSPacket LSPs are clumps of topology information organized by the originating router Always flooded intact, unchanged across all flooding hops (so LSP MTU is an architectural constantit must fit across all links) Small topology changes always yield entire LSPs (though packet size turns out to be much less of an issue than packet count) Implementations can attempt clever packing Shivkumar Kalyanaraman 22Comparison: Neighbor Establishment  Both protocols use periodic multicast Hello packets, “I heard you” mechanism to establish 2way communication  Both protocols have settable hello/holding timers to allow tradeoff between stability, overhead, and responsiveness  OSPF requires hello and holding timers to match on all routers on the same subnet (side effect of DR election algorithm) making it difficult to change timers without disruption  ISIS requires padding of Hello packets to full MTU size under some conditions (deprecated in practice)  OSPF requires routers to have matching MTUs in order to become adjacent (or LSA flooding may fail, since LSUpdates are built at each hop and may be MTUsized) Shivkumar Kalyanaraman 23Neighbor Adjacency Establishment  OSPF uses complex, multistate process to synchronize databases between neighbors  Intended to minimize transient routing problems by ensuring that a newborn router has nearly complete routing information before it begins carrying traffic  Accounts for a significant portion of OSPF’s implementation complexity  Partially a side effect of granular database (requires many DBD packets)  ISIS uses its regular flooding techniques to synchronize neighbors  Coarse DB granularity = easy (a few CSNPs) Shivkumar Kalyanaraman 24Designated Routers and Adjacency  Both protocols elect a DR on multiaccess networks to remove O(N2) link problem and to reduce flooding traffic  OSPF elects both a DR and a Backup DR, each of which becomes adjacent with all other routers  BDR takes over if DR fails  DRship is sticky, not deterministic  In ISIS all routers are adjacent (adjacency less stateful)  If DR dies, new DR must be elected, with short connectivity loss (synchronization is fast)  DRship is deterministic (highest priority, highest MAC address always wins)  DRship can be made sticky by cool priority hack (DR increases its DR priority) Shivkumar Kalyanaraman 25Comparison: LAN Flooding  OSPF uses multicast send, unicast ack from DR  Reduces flood traffic by 50 (uninteresting)  Requires perneighbor state (for retransmissions)  Interesting (but complex) acknowledgement suppression  Flood traffic grows as O(N)  ISIS uses multicast LSP from all routers, CSNP from DR  Periodic CSNPs ensure databases are synced (tractable because of coarse database granularity)  Flood traffic constant regardless of number of neighbors on LAN  But big LANs are uninteresting Shivkumar Kalyanaraman 26Comparison: Routes and Metrics  ISIS base spec used 6bit metrics on links  Allowed an uninteresting SPF optimization (CPUs are fast these days)  Proved difficult to assign meaningful metrics in large networks  Wide metric extension fixes this  Dual ISIS spec advertises only default into L1 areas  Interarea traffic routed suboptimally  Route leaking extension addresses this Shivkumar Kalyanaraman 27Comparison: Pragmatic Considerations  OSPF is much more widely understood Broadly deployed in enterprise market Many books of varying quality available Preserves our investment in terminology  ISIS is well understood within a niche Broadly deployed within the large ISP market Folks who build very large, very visible networks are comfortable with it Shivkumar Kalyanaraman 28
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