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OSPF

OSPF. CSC/ECE 573, Sections 001 Fall, 2012. OSPF: Open (= non-proprietary) SPF. RFC 2328 (OSPF version 2) Recommended interior (intra-domain) routing protocol for TCP/IP link state routing using Dijkstra's algorithm Goals converge faster than RIP exchange less information than RIP

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OSPF

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  1. OSPF CSC/ECE 573, Sections 001 Fall, 2012 Introduction

  2. OSPF: Open (= non-proprietary) SPF • RFC 2328 (OSPF version 2) • Recommended interior (intra-domain) routing protocol for TCP/IP • link state routing using Dijkstra's algorithm • Goals • converge faster than RIP • exchange less information than RIP • Runs directly over IP (not UDP or TCP); protocol number 89

  3. OSPF “advanced” features (not in RIP) • security: all OSPF messages authenticated (to prevent malicious intrusion) • multiple same-cost paths allowed (only one path in RIP) • For each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort; high for real time) • integrated uni- and multicast support: • Multicast OSPF (MOSPF) uses same topology data base as OSPF • hierarchical OSPF in large domains. Network Layer

  4. OSPF (cont’d) • Optimization metrics • hop-count • delay • throughput, etc. • When several equal-cost routes exist, can send traffic along each of them • for load-balancing • Reliability • flooding, with neighbor acknowledgments • reoriginate LSAs at 30 minute intervals • all LSAs are checksummed • can withdraw state using LSA with maximum age

  5. OSPF (cont’d) • Subnetting • designed to work with variable-length subnets and CIDR • Security • simple cleartext password • MD5 message digest, based on shared secret key

  6. Area Routing • Routing domain is split into areas • “backbone” and attached areas • areas do not overlap • 2-level hierarchy; each area must attach directly to backbone • Area = generalization of a subnet • its topology and details are not visible outside the area • each area runs a copy of the link-state protocol • smaller link-state databases • Routers at boundaries intercommunicate • summarize (aggregate) routing prefixes and advertise fewer routes

  7. Areas in an Autonomous System

  8. OSPF Areas (cont’d) • Exchange of summary LSA’s across the backbone: Distance Vector! • But using flooding as method of distribution • Virtual links to handle connections of border routers to the backbone area • Result: border routers do not have to be physically directly connected to each other • “tunnels” through non-border routers  virtual link

  9. OSPF Classes of Routers • Internal routers: wholly within an area • Area border routers: connect two or more areas • Backbone routers: on the backbone • AS boundary routers: talk to routers in other AS • The classes are allowed to overlap

  10. OSPF Classes of Routers (cont'd)

  11. Connections And Networks • OSPF supports three types of connections and networks • Point-to-point lines between exactly two routers • Multiaccess networks with broadcasting (e.g., most LANs) • Multiaccess networks without broadcasting (e.g., most WANs) • OSPF abstracts the collection of actual networks, routers, and lines into a directed graph • (hosts do not play a role in OSPF)

  12. LAN Abstraction • N routers connected to a broadcast LAN • Model as N*(N-1)/2 point-to-point connections  expensive! • size of link-state database (N routers, O(N2) links) • # of messages exchanged (O(N2) for N routers in LAN) • Instead, create one LSA for the LAN, containing links to all routers • N+1 nodes, N links, N messages on LAN • A “Designated Router” is responsible for originating the LSA representing the LAN • there may also be a “backup designated router”

  13. LAN Abstraction Example

  14. OSPF Network Representation Example

  15. OSPF Network Representation Example (cont'd)

  16. Message Types • Hello • at boot time, used to discover who the neighbors are; also used to periodically test reachability • Link state update • flooded to the whole network; provides the sender's cost to its neighbors • Link state ACK • ACKs link state update  reliable flooding

  17. Message Types (cont’d) • Database description • when a line between routers is brought up, gives sequence # of all link state entries a sender has • Link state request • after exchanging database description messages, sender requests link state information for links for which the other router has more recent info

  18. LSA Types: Router LSA • Lists router's interfaces, their cost, the network/router they connect to, the range of IP addresses directly accessible via the interfaces

  19. LSA Types: Network LSA • Performs the LAN abstraction: lists all routers connected to the network

  20. LSA Types: Summary Link • Link to border router, advertises IP addresses between areas

  21. External Link LSA • Link to another site, imports routing information from other AS

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