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Designing OSPF Networks

Explore OSPF protocols, advantages, and disadvantages. Learn the hierarchical organization and case studies for designing OSPF networks effectively. Understand OSPF functionalities and benefits for fast re-routing and reducing routing protocol traffic. Delve into OSPF background, features, and security considerations. Discover OSPF technical overview, including hierarchical organizations, fast convergence, load balancing, and IP subnetting support. Obtain insights on route summarization, authenticated routing updates, and OSPF external routes handling. Embrace OSPF areas, topology principles, and link state databases for optimal network design.

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Designing OSPF Networks

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  1. Designing OSPFNetworks 284_045/c2 1

  2. Amir Khan Consulting Engineerakhan@cisco.com 284_045/c2 2

  3. Agenda • Technical Overview • Protocol Functionality • Design Considerations • Case Studies

  4. OSPF Technical Overview • Background • Features • Hierarchical Organization

  5. Why OSPF: Advantages • Fast re-routing • Minimizes routing protocol traffic • Multi-vendor

  6. Why OSPF: Disadvantages • Topology restrictive • Not easily centrally controlled • Security transmitted in clear • No route filtering

  7. A Q 2 B Z 13 C X 13 Link State Technology Z’s Link States Y Q’s Link State Topology information iskept in a database separatefrom the routing table X’s Link State

  8. OSPF Background • Dynamic routing protocol • Link state or SPF technology • Developed by OSPF Working Group of IETF • Intra-autonomous system (IGP) • Designed expressly for TCP/IP Internet environment

  9. OSPF Background (Cont.) • Runs directly over IP (Protocol 89) • Each router maintains an identical database (within areas) • Each router constructs a tree of shortest paths by running SPF algorithm on the database • Tree provides route to each known destination • Cisco’s implementation is fully compliant with the specification as of software release 9.1 (November, 1992)

  10. OSPF Technical Overview • Background • Features • Hierarchical Organization

  11. Fast Convergence • Detection Plus LSA/SPF R2 Alternate Path X N1 N2 R1 R3 Primary Path

  12. Load Balancing • Equal cost multiple paths R2 T1 T1 N2 N1 R1 R4 T1 T1 R3

  13. FDDI Dual Ring Low Bandwidth Utilization • Only changes propagated • Multicast on multi-access broadcast networks LSA X R1 LSA

  14. FDDI Dual Ring Low Bandwidth Utilization ? • Database synchronization LSA Remote Site X R1 LSA R2 LSA

  15. Optimal Path Utilization The optimal path is determined by thesum of the interface costs Cost = 1 Cost = 1 N3 N2 R2 R3 R1 N1 N5 Cost = 10 R4 Cost = 10 N4

  16. IP Subneting Support • Network number, mask pair • Variable length subnet mask (VLSM) • Discontiguous subnets • Supernets/subnet prefixes

  17. Route Summarization • Prefix or all subnets • Prefix or all networks • ‘Area range’ command R2 Backbone Area 0 FDDI Dual Ring With summarization Network 1 Next Hop R1 R1 (ABR) Area 1 Without summarization Network 1.A 1.B 1.C Next Hop R1 R1 R1 1.A 1.B 1.C

  18. Authenticated Routing Updates • AuType 0: No authentication • AuType 1: Simple password • Password is transmitted in clear

  19. External Routes • Redistributed into OSPF • Flooded unaltered throughout the AS • OSPF supports two types of external metrics • Type 1 external metrics • Type 2 external metrics (Default) RIP IGRP EIGRP BGP etc. OSPF Redistribute

  20. to N1 External Cost = 1 Cost = 10 R1 to N1 External Cost = 2 R2 Cost = 8 R3 External Routes • Type 1 external metric Next Hop R1 R2 Network N1 N1 Type 1 11 10 Selected Route

  21. to N1 External Cost = 1 Cost = 10 R1 to N1 External Cost = 2 R2 Cost = 8 R3 External Routes • Type 2 external metric Next Hop R1 R2 Network N1 N1 Type 2 1 2 Selected Route

  22. FDDIDualRing External Routes • Forwarding Address on shared/common network • (Field in AS external links advertisement) R1 AS#2 N1 AS#1 BGP OSPF N3 N2 R3 R2 Network N3 Next Hop R3

  23. B D Route Tagging • Autonomous System B wants to • Propagate routes from A —> D, but NOT propagate routes from C —> D • OSPF tags routes with AS input • This info can be used when redistributing routes A C

  24. TOS Based Routing • IP header supports 3 bit priority field • IP header supports 4 special types of service • Bandwidth • Delay • MTU • Cost • Currently only TOS 0 supported

  25. Utilizes IP Multicast for Sending/Receiving Updates • Broadcast networks • DR and BDR —> AllSPFRouters (224.0.0.5) • All other routers —> AllDRRouters (224.0.0.6) • Hello packets sent to AllSPFRouters (Unicast on point-to-point and virtual links)

  26. OSPF Technical Overview • Background • Features • Hierarchical Organization

  27. Backbone Area #0 Area #1 Area #2 Area #3 Hierarchical Structure • Structure must exist or be created • Explicit topology has precedence over addressing

  28. OSPF Areas • OSPF areas • Group of contiguous hosts and networks • Per area topological database • Backbone area (contiguous) • Virtual links • Inter-area routing Area 2 Area 3 Area 0 Area 1 Area 4

  29. Backbone Area #0 Area #1 Area #2 Area #3 OSPF Areas • Rules • Backbone area must be present • All other areas must have connection to backbone • Backbone must be contiguous

  30. Backbone Area #0 Area #1 Area #2 Area #3 Why Areas • Topology of an area is invisible from outside of the area • Results in marked reduction in routing traffic

  31. Topology/Link State Database • A router has a separate LS database for each area to which it belongs • All routers belonging to the same area have identical database • SPF calculation is performed separately for each area • LSA flooding is bounded by area

  32. Area Link State Database • Area database is composed of: • Router links advertisements • Network links advertisements • Summary links advertisements (IP network, ASBR) • AS external advertisements (in non-stub areas)

  33. Area 1 Classification of Routers • Internal Router (IR) • Area Border Router (ABR) • Backbone Router (BR) • Autonomous System Border Router (ASBR) IR Area 2 Area 3 ABR/BR Area 0 IR/BR ASBR To other AS

  34. OSPF Address to Area Mapping • Area can be one or more networks • Area can be one or more subnets • Any combination of networks and subnets possible • (But bad in practice) • For summarization subnets must be grouped • Mask in area...range command consolidates

  35. Virtual Links • Virtual links configured between any two backbone routers that have an interface to a common non-backbone area • A router connected to two or more areas is considered to be a backbone router Area 3 Backbone Area 0 Backbone Area 0 Area 1 Area 2

  36. Agenda • Technical Overview • Protocol Functionality • Design Considerations • Case Studies

  37. Protocol Functionality • Bringing up adjacencies • Convergence • Subneting • Route summarization • Area classification

  38. OSPF Terminology • Hello protocol • Designated router • Router ID • Neighboring routers • Adjacency • Link state advertisement

  39. FDDI Dual Ring The Hello Protocol • Responsible for establishing and maintaining neighbor relationships • Elects designated router on multi-access networks Hello Hello Hello

  40. FDDI Dual Ring The Hello Packet • Router priority • Hello interval • Router dead interval • Network mask • Options: T-bit, E-bit • List of neighbors Hello Hello Hello

  41. Designated Router • One per multi-access network • Generates network links advertisements • Assists in database synchronization Backup Designated Router Designated Router Designated Router Backup Designated Router 284_045/c2 41

  42. Designated Router by Priority • Configured priority (per interface) • Else determined by highest router ID • Router ID is the highest IP address on the box 131.108.3.2 131.108.3.3 DR R1 Router ID = 144.254.3.5 R2 Router ID = 131.108.3.3 144.254.3.5

  43. Neighboring States • 2-way • Router sees itself in other Hello packets • DR selected from neighbors in state 2-way or greater 2-way DR BDR

  44. Neighboring States • Full • Routers are fully adjacent • Databases synchronized • Relationship to DR and BDR Full DR BDR

  45. When to Become Adjacent • Underlying network is point to point • Underlying network type is virtual link • The router itself is the designated router • The router itself is the backup designated router • The neighboring router is the designated router • The neighboring router is the backup designated router

  46. LSAs Propagate Along Adjacencies • LSAs acknowledged along adjacencies DR BDR

  47. Convergence • Detection Plus LSA/SPF R2 Alternate Path X N1 N2 R1 R3 Primary Path

  48. Convergence • Fault detection • Serial lines • Detection immediate for carrier loss • 2 to 3 times keepalive otherwisekeepalive 10 seconds by default • Token Ring and FDDI immediate • Ethernet • 2 to 3 times keepalive • Hello can supersede keepalive • Dead timer is 40 sec by default Alternate Path X N1 R1 Primary Path

  49. LSA Convergence • Finding a new route • LSA flooded throughout area • Acknowledgment based • Topology database synchronized • Each router derives routing table • Tree to each destination network X N1 R1

  50. Convergence • Finding a new route • Load balancing provides immediate convergence • Equal cost paths only R2 T1 T1 N2 N1 R1 R4 T1 T1 R3

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