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Designing OSPF Networks 284_045/c21. 2 Amir Khan Consulting Engineer

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Presentation on theme: "Designing OSPF Networks 284_045/c21. 2 Amir Khan Consulting Engineer"— Presentation transcript:

1 Designing OSPF Networks 284_045/c21

2 2 Amir Khan Consulting Engineer akhan@cisco.com

3 284_045/c23 Agenda Technical Overview Protocol Functionality Design Considerations Case Studies

4 284_045/c24 OSPF Technical Overview Background Features Hierarchical Organization

5 284_045/c25 Why OSPF: Advantages Fast re-routing Minimizes routing protocol traffic Multi-vendor

6 284_045/c26 Why OSPF: Disadvantages Topology restrictive Not easily centrally controlled Security transmitted in clear No route filtering

7 284_045/c27 Link State Technology Topology information is kept in a database separate from the routing table Q’s Link State X’s Link State Z’s Link States Y A B C Q Z X 2 13

8 284_045/c28 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

9 284_045/c29 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)

10 284_045/c210 OSPF Technical Overview Background Features Hierarchical Organization

11 284_045/c211 Fast Convergence Detection Plus LSA/SPF X R1R3 R2 Alternate Path Primary Path N1 N2

12 284_045/c212 Load Balancing Equal cost multiple paths R1R4 N2 N1 R3 R2 T1

13 284_045/c213 FDDI Dual Ring Low Bandwidth Utilization Only changes propagated Multicast on multi-access broadcast networks R1 LSA X

14 284_045/c214 FDDI Dual Ring Low Bandwidth Utilization ? Database synchronization R1 LSA X Remote Site R2 LSA

15 284_045/c215 Optimal Path Utilization N1 N2 N3 N4 N5 R1 R2 R3 R4 Cost = 1 Cost = 10 The optimal path is determined by the sum of the interface costs

16 284_045/c216 IP Subneting Support Network number, mask pair Variable length subnet mask (VLSM) Discontiguous subnets Supernets/subnet prefixes

17 284_045/c217 Route Summarization Prefix or all subnets Prefix or all networks ‘Area range’ command 1.A1.B1.C FDDI Dual Ring R1 (ABR) R2 Network 1 Next Hop R1 Network 1.A 1.B 1.C Next Hop R1 With summarization Without summarization Backbone Area 0 Area 1

18 284_045/c218 Authenticated Routing Updates AuType 0: No authentication AuType 1: Simple password Password is transmitted in clear

19 284_045/c219 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

20 284_045/c220 External Routes Type 1 external metric Network N1 Type 1 11 10 Next Hop R1 R2 Cost = 10 to N1 External Cost = 1 to N1 External Cost = 2 R2 R3 R1 Cost = 8 Selected Route

21 284_045/c221 External Routes Type 2 external metric Network N1 Type 2 1 2 Next Hop R1 R2 Cost = 10 to N1 External Cost = 1 to N1 External Cost = 2 R2 R3 R1 Cost = 8 Selected Route

22 284_045/c222 External Routes Forwarding Address on shared/common network (Field in AS external links advertisement) AS#2 R1 R2 R3 AS#1 FDDI Dual Ring N1 N2 N3 OSPF BGP Network N3 Next Hop R3

23 284_045/c223 Route Tagging A B C D 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

24 284_045/c224 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

25 284_045/c225 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)

26 284_045/c226 OSPF Technical Overview Background Features Hierarchical Organization

27 284_045/c227 Backbone Area #0 Area #1Area #2Area #3 Hierarchical Structure Structure must exist or be created Explicit topology has precedence over addressing

28 284_045/c228 OSPF Areas OSPF areas Group of contiguous hosts and networks Per area topological database Backbone area (contiguous) Virtual links Inter-area routing Area 1 Area 4 Area 0 Area 2 Area 3

29 284_045/c229 OSPF Areas Rules Backbone area must be present All other areas must have connection to backbone Backbone must be contiguous Backbone Area #0 Area #1Area #2Area #3

30 284_045/c230 Backbone Area #0 Area #1Area #2Area #3 Why Areas Topology of an area is invisible from outside of the area Results in marked reduction in routing traffic

31 284_045/c231 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

32 284_045/c232 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)

33 284_045/c233 Area 1 Area 0 Area 2 Area 3 Classification of Routers IR ABR/BR IR/BR To other AS ASBR Internal Router (IR) Area Border Router (ABR) Backbone Router (BR) Autonomous System Border Router (ASBR)

34 284_045/c234 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

35 284_045/c235 Virtual Links Backbone Area 0 Backbone Area 3 Area 1 Area 2 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

36 284_045/c236 Agenda Technical Overview Protocol Functionality Design Considerations Case Studies

37 284_045/c237 Protocol Functionality Bringing up adjacencies Convergence Subneting Route summarization Area classification

38 284_045/c238 OSPF Terminology Hello protocol Designated router Router ID Neighboring routers Adjacency Link state advertisement

39 284_045/c239 The Hello Protocol Responsible for establishing and maintaining neighbor relationships Elects designated router on multi-access networks FDDI Dual Ring Hello

40 284_045/c240 The Hello Packet Router priority Hello interval Router dead interval Network mask Options: T-bit, E-bit List of neighbors FDDI Dual Ring Hello

41 Designated Router One per multi-access network Generates network links advertisements Assists in database synchronization Designated Router Designated Router Backup Designated Router Backup Designated Router 284_045/c241

42 284_045/c242 Designated Router by Priority Configured priority (per interface) Else determined by highest router ID Router ID is the highest IP address on the box 144.254.3.5 R2 Router ID = 131.108.3.3 131.108.3.2 131.108.3.3 R1 Router ID = 144.254.3.5 DR

43 284_045/c243 Neighboring States 2-way Router sees itself in other Hello packets DR selected from neighbors in state 2-way or greater DRBDR 2-way

44 284_045/c244 Neighboring States Full Routers are fully adjacent Databases synchronized Relationship to DR and BDR DRBDR Full

45 284_045/c245 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

46 284_045/c246 LSAs Propagate Along Adjacencies LSAs acknowledged along adjacencies DRBDR

47 284_045/c247 Convergence Detection Plus LSA/SPF X R1R3 R2 N2 Alternate Path Primary Path N1

48 284_045/c248 Convergence Fault detection Serial lines Detection immediate for carrier loss 2 to 3 times keepalive otherwise keepalive 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 X R1 N1 Primary Path Alternate Path

49 284_045/c249 Convergence Finding a new route LSA flooded throughout area Acknowledgment based Topology database synchronized Each router derives routing table Tree to each destination network LSA X R1 N1

50 284_045/c250 Convergence Finding a new route Load balancing provides immediate convergence Equal cost paths only R1R1 R4 N2 N1 R3 R2 T1

51 284_045/c251 Variable Length Subneting (IP).10..6.50..13.60..254. ‘Sub-subnet’ subnet.254. for serial links Use mask 255.255.255.252 for serial links ‘Sub-subnet’ Mask Boundary 131.108.254.XXXXXXYYSubnetHosts 1004 015 106 11.9.10.14.5 Serial Addresses 131.108.254.5.6.9.10.13.14. A C B

52 284_045/c252 VLSM Concerns Pick one regular subnet Further subnet that with extended mask Keep sub-subnets together in same area 131.108.0.0 255.255.255.0 255.255.255.252 Class B 254 subnets 62 extra subnets

53 284_045/c253 Discontiguous Subnets Area 1 network 131.108.0.0 subnets 17-31 range 255.255.240.0 Area 2 network 131.108.0.0 subnets 33-47 range 255.255.240.0 Area 3 network 131.108.0.0 subnets 49-63 range 255.255.240.0 Area 0 network 192.117.49.0 range 255.255.255.0

54 284_045/c254 Backbone Area #0 Area #1Area #2Area #3 OSPF Link Summarization

55 284_045/c255 Not Summarized: Specific Links Backbone Area #0 External links 1.A 1.C 1.B 1.D 3.D 3.A 3.C 3.B 1.A 1.B 1.C 1.D 3.A 3.B 3.C 3.D 2.A 2.B 2.C 2.A 2.C 2.B Specific link LSA advertised out Link state changes propagate out ASBR

56 284_045/c256 Summarized: Summary Links Backbone Area #0 ASBR External links 1.A 1.C 1.B 1.D 3.D 3.A 3.C 3.B 2.A 2.B Only summary LSA advertised out Link state changes do not propagate 1 3 2

57 284_045/c257 Backbone Area #0 External links 1.A 1.C 1.B 1.D 3.D 3.A 3.C 3.B 2.A 2.C 2.B ASBR Not Summarized: Specific Links 2.A 2.B 2.C 3.A 3.B 3.C 3.D 1.A 1.B 1.C 1.D 3.A 3.B 3.C 3.D 1.A 1.B 1.C 1.D 2.A 2.B 2.C Specific link LSA advertised in Link state changes propagate in

58 284_045/c258 Summarized: Summary Links Backbone Area #0 ASBR External links 1.A 1.C 1.B 1.D 3.D 3.A 3.C 3.B 2.A 2.B 2,3 1,3 Only summary LSA advertised out Link state changes do not propagate 1,2

59 284_045/c259 Regular Area (Not a Stub) Summary LSA from other areas injected Specific links from other areas injected Can cause routing trouble—link flaps Solution: configure area range Default external injected Configure default-info originate on AS border External links injected AS border not default-info originate

60 284_045/c260 Normal Stub Area Summary LSAs from other areas injected Default LSA injected into area Represents external links Represents non-summarized internal links Default path to closest area border router Define all routers in area as stub (area x stub) Specific link flaps will not be injected

61 284_045/c261 Totally Stubby Area Configure ‘area x stub no-summary’ Default LSA injected into area Represents all external links Represents all summarized internal links Represents non-sumarized internal links Default path to closest area border router Define all routers totally stubby Specific link flaps will not be injected

62 284_045/c262 Agenda Technical Overview Protocol Functionality Design Considerations Case Studies

63 284_045/c263 Design Considerations Network hierarchy Addressing

64 284_045/c264 Hierarchical Network Structure Distribution Core Access Optimal Transport Between Sites Policy Based Connectivity Local/Remote Workgroup Access

65 284_045/c265 Hierarchical Network Structure Distribution Access Core

66 284_045/c266 OSPF Network Topology Backbone Area 0 Area 1 Area 2

67 284_045/c267 Addressing Area 1 network 131.108.0.0 subnets 17-31 range 255.255.240.0 Area 2 network 131.108.0.0 subnets 33-47 range 255.255.240.0 Area 3 network 131.108.0.0 subnets 49-63 range 255.255.240.0 Area 0 network 192.117.49.0 range 255.255.255.0 Assign contiguous ranges of subnets per area to facilitate summarization

68 284_045/c268 OSPFRIP Route Redistribution Transferring routes between routing protocols Redistribute OSPF routes into RIP and vice versa

69 284_045/c269 Route Redistribution Example UNIX host running routed Cisco router redistributes RIP into OSPF and vice versa OSPF Domain

70 284_045/c270 Good OSPF Backbone Design Avoid large mesh backbones Best—collapsed LAN backbone

71 284_045/c271 Scalable OSPF Network Design Area hierarchy Stub areas Addressing Route summarization

72 284_045/c272 Agenda Technical Overview Protocol Functionality Design Considerations Case Studies

73 284_045/c273 Case Study Add site to an existing network Minimize impact on existing network Topology and addressing considerations Backbone Area #0 1.A 1.C 1.B 1.D 2.A 2.C 2.B Area 1 Area 2

74 284_045/c274 Case Study Option 1: Place in the backbone Jeopardize backbone stability Increase non-transit traffic Option 2: Place in existing area 2 Adjust area 2’s address space Increase traffic in Europe

75 284_045/c275 Case Study Option 3: Create new area Requires unique address space Logically easy Optimizes routing

76 284_045/c276 Case Study Option 3: Create new area Requires unique address space Logically easy, optimizes routing Backbone Area #0 1.A 1.C 1.B 1.D 2.A 2.C 2.B Area 1 Area 2 Area 3

77 284_045/c277 OSPF Configuration Commands Router command Router subcommand Interface subcommands

78 284_045/c278 Router Command router ospf {as} Multiple OSPF processes can be configured Autonomous system must be on unique interfaces as# not transmitted

79 284_045/c279 Router Sub-commands NETWORK AREA AREA STUB {no-summary} AREA AUTHENTICATION AREA DEFAULT_COST AREA VIRTUAL-LINK... AREA RANGE

80 284_045/c280 Interface Subcommands IP OSPF COST IP OSPF PRIORITY IP OSPF HELLO-INTERVAL IP OSPF DEAD-INTERVAL IP OSPF AUTHENTICATION-KEY

81 284_045/c281 Redistributing Routes into OSPF ROUTER OSPF REDISTRIBUTE {protocol} { "@context": "http://schema.org", "@type": "ImageObject", "contentUrl": "http://images.slideplayer.com/12/3351409/slides/slide_81.jpg", "name": "284_045/c281 Redistributing Routes into OSPF ROUTER OSPF REDISTRIBUTE {protocol}

82 284_045/c282 Interface Subcommands IP OSPF COST IP OSPF PRIORITY IP OSPF HELLO-INTERVAL IP OSPF DEAD-INTERVAL IP OSPF AUTHENTICATION-KEY

83 284_045/c283 Supporting Nonbroadcast Multiaccess Environments X.25, Frame Relay, ATM etc. ROUTER OSPF NEIGHBOR n.n.n.n INTERFACE


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