1. Designing OSPF Networks
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2. Consulting Engineer akhan@cisco.com
Amir Khan
Consulting Engineer
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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. Link State Technology Z’s Link States A B C Q Z X 2 13 Y
Q’s Link State
Topology information is kept in a database separate from 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. X Fast Convergence Detection Plus LSA/SPF R2 Alternate Path N1 N2 R1
Time it takes to start using a new route when a link fails and alternates routes are available X N1 N2 R1 R3
Primary Path

12. Load Balancing Equal cost multiple paths R2 T1 T1 N2 N1 R1 T1 R4 T1 R3
Time it takes to start using a new route when a link fails and alternates routes are available T1 T1 N2 N1 R1 T1 R4 T1 R3

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

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

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

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
FDDI
Dual Ring
Backbone
Area 0
With
summarization
Network 1
Next Hop R1
R1 (ABR)
Area 1
Without
summarization
Network
1.A
1.B
1.C
Next Hop 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. External Routes Type 1 external metric Cost = 10 to N1
External Cost = 1
External Cost = 2 R2 R3 R1
Cost = 8
Network N1
Type 1 11 10
Next Hop R1 R2
Selected Route

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

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

23. Route Tagging Autonomous System B wants toC 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. 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 ( )
All other routers —> AllDRRouters ( )
Hello packets sent to AllSPFRouters (Unicast on point-to-point and virtual links)

26. OSPF Technical Overview
Background
Features
Hierarchical Organization

27. Hierarchical Structure
Backbone
Area #0
Area #1
Area #2
Area #3
Structure must exist or created
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
Router ID: A 32-bit number assigned to each router running the OSPF protocol. This number uniquely identifies the router within an Autonomous System. Router use this number to identify themselves when generating updates.
Neighboring routers: Two routers that have interfaces to a common network. On multi-access networks, neighbors are dynamically discovered by OSPFs Hello Protocol.
Adjacency: A relationship formed between selected neighboring routers for the purpose of exchanging routing information. Not every pair of neighboring routers become adjacent.
Link state advertisement: Describes the local state of the router or network. This includes the state of the router’s interfaces and adjacencies. Each link state advertisement is flooded throughout the routing domain. The collected link state advertisements of all routers and networks forms the protocol’s topological database.
Hello protocol: The part of the OSPF protocol used to establish and maintain neighboring relationships. On multi-access networks the Hello Protocol can also dynamically discover neighboring routers.
Designated router: Each multi-access network that has at least two attached routers has a Designated Router. The Designated Router generates a link state advertisement for the multi-access network and has other special responsibilities in the running of the protocol. The Designated Router is elected by the Hello Protocol.
Area 0
Area 1
Area 4

29. OSPF Areas Rules Backbone area must be present
All other areas must have connection to backbone
Backbone must be contiguous

30. Why Areas Topology of an area is invisible from outside of the area
Backbone
Area #0
Area #1
Area #2
Area #3
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. Classification of RoutersIR
Area 2
Area 3
ABR/BR
Area 0
IR/BR
Area 1
ASBR
Internal Router (IR)
Area Border Router (ABR)
Backbone Router (BR)
Autonomous System Border Router (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 Area 3 Backbone Area 0 Backbone Area 0 Area 1
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 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
Router ID: A 32-bit number assigned to each router running the OSPF protocol. This number uniquely identifies the router within an Autonomous System. Router use this number to identify themselves when generating updates.
Neighboring routers: Two routers that have interfaces to a common network. On multi-access networks, neighbors are dynamically discovered by OSPFs Hello Protocol.
Adjacency: A relationship formed between selected neighboring routers for the purpose of exchanging routing information. Not every pair of neighboring routers become adjacent.
Link state advertisement: Describes the local state of the router or network. This includes the state of the router’s interfaces and adjacencies. Each link state advertisement is flooded throughout the routing domain. The collected link state advertisement of all routers and networks forms the protocol’s topological database.
Hello protocol: The part of the OSPF protocol used to establish and maintain neighboring relationships. On multi-access networks the Hello Protocol can also dynamically discover neighboring routers.
Designated router: Each multi-access network that has at least two attached routers has a Designated Router. The Designated Router generates a link state advertisement for the multi-access network and has other special responsibilities in the running of the protocol. The Designated Router is elected by the Hello Protocol.

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

40. The Hello Packet Router priority Hello interval Router dead interval
Network mask
Options: T-bit, E-bit
List of neighbors
FDDI
Dual Ring
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
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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 DR
R1 Router ID =
R2 Router ID =

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 AdjacenciesDR
BDR
LSAs acknowledged along adjacencies

47. X Convergence Detection Plus LSA/SPF R2 Alternate Path N1 N2 R1 R3
Time it takes to start using a new route when a link fails and alternates routes are available X N1 N2 R1 R3
Primary Path

48. X 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
Alternate Path X N1 R1
Primary Path

49. X 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 N1 R1

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

51. Variable Length Subneting (IP).5
.50.
Serial Addresses .6 .9
.10
.13
.14 .
.13 B .6
.10. .9
.14
.254.
.60.
.10 C
‘Sub-subnet’ subnet for serial links
Use mask for serial links
‘Sub-subnet’ Mask Boundary
XXXXXX YY Subnet Hosts
1 00 4
01 5
10 6 11

52. VLSM Concerns Pick one regular subnet
Further subnet that with extended mask
Keep sub-subnets together in same area
Class B
254 subnets
62 extra subnets

53. Discontiguous Subnets
Area 0
network
range
Area 1
network
subnets 17-31
range
Area 2
network
subnets 33-47
range
Area 3
network
subnets 49-63
range

54. OSPF Link Summarization
Backbone
Area #0
Area #1
Area #2
Area #3

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

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

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

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

59. 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. 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. 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. Agenda Technical Overview Protocol Functionality Design Considerations
Case Studies

63. Design Considerations
Network hierarchy
Addressing

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

65. Hierarchical Network Structure
Core
Distribution
Access

66. OSPF Network Topology
Backbone
Area 0
Area 1
Area 2

67. Addressing
Area 0
network
range
Area 1
network
subnets 17-31
range
Area 2
network
subnets 33-47
range
Area 3
network
subnets 49-63
range
Assign contiguous ranges of subnets per area to facilitate summarization

68. Route Redistribution Transferring routes between routing protocols
OSPF
RIP
Structure must exist or created
Redistribute OSPF routes into RIP
and vice versa
Transferring routes between routing protocols

69. Route Redistribution Example
Structure must exist or created
OSPF Domain
UNIX host running routed
Cisco router redistributes RIP into OSPF and vice versa

70. Good OSPF Backbone Design
Avoid large mesh backbones
Best—collapsed LAN backbone

71. Scalable OSPF Network Design
Area hierarchy
Stub areas
Addressing
Route summarization

72. Agenda Technical Overview Protocol Functionality Design Considerations
Case Studies

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

74. 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. Case Study Option 3: Create new area Requires unique address space
Logically easy
Optimizes routing

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

77. OSPF Configuration Commands
Router command
Router subcommand
Interface subcommands

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

79. Router Sub-commands
NETWORK <n.n.n.n> <mask> AREA <area-id>
AREA <area-id> STUB {no-summary}
AREA <area-id> AUTHENTICATION
AREA <area-id> DEFAULT_COST <cost>
AREA <area-id> VIRTUAL-LINK <router-id>...
AREA <area-id> RANGE <address mask>

80. Interface Subcommands
IP OSPF COST <cost>
IP OSPF PRIORITY <8-bit-number>
IP OSPF HELLO-INTERVAL <number-of-seconds>
IP OSPF DEAD-INTERVAL <number-of-seconds>
IP OSPF AUTHENTICATION-KEY <8-bytes-of-passwd>

81. Redistributing Routes into OSPF
ROUTER OSPF <as#x>
REDISTRIBUTE {protocol} <as#y>
<metric>
<metric-type (1 ro 2)
<tag>
<subnets>
<default>

82. Interface Subcommands
IP OSPF COST <cost>
IP OSPF PRIORITY <8-bit-number>
IP OSPF HELLO-INTERVAL <number-of-seconds>
IP OSPF DEAD-INTERVAL <number-of-seconds>
IP OSPF AUTHENTICATION-KEY <8-bytes-of-passwd>

83. Supporting Nonbroadcast Multiaccess Environments
X.25, Frame Relay, ATM etc.
ROUTER OSPF <as#>
NEIGHBOR n.n.n.n INTERFACE <x> <y>