Open Shortest Path First Protocol (OSPF)

1
Open Shortest Path First Protocol(OSPF)

2
Overview

• Introduction
• Motivation
• OSPF Basics
• Hierarchical Routing in OSPF
• Summary

3
Introduction

• Development began in 1987
• OSPF Working Group (part of IETF)
• OSPFv2 first established in 1991
• Many new features added since then
• Updated OSPFv2 specification in RFC 2178

4
Motivation

• Original IGP used was RIP
• Based on Bellman-Ford Algorithm
• Worked well in small systems
• Suffered from problems of Distance Vector
  Protocol
• Count to Infinity Problem
• Slow Convergence

5
Motivation

• Problems with Distance Vector Protocol
• Large update packets
• Slow response to topological changes
• Need for a Link State Protocol
• A long list of functional requirements follows

6
Functional Requirements of OSPF

• Faster Convergence and less consumption of
  network resources
• A more descriptive routing metric
• configurable
• value ranges between 1 and 65,535
• no restriction on network diameters
• Equal-cost multipath
• a way to do load balancing

7
Functional Requirements(contd.)

• Routing Hierarchy
• support large routing domains
• Separate internal and external routes
• Support of flexible subnetting schemes
• route to arbitrary address,mask combinations
  using VLSMs
• Security
• Type of Service Routing

8
OSPF Basicsthe essence

• Distributed, replicated database model
• describes complete routing topology
• Link state advertisements
• carry local piece of routing topology
• Distribution of LSAs using reliable flooding
• Link state database
• identical for all the routers

9
Link State Advertisements(LSAs)
LS Age
Options
LS Type
Link State ID
Advertising Router
LS Sequence Number
LS Checksum
Length
0
16
LSA Header
10
LSAs contd.

• Identifying LSAs
• LS type field
• Link State ID field
• mostly carries addressing information
• e.g. IP address of externally reachable network
• Advertising Router field
• originating routers OSPF router ID

11
LSAs contd.

• Identifying LSA instances
• needed to update self-originated LSAs
• LS Sequence Number field
• 32 bit values
• monotonically increasing until some max value
• 600 years to roll over!
• LSA checksum and LS Age guard against potential
  problems

12
LSAs contd.

• Verifying LSA contents
• LS Checksum field
• computed by the originating router and left
  unchanged thereafter
• LS age field not included in checksum
• Removing LSAs from databases
• LS Age field
• ranges from 0 to 30 min.
• Max Age LSAs used to delete outdated LSAs

13
LSAs contd.

• Other LSA Header fields
• Options field
• sometimes used to give special treatment during
  flooding or routing calculations
• Length field
• includes LSA header and contents
• ranges from 20-65535 bytes

14
Sample Router LSA
10.1.1.1
10.1.1.2
10.1.1.4
10.1.1.6
10.1.1.3
10.1.1.5
15
Sample Router LSA contd..
LS Age
0 seconds
Options
LS Type
E-bit,LS Type 1
10.1.1.1
Link State ID
Advertising Router
10.1.1.1
LS Sequence Number
0x80000006
LS Checksum
0x9b47
Length
60 bytes
Router Type
0
0 (ordinary)
of links
3
Link ID
10.1.1.3
Link Data
Ifindex 2 (unnumbered link)
Link 1
Link Type
TOS Metrics
1(point to point), 0
Metric
5
16
Link State Database

• Collection of all OSPF LSAs
• databases exchanged between neighbors
• synchronization thru reliable flooding
• gives the complete routing topology
• each OSPF router has identical link-state
  database

17
Link State Database contd..

• Example of a link state database

LS Type
Link State ID
Adv Router
LS Checksum
LS Seq No
LS Age
Router LSA
10.1.1.1
10.1.1.1
0x9b47
0x80000006
0
..
...
..
..
.
...
18
Communication between OSPF Routers

• OSPF packets encapsulated in IP packets
• standard 24 byte header
• OSPF packet type field
• OSPF router ID of sender
• Packet checksum
• Authentication fields
• OSPF Area ID

19
Neighbor Discovery and Maintenance

• OSPF Hello Protocol
• Hello packets sent out every 10 seconds
• helps to detect failed neighbors
• RouterDeadInterval (default 40 seconds)
• also ensures that link is bidirectional
• neighboring routers agree on intervals
• hello interval set so that a link is not
  accidentally brought down

20
Database Synchronization

• Crucial to ensure correct and loop free routing
• must be done before 2 neighbors start
  communication
• also whenever new LSAs are introduced
• uses reliable flooding
• each router sends LSA headers to its neighbor
  when connection comes up
• requests only those LSAs which are recent

21
Database Exchange

• Neighboring routers first exchange hellos
• a database description packet packet establishes
  the sequence number
• the other router sends LSA headers
• sequence number incremented for every pair od
  database description packets
• implicit acknowledgement for the previous pair
• after examining LSA headers explicit request sent
  for complete LSAs

22
Reliable Flooding

• Starts when a router wants to update
  self-originated LSAs
• Link State Update packets
• Neighbor installs more recent LSAs into its
  database
• floods out on all interfaces except the one on
  which it arrived
• reliability-retransmissions until acks received

23
Reliable Flooding (contd..)
10.1.1.1
10.1.1.2
10.1.1.4
10.1.1.6
Time T1
u
u
u
10.1.1.3
10.1.1.5
24
Reliable Flooding (contd..)
10.1.1.1
10.1.1.2
10.1.1.4
10.1.1.6
u
u
Time T2
u
u
u
10.1.1.3
10.1.1.5
25
Reliable Flooding (contd..)
10.1.1.1
10.1.1.2
10.1.1.4
10.1.1.6
u
Time T3
u
10.1.1.3
10.1.1.5
26
Reliable Flooding (contd..)
10.1.1.1
10.1.1.2
10.1.1.4
10.1.1.6
Time T3
ack
ack
ack
ack
ack
10.1.1.3
10.1.1.5
27
Reliable Flooding(contd..)

• Robustness
• updates flooded over all the links , so failure
  of any link doesnt affect database
  synchronization
• LSAs refreshed every 30 minutes
• LSA checksum field detects corruption
• flooding loops avoided by LS Age field
• MinLSInterval limits rate of LSA origination
• Receivers can refuse to accept LSA updates if
  they received an update less than a second ago

28
Routing Calculations

• Link costs configurable by administrator
• Smaller values for more preferred links
• must make sense to add link costs
• different costs for each link direction possible
• Dijkstras shortest path algorithm
• incrementally calculates tree of shortest paths
• each link in the network examined once
• computes multiple shortest paths (equal-cost
  multipath)

29
Hierarchical Routing

• Technique used to build large networks
• minimizes consumption of network resources such
  as
• router memory
• router computing resources
• link bandwidth
• with flat routing linear increase in routing
  table size
• with hierarchical, size increases logarithmically

30
an example
10.3
10.3.3
10.0.0.0/8
10.3.2
10.3.1
10.1.3
10.2.3
10.1
10.2
10.1.1
10.1.2
10.2.2
10.2.1
31
example contd..

• Consider a router in 10.1.1
• assume 16 entries in each of the first level
  partitions
• with flat routing, 916 144 entries/router
• with 3 level hierarchy, the router has 16 entries
  within 10.1.1.0/24 entries for 10.1.2.0/24,
  10.1.3.0/24,10.1.0.0/16 for a total of 19
  entries.
• Marked reduction in routing table size
• but might lead to suboptimal routing

32
OSPF Areas

• Two-level hierarchical routing scheme through the
  use of areas
• areas identified by 32-bit id
• each area has its own link state database which
  is a collection of network-LSAs and router-LSAs
• areas topology hidden from all other areas
• interconnection of areas through area border
  routers (ABRs)
• ABR leaks IP addressing information to other
  areas through summary LSAs

33
Sample Area Configuration
Area 0.0.0.1
10.2.1.0/24
10.2.2.0/24
1
3
1
I
J
3
3
B
C
Area 0.0.0.2
Area 0.0.0.3
10.1.2.0/24
1
1
1
E
G
2
2
1
3
10.3.7.0/24
3
Area 0.0.0.0
1
A
D
A
A
3
10.8.2.0/24
1
10.1.1.0/24
3
H
F
3
1
1
34
OSPF Areas contd..

• Example of Summary LSA(router B)

LS Age
0
Options
LS Type
0x2, Type 3(summary-LSA)
Link State ID
10.2.0.0
Advertising Router
Router Bs router ID
LS Sequence Number
0x80000001
LS Checksum
Length
28 bytes
Network Mask
255.255.0.0
TOS
TOS 0 (normal)
Metric
Cost of 7
35
OSPF Areas contd..

• Reduction in link state databases of an area
• reduction in amount of flooding traffic needed
  for synchronization
• reduction in the cost of the shortest path
  calculations
• increased robustness
• routing protection
• Hidden prefixes

36
Area Organization

• All the areas are connected to area 0.0.0.0 also
  called the backbone area
• need not have a direct physical connection though
• virtual links provide logical link to backbone
• summary LSAs tunneled across non backbone areas
• exchange of routing information between areas
  using Distance Vector Protocol
• absence of redundant paths between areas
• not subject to convergence problems

37
Incorporating external routing information

• Special routers called AS boundary routers at the
  edge of OSPF domain
• ASBRs originate AS-External LSAs
• only routes for which the choice of an ASBR makes
  sense are imported
• otherwise default routes are used
• AS external LSAs similar to Summary LSAs with 2
  additional fields
• Forwarding address
• external route tag

38
Interaction with areas

• AS-External LSAs flooded across borders
• ASBR summary LSAs used to know the location of
  the originator of AS-External LSA
• Link State ID of ASBR Summary LSA set to the OSPF
  router ID of the ASBR whose location is
  advertised
• similar to summary LSA in all other respects

39
OSPF Area Types

• Restrict the amount of external routing
  information within an area
• used when resources especially router memory is
  very limited
• two types of restricted areas
• Stub Areas
• NSSAs or Not-So-Stubby-Areas

40
OSPF Area Types

• Stub Areas
• dont support ASBRs and hence no AS-External-LSAs
• routing to external destinations based on default
  routes originated by the areas border routers
• summary LSAs also made optional
• must lie on the edge of OSPF routing domain
• inter-area routing may also be based on default
  routes
• improved scaling
• but not preferred due to the possibility of
  suboptimal routes

41
OSPF Area Types contd..

• NSSAs
• import small amount of routing information
• this information flooded to other areas by the
  NSSA Border router
• Use Type-7 LSAs to import external routing
  information
• translated into AS-External-LSA at the NSSA
  Border
• one-way filter

42
Summary

• Why OSPF is needed in the Internet?
• The basics of the protocol
• The Link state Advertisements
• Neighbor Discovery (Hello Protocol)
• Database Synchronization and reliable flooding
• Hierarchical Routing in OSPF
• OSPF Areas and Area Organization
• Interaction with External Routing Information
• OSPF Area Types viz. Stub Areas and NSSAs