1ISP/IXP Workshops © 1999, Cisco Systems, Inc.
Introduction to OSPFIntroduction to OSPFIntroduction to OSPF
ISP/IXP WorkshopsISP/IXP WorkshopsISP/IXP Workshops
2ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
OSPFOSPFOSPF
• Dynamic Routing Protocol
• Link State technology
• Runs over IP, protocol 89
• Designed by IETF for TCP/IP
• Supports VLSM
• Multi-vendor
• Fast rerouting
3ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
OSPF (continued)OSPF (continued)OSPF (continued)
• Minimises routing protocol traffic
• Low bandwidth requirements
• Supports different types of areas
• Route summarisation and authentication
• Cisco’s implementation is fully compliantwith the specification OSPF v2.
4ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Link StateLink State
Topology Information IsKept in a DatabaseSeparate from the
Routing Table
AABBCC
2213131313
QQZZXX
X’s Link State
ZZ
XX
YYQQ
Z’s Link State
Q’s Link State
5ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Link State RoutingLink State Routing
• Neighbour discovery
• Constructing an LSP
• Distribute LSP
• Compute routes
• On network failureNew LSPs flooded
All routers recompute routing tables
6ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
FDDIDual Ring
Low Bandwidth UtilisationLow Bandwidth Utilisation
• Only changes propagated
• Multicast on multi-access broadcastnetworks
R1
LSA
XLSA
7ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
FDDI Dual Ring
FDDI Dual Ring
Optimal Path UtilisationOptimal Path Utilisation
N1
N2 N3
N4
N5R1
R2
R3
R4
Cost = 1 Cost = 1
Cost = 10
Cost = 10
The optimal path is determined by thesum of the interface costs
8ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Fast ConvergenceFast Convergence
• Detection Plus LSA/SPF
XR1 R3
R2
N2
Alternate Path
Primary Path
N1
9ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Fast ConvergenceFast Convergence
• Finding a new route
LSA flooded throughout area
Acknowledgement based
Topology databasesynchronised
Each router derives routingtable to destination networks
LSA
XR1
N1
10ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Utilises IP Multicast forUtilises IP Multicast forSending/Receiving UpdatesSending/Receiving Updates
• Broadcast networksDR 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 virtuallinks)
11ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
OSPF AreasOSPF Areas
• Group of contiguoushosts and networks
• Per area topologicaldatabase
Invisible outside the area
Reduction in routing traffic
• Backbone areacontiguous
All other areas must beconnected to the backbone
• Virtual Links
Area 1Area 4
Area 0Backbone Area
Area 2 Area 3
12ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Classification of RoutersClassification of Routers
• Internal Router (IR)
• Area Border Router(ABR)
• Backbone Router (BR)
• Autonomous SystemBorder Router (ASBR)
Area 1
IR/BRArea 0
Area 2 Area 3
IR
ABR/BR
To other AS
ASBR
13ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
OSPF Route TypesOSPF Route Types
Intra-area Route
all routes inside an area
Inter-area Route
routes advertised from one areato another by an Area BorderRouter
External Route
routes imported into OSPF fromother protocol or static routes
Area 0Area 2 Area 3
ABR
To other AS
ASBR
14ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Inter-Area RouteInter-Area RouteSummarisationSummarisation
• Prefix or all subnets
• Prefix or all networks
• ‘Area range’ command
1.A 1.B 1.C
FDDIDual Ring
R1 (ABR)
R2
Network1
Next HopR1
Network1.A1.B1.C
Next HopR1R1R1
With summarisation
Withoutsummarisation
BackboneArea 0
Area 1
15ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
External RoutesExternal Routes
• Redistributed into OSPF
• Flooded unaltered throughout the AS
• OSPF supports two types of externalmetrics
Type 1 external metrics
Type 2 external metrics (Default)
RIPIGRPEIGRPBGPetc.
OSPF
Redistribute
16ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
External RoutesExternal Routes
• Type 1 external metric: metrics are addedto the summarised internal link cost
NetworkN1N1
Type 11110
Next HopR2R3
Cost = 10
to N1External Cost = 1
to N1External Cost = 2R2
R3
R1
Cost = 8
Selected Route
17ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
External RoutesExternal Routes
• Type 2 external metric: metrics are comparedwithout adding to the internal link cost
NetworkN1N1
Type 212
Next HopR2R3
Cost = 10
to N1External Cost = 1
to N1External Cost = 2R2
R3
R1
Cost = 8
Selected Route
18ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Topology/Link State DatabaseTopology/Link State Database
• A router has a separate LS databasefor each area to which it belongs
• All routers belonging to the samearea have identical database
• SPF calculation is performed separatelyfor each area
• LSA flooding is bounded by area
19ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Protocol FunctionalityProtocol Functionality
• Bringing up adjacencies
• LSA types
• Area classification
20ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
The Hello ProtocolThe Hello Protocol
• Responsible for establishing and maintainingneighbour relationships
• Elects designated router on multi-accessnetworks
FDDIDual Ring
Hello
HelloHello
21ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
The Hello PacketThe Hello Packet
• Router priority
• Hello interval
• Router deadinterval
• Network mask
• Options: T-bit, E-bit
• List of neighbours
FDDIDual Ring
Hello
HelloHello
22ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Designated RouterDesignated Router
• One per multi-access network
Generates network links advertisements
Assists in database synchronization
Designated Router
Designated Router
BackupDesignated Router
BackupDesignated
Router
23ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Designated Router by PriorityDesignated Router by Priority
• Configured priority (per interface)
• Else determined by highest router ID
Router ID is the loopback interface address, ifconfigured, otherwise the highest IP address
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
24ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Neighbouring StatesNeighbouring States
• 2-way
Router sees itself in other Hello packets
DR selected from neighbours in state2-way or greater
DR BDR
2-way
25ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Neighbouring StatesNeighbouring States
• FullRouters are fully adjacent
Databases synchronised
Relationship to DR andBDR
DR BDR
Full
26ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
When to Become AdjacentWhen 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 designatedrouter
• The neighbouring router is the designatedrouter
• The neighbouring router is the backupdesignated router
27ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
LSAsLSAs Propagate Along Propagate AlongAdjacenciesAdjacencies
• LSAs acknowledged alongadjacencies
DR BDR
28ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Routing Protocol PacketsRouting Protocol Packets
• Share a common protocol header
• Routing protocol packets are sent with typeof service (TOS) of 0
• Five types of OSPF routing protocol packetsHello - packet type 1
Database description - packet type 2
Link-state request - packet type 3
Link-state update - packet type 4
Link-state acknowledgement - packet type 5
29ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Different Types ofDifferent Types of LSAs LSAs
• Five distinct type of LSAs
Type 1 : Router LSA
Type 2 : Network LSA
Type 3 and 4: Summary LSA
Type 5 and 7: External LSA
30ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Router LSA (Type 1)Router LSA (Type 1)
• Describes the state and cost of therouter’s links to the area
• All of the router’s links in an area must bedescribed in a single LSA
• Flooded throughout the particular areaand no more
• Router indicates whether it is an ASBR,ABR, or end point of virtual link
31ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Network LSA (Type 2)Network LSA (Type 2)
• Generated for every transit broadcastand NBMA network
• Describes all the routers attached to thenetwork
• Only the designated router originatesthis LSA
• Flooded throughout the area and nomore
32ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Summary LSA (Type 3 and 4)Summary LSA (Type 3 and 4)
• Describes the destination outside thearea but still in the AS
• Flooded throughout a single area
• Originated by an ABR
• Only intra-area routes are advertisedinto the backbone
• Type 4 is the information about theASBR
33ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
External LSA (Type 5)External LSA (Type 5)
• Defines routes to destination externalto the AS
• Default route is also sent as external
• Two types of external LSA:E1: Consider the total cost up to the externaldestination
E2: Considers only the cost of the outgoinginterface to the external destination
34ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Not Summarised: Specific LinksNot Summarised: Specific Links
BackboneArea #0
External links
1.A
1.C
1.B
1.D
TokenRing
TokenRing
TokenRing
TokenRing
3.D
3.A
3.C
3.B
1.A1.B1.C1.D
3.A3.B3.C3.D
2.A2.B2.C
2.A
2.C
2.B
TokenRing
TokenRing
• Specific link LSA advertised out• Link state changes propagate out ASBR
35ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Summarised: Summary LinksSummarised: Summary Links
BackboneArea #0
ASBR
External links
1.A
1.C
1.B
1.D
TokenRing
TokenRing
TokenRing
TokenRing
3.D
3.A
3.C
3.B
2.A
2.B
TokenRing
TokenRing
• Only summary LSA advertised out• Link state changes do not
propagate
1 3
2
36ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
BackboneArea #0
External links
1.A
1.C
1.B
1.D
TokenRing
TokenRing
TokenRing
TokenRing
3.D
3.A
3.C
3.B
2.A
2.C
2.B
TokenRing
TokenRing
ASBR
Not Summarised: Specific LinksNot Summarised: Specific Links
2.A2.B2.C3.A3.B3.C3.D
1.A1.B1.C1.D3.A3.B3.C3.D
1.A1.B1.C1.D2.A2.B2.C
• Specific link LSA advertised in• Link state changes propagate in
37ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Summarised: Summary LinksSummarised: Summary Links
BackboneArea #0
3.D
3.A
2.B
• Only summary LSA advertised in• Link state changes do not
propagate ASBR
External links
1.A
1.C
1.B
1.D
TokenRing
TokenRing
TokenRing
TokenRing
3.C
3.B
2.A
TokenRing
TokenRing
2,3
1,3
1,2
38ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
TokenRing
TokenRing
TokenRing
TokenRing
Regular Area (Not a Stub)Regular Area (Not a Stub)
From area 1’s viewpoint
• Summary networks from other areas injected• External networks injected, for example
network X.1ASBR
External Networks
1.A
1.C
1.B
1.DTokenRing
TokenRing
3.C
3.B
2.A
2,3
1,3
1,2X.1
X.1
X.1
X.1
2.D2.C
2.B
3.A
3.D
39ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
TokenRing
TokenRing
TokenRing
TokenRing
Normal Stub AreaNormal Stub Area
From area 1’s viewpoint
• Summary networks from other areas injected
• Default network injected into the area - represents external links
• Default path to closest area border router
• Define all routers in the area as stubarea x stub command ASBR
External Networks
1.A
1.C
1.B
1.DTokenRing
TokenRing
3.C
3.B
2.A
2,3 & Default
1,3
1,2X.1
X.1
X.1
X.1
2.D2.C
2.B
3.A
3.D
40ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
TokenRing
TokenRing
TokenRing
TokenRing
Totally Stubby AreaTotally Stubby Area
From area 1’s viewpoint
• Only a default network is injected into the areaRepresents external networks and all inter-area routes
• Default path to closest area border router
• Define all routers in the area as totally stubbyarea x stub no-summary command ASBR
External Networks
1.A
1.C
1.B
1.DTokenRing
TokenRing
3.C
3.B
2.A
Default 2&3
1,3
1,2X.1
X.1
X.1
X.1
2.D2.C
2.B
3.A
3.D
41ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
TokenRing
TokenRing
TokenRing
TokenRing
Not-So-Stubby AreaNot-So-Stubby Area
• Capable of importing external routes in a limitedfashion
• Type-7 LSA’s carry external information within anNSSA
• NSSA Border routers translate selected type-7LSAs into type-5 external network LSAs ASBR
External Networks
1.A
1.C
1.B
1.DTokenRing
TokenRing
3.C
3.B
2.A
Default 2&3
1,3
1,2X.1
X.1, X.2
X.1, X.2X.1
2.D2.C
2.B
3.A
3.DExternalNetworks
X.2
42ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
AddressingAddressing
Area 1network 131.108.0.0subnets 17-31range 255.255.240.0
Area 2network 131.108.0.0subnets 33-47range 255.255.240.0
Area 3network 131.108.0.0subnets 49-63range 255.255.240.0
Area 0network 192.117.49.0range 255.255.255.0
Assign contiguous ranges of subnets per area to facilitate summarisation
43ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
SummarySummary
• Scalable OSPF Network Design
Area hierarchy
Stub areas
Contiguous addressing
Route summarisation
44ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Redistributing Routes intoRedistributing Routes intoOSPFOSPF
ROUTER OSPF <pid#x>
REDISTRIBUTE {protocol} <as#y>
<metric>
<metric-type (1 or 2)
<tag>
<subnets>
45ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Router Sub-commandsRouter 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>
46ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com
Interface SubcommandsInterface 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-password>
47