© Synergon Informatika Rt., 1999 Chapter 9 Configuring Open Shortest Path First

2 Objectives Upon completion of this chapter, you will be able to perform the following tasks: Describe OSPF features and operation Configure OSPF for proper operation Use Cisco IOS summarization and stub-area features for OSPF Verify OSPF operation

3 OSPF Features and Operation

4 OSPF Overview  Standard IP link-state routing protocol  Designed to overcome RIP limitations

5 OSPF Features  Has no hop count limitation  Supports VLSM  Uses multicast addressing for updates  Has faster convergence  Allows for routing authentication  Supports hierarchical routing

6 OSPF Hierarchical Routing  Consist of areas and autonomous systems  Minimizes routing update traffic Area 0 Area 1Area 2 Autonomous System

7 OSPF Router Types Area 1 Backbone Area 0 Area 2 External AS Area Border Router AS Boundary Router Internal Router Backbone Router

8 The Backbone and Virtual Links  Backbone center of communication  Virtual links provide path to backbone  Avoid configuring virtual links if possible Virtual Link Area 0 (Backbone) Area 1 Area 2 Area 3

9 Another Use for Virtual Links  Link discontinuous backbone - Merged networks - Redundancy Area 3 Area 0

10 The Link-State Database  Represents the network topology  Shared with OSPF routers in same area Adjacent Link-State Database

11 Link-State Advertisements (LSAs)  Sourced by router connected to link  Flooded by all other routers in area  Transmitted at each link-state change LSA

12 Link-State Advertisement Operation LSA LSA received Add to database Flood LSA Run SPF to calculate new table Reset timer for entry Yes No Is entry in topological database?

13 Types of Link-State Packets O — OSPF Derived Intra-Area (Router LSA) IA — Inter-Area (Summary LSA) E1 — Type 1 External Route E2 — Type 2 External Route Routing Table External AS ASBR DR Area 1 Area 0 ABR Network Router External Summary

14 Types of External Routes  Type-1 (E1) metric — external + internal  Type-2 (E2) metric — external cost only 55 Area 1Area 0 R1R2R3 N1 cost = 30 N2 cost = 20 N1 cost = 25 N2 cost = 20 Cost = N2 (E2) N1 (E1)

15 OSPF Network Types Point-to-Point Broadcast Multiaccess X.25 Frame Relay Nonbroadcast Multiaccess

16 OSPF Router ID  Number by which the router is known to OSPF - Default: The highest IP address on an active interface - Can be overridden by a loopback interface Network Bogus Loopback Address Ex: Not in OSPF table Saves address space Bogus Loopback Address Ex: Not in OSPF table Saves address space Real Loopback Address Ex: In OSPF table Uses address space Real Loopback Address Ex: In OSPF table Uses address space

17 Neighbors  Form using the Hello protocol Hello

18 Adjacencies  Hellos elect designated router (DR) and backup designated router (BRD)  Each router forms adjacency with DR and BDR DRBDR

19 Designated Router Election  Hello packets exchanged via IP multicast  Router with highest OSPF priority elected Hello DRBDR P=3P=2 P=1 P=0

20 Multiaccess Network  New router announces presence  DR and BDR respond Adding a Router New Router DR BDR Hello

21 Multiaccess Network (cont.)  New router sends LSAs to DR and BDR  BDR waits for DR to respond Adding a Router New Router DR BDR LSA

22 Multiaccess Network (cont.)  DR transmits LSAs to other routers  DR must receive ACK from all routers Adding a Router New Router DR BDR LSA

23 Multiaccess Network (cont.)  Source router tells DR on  DR tells others on  Other routers flood LSAs Link-State Change New Router DR BDR X X LSA

24 Configuring OSPF

25 OSPF Basic Configuration Commands Router (config) # router ospf process-id  Enables on OSPF routing process Router (config-router ) # network address wildcard-mask area area-id Selects participating interfaces

26 OSPF Basic Configuration Example router ospf 63 network area 1 network area 0 network area 1 router ospf 63 network area 1 network area 0 network area T0 E0 E1 E2 Area 1 Area 0

27 OSPF Virtual Link Command Router (config-router) # Area area-id virtual-link router-id  Creates a virtual link

28 OSPF Virtual Link Example R2: router ospf 63 area 1 virtual-link R2: router ospf 63 area 1 virtual-link Area 1 Area 0 Area 3 Router ID Router ID R1: router ospf 100 area 1 virtual-link R1: router ospf 100 area 1 virtual-link

29 Vendor Interoperability Assigns a cost to an outgoing interface May be required for interoperability Use default cost between Cisco devices Traffic Non-Cisco Cisco Router (config-if ) # Ip ospf cost cost

30 Using Route Summarization, Stub Areas, and Redistribution

31 OSPF Route Summarization Minimizes routing table entries Localizes impact of a topology change Area 1 ABRs Summarization Area 0 Backbone

32 Route Summarization (cont.) Interarea (IA) summary link carries mask One entry can represent several subnet Area 1 Area 0 ABR Summarization O O O O O O O IA Routing Table for C Routing Table for B ABC

33 Route Summarization Issue Existing Subnet Mask = Valid Address = rd Subnet Summary Route Mask = Invalid Subnet Address = zero Some addresses may need reallocating

34 Route Summarization Commands Router (config-router ) # Area area-id range address mask  Consolidates IA router on an ABR Router (config-router ) # Summary-address address mask Consolidates external routes on an ASBR

35 Route Summarization Example Area 0 Area 1 Area Interface Addresses ( mask) Interface Addresses ( mask) R1# router ospf 100 network area 1 network area 0 area 0 range area 1 range R1# router ospf 100 network area 1 network area 0 area 0 range area 1 range R2# router ospf 100 network area 2 network area 0 area 0 range area 2 range R2# router ospf 100 network area 2 network area 0 area 0 range area 2 range R1 R2

36 Stub Areas Hide external routes, reduce database Consolidate external links Area 1 Area 0 Area BGP Stub Area External AS

37 Stub Area Restrictions External AS Area 2 single exit point or, if multiple exit points, suboptimal path acceptable Transit area for virtual links disallowed An ASBR cannot be internal to stube Single Exit Point

38 Totally Stubby Areas Block external and summary routes Know only intra-area and default routes Area 1 Area 0 Area BGP Stub Area External AS Summary (IA) Route

39 OSPF Stub Area Commands Router (config-router ) # Area area-id stub [no-summary]  Creates a stub network Router (config-router ) # Area area-id default-cost cost Specifies cost for default route sent into stub area

40 OSPF Stub Area Example External AS Area 0 Stub Area E0S0 R3 R4 R3# interface Ethernet 0 ip address interface Serial 0 ip address router ospf 100 network area 0 network area 2 area 2 stub R3# interface Ethernet 0 ip address interface Serial 0 ip address router ospf 100 network area 0 network area 2 area 2 stub R4# interface Serial 0 ip address router ospf 15 network area 2 area 2 stub R4# interface Serial 0 ip address router ospf 15 network area 2 area 2 stub

41 OSPF Totally Stubby Example External AS Area 0 Stub Area E0S0 R3 R4 R3# router ospf 100 network area 0 network area 2 area 2 stub no-summary area 2 stub default-cost 20 R3# router ospf 100 network area 0 network area 2 area 2 stub no-summary area 2 stub default-cost 20 R4# router ospf 15 network area 2 area 2 stub R4# router ospf 15 network area 2 area 2 stub

42 OSPF Route Redistribution RIP IGRP Enhanced IGRP IS-IS OSPF BGP EGP Allows routing-information exchange between OSPF and other routing protocols

43 OSPF redistribution Command Router (config-router ) # Redistribute protocol [ process-id ] [ metric value ] [ metric-type value ] [ subnet ]  Redistributes routes from OSPF into other routing protocols (and vice versa)

44 OSPF Redistribution Example 1 RIP OSPF Area S0 S1 R Redistribution between RIP and OSPF router ospf 109 network area 0 network area 0 redistribute rip subnets metric-type 1 metric 20 router rip network passive-interface serial 0 passive-interface serial 1 default-metric 10 redistribute ospf match internal external 1 external 2 router ospf 109 network area 0 network area 0 redistribute rip subnets metric-type 1 metric 20 router rip network passive-interface serial 0 passive-interface serial 1 default-metric 10 redistribute ospf match internal external 1 external 2

45 Redistribution Example 2 “Back door” creates potential loop RIP OSPF Area 0 R1 R2 R router ospf 109 network area 0 network area 0 redistribute rip subnets metric-type 1 metric 20 distribute-list 11 out rip access-list 11 permit router ospf 109 network area 0 network area 0 redistribute rip subnets metric-type 1 metric 20 distribute-list 11 out rip access-list 11 permit

46 Verifying OSPF Operation

47 Show ip ospf interface Command Verifies interfaces are in correct areas Router# show ip ospf interface e 0 Ethernet 0 is up, line protocol is up Internet Address , Area Process ID 10, Router ID , Network Type BROADCAST, Cost: 10 Transmit Delay is 1 sec, State BDR, Priority l Designated Router (ID) , Interfece address Backup Designated router (ID) , interface address Timer intervale configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 0:00:02 Neighbor Count is 3, Adjacent neighbor count is 3 Adjacent with neighbor (Designated Router) Loopback0 is up, line protocol is up Internet address , Area 1 Process ID 10, Router ID , Network Type LOOPBACK, Cost: 1 Loopback interface is treated as a stub Host Router# show ip ospf interface e 0 Ethernet 0 is up, line protocol is up Internet Address , Area Process ID 10, Router ID , Network Type BROADCAST, Cost: 10 Transmit Delay is 1 sec, State BDR, Priority l Designated Router (ID) , Interfece address Backup Designated router (ID) , interface address Timer intervale configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 0:00:02 Neighbor Count is 3, Adjacent neighbor count is 3 Adjacent with neighbor (Designated Router) Loopback0 is up, line protocol is up Internet address , Area 1 Process ID 10, Router ID , Network Type LOOPBACK, Cost: 1 Loopback interface is treated as a stub Host

48 Show ip ospf Command Router # show ip ospf Routing Process "ospf 1" with ID Supports only single TOS (TOS0) routes SPF schedule delay 5 secs, Hold time between two SPFs 10 secs Number of areas in this router is 1 Area 23 Number of interfaces in this area is 3 Area has no authentication SPF algorithm executed 19 times Area ranges are Link State Update Interval is 0:30:00 and due in 0:04:55 Link State Age Interval is 0:20:00 and due in 0:04:55 Router # show ip ospf Routing Process "ospf 1" with ID Supports only single TOS (TOS0) routes SPF schedule delay 5 secs, Hold time between two SPFs 10 secs Number of areas in this router is 1 Area 23 Number of interfaces in this area is 3 Area has no authentication SPF algorithm executed 19 times Area ranges are Link State Update Interval is 0:30:00 and due in 0:04:55 Link State Age Interval is 0:20:00 and due in 0:04:55 Displays general information about the OSPF routing process

49 show ip ospf database Command Router# show ip ospf database OSPF Router with ID ( ) (Process ID 1) Router Link States (Area 23) Link IDADVAgeSeq#ChecksumLink Routercount x x x xEllC x x835E x x80A11 Net Link States (Area 23) Link IDADV RouterAgeSeg#Checksum x x2FCE x xFB29 Router# show ip ospf database OSPF Router with ID ( ) (Process ID 1) Router Link States (Area 23) Link IDADVAgeSeq#ChecksumLink Routercount x x x xEllC x x835E x x80A11 Net Link States (Area 23) Link IDADV RouterAgeSeg#Checksum x x2FCE x xFB29

50 show ip protocol Command Router> show ip protocol Routing Protocol is "ospf 300" Sendinq updates every 0 seconds Invalid after 0 seconds, hold down 0, flushed after 0 Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Redistributing: ospf 300 Routing for Networks: / / Routing Information Sources: GatewayDistance Last Update 14d d :17: :17: :17: d d :17: :17: :17:33 Distance: (default is 1l0) - - More - - Router> show ip protocol Routing Protocol is "ospf 300" Sendinq updates every 0 seconds Invalid after 0 seconds, hold down 0, flushed after 0 Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Redistributing: ospf 300 Routing for Networks: / / Routing Information Sources: GatewayDistance Last Update 14d d :17: :17: :17: d d :17: :17: :17:33 Distance: (default is 1l0) - - More - -

51 Other OSPF show Commands Router# show ip ospf virtual-links Displays parameters about OSPF virtual links Router# show ip ospf neighbor detail Displays neighbor information per interface Router# show ip ospf border-routers Displays routes to the ABR and ASBR

52 Summary OSPF is a scalable, standards-based link-state routing protocol OSPF features: Hierarchical design VLSM support Vendor interoperability Route summarization Route redistribution