Link State. 1 Flooding of link-state information. 5 Routing Table. 3 SPF Algorithm. 2 Building a Topological Database. 4 SPF Tree

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2 Link State 1 Flooding of link-state information 5 Routing Table 2 Building a Topological Database 3 SPF Algorithm 4 SPF Tree

3 OSPF Hello Protocol OSPF routers send Hellos on OSPF enabled interfaces: Default every 10 seconds on multi-access and point-to-point segments Default every 30 seconds on NBMA segments Most cases OSPF Hello packets are sent as multicast to ALLSPFRouters ( ) HelloInterval - Cisco default = 10 seconds or 30 seconds and can be changed with the command ip ospf hello-interval. RouterDeadInterval t - The period in seconds that t the router will wait to hear a Hello from a neighbor before declaring the neighbor down. Cisco uses a default of four-times the HelloInterval and can be changed with the command ip ospf dead-interval. Note: For routers to become adjacent, the Hello, DeadInterval and network types must be identical between routers OR Hello packets get dropped!

4 Network Types show ip ospf interface Unless you are configuring an NBMA network like Frame Relay, this won t be an issue. Many administrators prefer to use point-to-point or point-to-multipoint for NMBA to avoid the DR/BDR and full-mesh issues.

5 Configuring Basic OSPF Router(config)# router ospf process-id [vrf vpn-name] ] Enable one or more OSPF routing processes. Router(config-router)# network ip-address wildcard-mask area area-id Define the interfaces that OSPF will run on. Router(config-if)# if)# ip ospf process-id area area-id [secondaries none] Optional method to enable OSPF explicitly on an interface.

6 Configuring OSPF

7 OSPF s Metric is Cost (Bandwidth) Cisco default interface costs: 56-kbps serial link = kbps serial link = kbps serial link = 781 T1 (1.544-Mbps serial link) = 64 E1 (2.048-Mbps serial link) = 48 4-Mbps Token Ring = 25 Ethernet = Mbps Token Ring = 6 Fast Ethernet = 1 Problem: Gigabit Ethernet and faster = 1 Cost = 100,000,000/Bandwidth /B Notes: Cisco routers default to T1 (1.544 Mbps) on all serial interfaces and require manual modification with the bandwidth command. ospf auto-cost reference-bandwidth reference-bandwidth can be used to modify the reference-bandwidth for higher speed interfaces

8 Changing the Cost Metric The cost, or metric, is an indication of the overhead to send packets over an interface. Default = (100 Mbps)/(bandwidth in Mbps). RouterA(config-router)# auto-cost reference-bandwidth ref-bw Sets the reference bandwidth to values other than 100 Mbps (legal values range from 1 to 4,294,967 in Mbps). RouterA(config-if)# if)# ip ospf cost interface-cost Overrides the default cost calculation Values from 1 to Overrides the default cost calculation. Values from 1 to can be defined.

9 OSPF Router ID The router is known to OSPF by the OSPF router ID number. LSDBs use the OSPF router ID to differentiate one router from the next. By default, the router ID is the highest IP address on an active interface at the moment of OSPF process startup. If no interface is up when the OSPF process starts, you will get the following error message: R1(config)#router ospf 1 2w1d: %OSPF-4-NORTRID: OSPF process 1 cannot start. If a loopback interface exists, the router ID is the highest IP address on any active loopback interface. The OSPF router-id command can be used to override the OSPF router ID. Using a loopback interface or a router-id command is recommended for stability.

10 OSPF router-id Command Router(config-router)# router-id ip-address This command is configured under the router ospf [process-id] command. Any unique arbitrary 32-bit value in an IP address format (dotted d decimal) can be used. If this command is used on an OSPF process that is already active, then the new router ID takes effect after the next reload or after a manual restarting ti of the OSPF process using: Router#clear ip ospf process Router(config)#router ospf 1 Router(config-router)#router-id Router#clear ip ospf process

11 OSPF over NBMA Topology Modes of Operation RFC compliant modes are as follows: Nonbroadcast (NBMA) Point-to-multipoint Additional modes from Cisco are as follows: Point-to-multipoint nonbroadcast Broadcast Point-to-point Router(config-if)# if)# ip ospf network [{broadcast non-broadcast point-tomultipoint [non-broadcast] point-to-point}] This interface command defines OSPF network type.

12 RFC-compliant Non-broadcast Mode One IP subnet. Neighbors must be manually configured. DR and BDR elected. DR and BDR need to have full connectivity with all other routers. Typically used in a full mesh topology. RTB(config-if)#ip ospf network non-broadcast RTB(config-router)#network area 0 RTB(config-router)#neighbor RTB(config-router)#neighbor

13 RFC-compliant Point-to-Multipoint Mode One IP subnet. Uses multicast OSPF hello packet to automatically discover neighbors. DR and BDR not required Typically used in a partialmesh or hub-and-spoke topology. RTB(config-if)#ip ospf network point-to-multipoint RTB(config-router)#network area 0

14 Cisco s Point-to-Multipoint Non-broadcast mode Cisco extension to RFCcompliant point-to-multipoint mode Must statically define neighbors, like nonbroadcast mode Like point-to-multipoint to mode, DR/BDR not elected Used in special cases where neighbors cannot be utomatically discovered RTB(config-if)#ip ospf network point-to-multipoint non-broadcast RTB(config-router)#network area 0 RTB(config-router)#neighbor cost 10 RTB(config-router)#neighbor cost 20

15 Cisco s s Broadcast Mode Makes a WAN interface appear to be a LAN One IP subnet Uses multicast hellos to discover neighbors DR and BDR elected Requires a full mesh. RTB(config-if)#ip ospf network broadcast RTB(config-router)#network area 0

16 Cisco s Point-to-Point mode One IP subnet per subinterface pair No DR or BDR election Used when only two routers need to form an adjacency on a pair of interfaces Same properties as any physical pointto-point physical interface RTB(config)#interface serial 0/0.1 RTB(config-subif)#ip address RTB(config-subif)#interface serial 0/0.2 RTB(config-subif)#ip address RTB(config-router)#network area 0 RTB(config-router)#network area 0

17 The neighbor command options Router(config-router)# neighbor ip-address [priority number] [poll-interval number] [cost number] [database-filter all] Used to statically define neighbor relationships in an NBMA network Option priority number poll-interval number cost number database-filter all Description Specifies the priority of neighbor. Default is zero, which means neighbor can never be DR. Amount of time a NBMA interface waits before sending hellos to the neighbor even if the neighbor is inactive. Assigns a cost to the neighbor from 1 to Neighbors with no specific cost configured will assume the cost of the interface, based on the ip ospf cost command. (Not used with NBMA) Filters outgoing LSAs to an OSPF neighbor

18 OSPF over NBMA Topology Summary

19 Issues with large OSPF nets Large link-state table Each router maintains a LSDB for all links in the area The LSDB requires the use of memory Frequent SPF calculations A topology change in an area causes each router to re-run SPF to rebuild the SPF tree and the routing table. A flapping link will affect an entire area. SPF re-calculations l are done only for changes within that t area. Large routing table Typically, y, the larger the area the larger the routing table. A larger routing table requires more memory and takes more time to perform the route look-ups.

20 OSPF uses Areas Hierarchical routing enables you to separate large internetworks (autonomous systems) into smaller internetworks that are called areas. With this technique, routing still occurs between the areas (called inter-area routing), but many of the smaller internal routing operations, such as recalculating the database re-running the SPF algorithm, are restricted within an area.

21 OSPF Router Types

22 LSA Types

23 Link Types Link Type 1 Description Link-state ID Point-to-point connection to another router Neighboring router ID 2 Connection to a transit network IP address of DR 3 Connection to a stub network IP network/subnet number 4 Virtual link Neighboring router ID

24 Interpreting the OSPF Database RouterA#show ip ospf database OSPF Router with ID ( ) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count x x00401A x x003A1B 003A1B x800002D7 0x00EEA9 2 Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum x x004EC9 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum x x00FB x x00F516 <output omitted> Link ID: Identifies each LSA. ADV router: Advertising router; the source router of the LSA. Age: The maximum age counter in seconds; the maximum age is 1 hour or 3,600 seconds. Seq#: Sequence number of the LSA; this number begins at 0x and increases with each update of the LSA. Checksum: Checksum of the individual LSA to ensure reliable receipt of that LSA. Link count: Total number of directly attached links, used only on router LSAs. The link count includes all pointto-point, transit, and stub links. Each point-to-point serial link counts as two; all other links count as one, including Ethernet links.

25 LSA Types LSA Types 1 through 5 We will look at these in detail as we discuss areas in this chapter. LSA Type 6 MOSPF (Multicast OSPF) Not supported by Cisco. MOSPF enhances OSPF by letting routers use their link-state t databases to build multicast distribution trees for the forwarding of multicast traffic. LSA Type 7 NSSA External Link Entry Originated by an ASBR connected to an NSSA. Type 7 messages can be flooded throughout NSSAs and translated into LSA Type 5 messages by ABRs. Routes learned via Type-7 LSAs are denoted by either a N1 or and N2 in the routing table. (Compare to E1 and E2). We will discuss this more later when we look at NSSA areas.

26 Area Types Standard or Normal Areas Backbone Non-Backbone Stub Stub Area Totally Stubby Area (TSA) Not-so-stubby-area (NSSA)

27 Area Types

28 Overview of Normal Areas Routes Received on all OSPF Routers Receives all routes from within A.S.: Within the local area (Intra-Area) LSA 1 and LSA 2 From other areas (Inter-Area) LSA 3, LSA 4, LSA 5. Receives all routes from External A.S. s: From external AS s LSA 5 As long as routes are being redistributed by the ASBR. Default Routes Received only if default-information-originate originate command was used (later) If default-information-originate command is not used, then the default route is not received.

29 Part I - LSAs using all normal areas / / / /24 Multi Area OSPF Normal Areas Lo - RouterID /32 ASBR.1 Lo - RouterID /32.1 ABR /24 Area Pri 200 Pri /24 Area /16 ABR-2.5 Lo - RouterID / /30.6 Internal /24 Lo - RouterID /32 Area 1

30 1. OSPF Multi-Areas - All Normal Areas ASBR router ospf 1 redistribute static network area 0! ABR-1 interface FastEthernet0/0 ip address ip ospf priority 200 router ospf 1 network area 0 network area 51 ABR-2 interface FastEthernet0 ip address ip ospf priority 100! router ospf 1 network area 0 network area 1 Internal router ospf 1 network area 1 ABR contains network statements for each area it belongs to, using the proper area value.

31 LSA 1 - Router Link States LSA 1 s being sent within Area / / / /24 Multi Area OSPF Normal Areas Lo - RouterID /32.1 ABR /2416 Area 51 LSA 1 ASBR Pri /24 Area /16 LSA 1 Lo - RouterID /32 Pri 100 LSA 1 ABR-2.5 Lo - RouterID / /30.6 Internal /24 Lo - RouterID /32 Area 1

32 LSA 1 - Router Link States LSA 1 s being sent within other areas / / / /24 Multi Area OSPF Normal Areas Lo - RouterID /32 LSA 1.1 ABR /2416 Area 51 LSA 1 ASBR Pri /24 Area /16 LSA 1 Lo - RouterID /32 Pri 100 LSA 1 ABR-2.5 Lo - RouterID /32 LSA /30 LSA 1.6 LSA 1 Internal /24 Lo - RouterID /32 Area 1

33 LSA 1 - Router Link States For Router Links: The Link State ID is always the same as the Advertising Router Advertising Router is the Router ID of the router that created this LSA 1 Internal#show ip ospf data OSPF Router with ID ( ) (Process ID 1) Router Link States (Area 1) (LSA 1 - Links in the area to which this router belongs.) Link ID ADV Router Age Seq# Checksum Link count x xCE x xFD44 3 Conclusion: Router Link States (LSA1 s) should display all the Router IDs of routers in that area including its own IDs of routers in that area, including its own. reminder: LSA 1 -> my one area

34 LSA 1 - Router Link States ABR-2 - show ip route /1616 0/16 is variably subnetted, 4 subnets, 3 masks O IA /32 [110/2] via , 00:11:44, FastEthernet0 O /24 [110/782] via , 00:12:29, Serial0 C /30 is directly connected, Serial0 C /24 is directly connected, FastEthernet0 O E /8 [110/20] via , 00:11:44, FastEthernet0 O E /8 [110/20] via , 00:11:44, FastEthernet0 O E /8 [110/20] via , 00:11:44, FastEthernet /32 is subnetted, 1 subnets C is directly connected, Loopback1 Denoted by just an O in the routing table, or a C Why is there only just an O for this network and not the other networks? Directly connected or via another area.

35 LSA 2 - Network Link States LSA 2 Network LSA Generated by the DR on every multi-access network Denoted by just an O in the routing table or C if the network is directly connected. Flooded only within the originating area. LSA 2 s are in link state database for all routers within an area. ABR may include a set of LSA 2s for each area it belongs to.

36 LSA 2 - Network Link States No LSA 2 s for ABR-1 in Area 51, or for Internal because no other routers on multiaccess segment. Lo - RouterID / /24 Area 51 ABR / / /8 ASBR /24 LSA 2 DR LSA Pri 200 LSA 2 flooded /24 Area /16 Lo - RouterID /32 Pri 100 LSA 2 flooded DR Multi Area OSPF Normal Areas ABR-2.5 Lo - RouterID / /30.6 Internal LSA /24 Lo - RouterID /32 Area 1

37 LSA 2 - Network Link States ASBR#show ip ospf data OSPF Router with ID ( ) (Process ID 1) Net Link States (Area 0) (LSA 2 - Generated by the DR) Link ID ADV Router Age Seq# Checksum x D 0xCFE8 Link ID = IP address of DR on MultiAccess Network ADV Router = Router ID of DR Conclusion: Net Link States (LSA2 s) should display the RouterIDs of the DRs on all multi-access networks in the area and their IP addresses. reminder: LSA 2 -> Generated by the D R 1 2

38 LSA 3 Summary Net Link States LSA 1 s are sent as LSA 3 s into other areas by the ABRs / / / /24 Multi Area OSPF Normal Areas Lo - RouterID / /24 Area 51 ABR-1 LSA 3 LSA 1 ASBR.1 LSA Pri /24 Area /16 Lo - RouterID /32 Pri 100 LSA 3 LSA 1 ABR-2.5 Lo - RouterID / /30.6 Internal /24 Lo - RouterID /32 Area 1

39 LSA 3 Summary Net Link States Don t forget about the LSA 1 s from Area / / / /24 Multi Area OSPF Normal Areas LSA 3 Lo - RouterID /32.1 ABR /2416 Area 51 LSA 1 ASBR Pri /24 Area /16 LSA 1 Lo - RouterID /32 Pri 100 LSA 3 LSA 1 ABR-2.5 Lo - RouterID / /30.6 Internal /24 Lo - RouterID /32 Area 1

40 LSA 3 Summary Net Link States ASBR ASBR# show ip ospf database Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum (Area 1 networks - Advertising Router ABR-2) x xD x xA746 (Area 51 networks - Advertising Router ABR-1) x xA832 Conclusion: We should see networks in other areas and the ABR advertising Conclusion: We should see networks in other areas and the ABR advertising that route. reminder: LSA 3 -> networks sent by the A B R 1 2 3

41 LSA 3 Summary Net Link States ASBR ASBR# show ip route /16 is variably subnetted, 4 subnets, 3 masks O IA /32 [110/2] via , 00:02:54, FastEthernet0/0 O IA /24 [110/783] via , 00:02:54, FastEthernet0/0 O IA /30 [110/782] via , 00:02:54, FastEthernet0/0 C /24 is directly connected, FastEthernet0/ /24 is subnetted, 1 subnets C is directly connected, FastEthernet0/1 S /8 is directly connected, Null0 S /8 is directly connected, Null /32 is subnetted, 1 subnets C is directly connected, Loopback0 S /8 is directly connected, Null0

42 LSA 4 ASBR Summary Link States LSA 4 ASBR Summary LSA Originated by the ABR. Flooded throughout the backbone area to the other ABRs. Describes the reachability to the ASBRs Advertises an ASBR (Router ID) not a network Included in routing table as an IA route. Same format as a LSA 3 - Summary LSA, except LSA 4 ASBR Summary LSA. Exceptions Not flooded to Stub and Totally Stubby networks.

43 Lo - RouterID /32 LSA /2416 Area / / /8 Normal Areas ASBR /24 LSA 5 s flooded Lo - RouterID / ABR-1 Pri 200 Pri 100 ABR-2 LSA /24 Area /16 LSA 4 Internal Lo - RouterID / /30.6 LSA 4.1 Lo - RouterID / / /24 Area 1 Area 1 Flooded throughout t the backbone area to the other ABRs. Describes the reachability to the ASBRs

44 LSA 4 ASBR Summary Link States ABR-2 ABR-2# show ip ospf database Summary ASB Link States (Area 1) LSA 4 - Reachability to ASBR. Not flooded to Stub and Totally Stubby networks. Link ID ADV Router Age Seq# Checksum x x93CC Conclusion: Routers in non-area 0, should see Router ID of ASBR and its ABR to get there. reminder: LSA 4 -> Reachability to the A S B R

45 LSA 5 - AS External Link States LSA 5 AS External LSA Originated by the ASBR. Describes destination networks external to the Autonomous System Flooded throughout the OSPF AS, except to stub and totally stubby areas Denoted in routing table as E1 or E2 (default) route (see later) ASBR Router which redistributes routes into the OSPF domain. Exceptions Not flooded to Stub and Totally Stubby networks. More on this later

46 ASBR router ospf 1 redistribute static network area 0 ip route Null0 ip route Null0 ip route Null0 LSA 5 Lo - RouterID / / /24 Area 51 LSA 5 ABR / / /8 Normal Areas ASBR / Pri /24 Area /16 Lo - RouterID /32 LSA 5 s 5s flooded Pri 100 LSA 5 LSA 5 ABR-2.5 Lo - RouterID / /30.6 Internal Redistribute command creates an ASBR router. This flood is originated by the ASBR. LSA /24 Area 1 Lo - RouterID /32 It describes destination networks external to the OSPF Routing Domain It s flooded throughout the OSPF AS, except to stub and totally stubby areas

47 LSA 5 - AS External Link States ABR-2 ABR-2# show ip ospf database AS External Link States LSA 5 - External Networks originated by the ASBR, Flooded throughout A.S. except to Stub and Totally Stubby Link ID ADV Router Age Seq# Checksum Tag x x3FEA x x32F x x Conclusion: All Routers should see External networks and the Router ID of ASBR to get there. reminder: LSA 5 -> O T H E R networks

48 LSA 5 - AS External Link States ABR-2 ABR-2# show ip route /16 is variably subnetted, 4 subnets, 3 masks O IA /32 [110/2] via , 00:11:44, FastEthernet0 O /24 [110/782] via , 00:12:29, Serial0 C /30 is directly connected, Serial0 C /24 is directly connected, FastEthernet0 O E /8 [110/20] via , 00:11:44, FastEthernet0 O E /8 [110/20] via , 00:11:44, FastEthernet0 O E /8 [110/20] via , 00:11:44, FastEthernet /32 is subnetted, 1 subnets C is directly connected, Loopback1 Designated by E2 Notice that the cost is 20 for all three routes, we will see why later. It has to do with E2 routes and where the default cost is 20. Redistribute command: If a value is not specified for the metric option, and no value is specified using the default-metric command, the default metric value is 0, except for OSPF where the default cost is 20.

49 LSA 5 - AS External Link States E1 vs. E2 External Routes External routes fall under two categories: external type 1 external type 2 The difference between the two is in the way the cost (metric) of the route is being calculated. The cost of a type 2 route is always the external cost, irrespective of the interior cost to reach that route. A type 1 cost is the addition of the external cost and the internal cost used to reach that route. A type 1 route is always preferred over a type 2 route for the same destination. More later

50 Stub Areas Considerations for both Stub and Totally Stubby Areas An area could be qualified a stub when: There is a single exit point (a single ABR) from that area. If routing to outside of the area does not have to take an optimal path. The area is not needed as a transit area for virtual links (later). The ASBR is not within the area The area is not the backbone area (area 0) Stub areas will result in memory and processing savings, depending upon the size of the network.

51 Stub Areas ABR-2 router ospf 1 network area 0 network area 1 area 1 stub << Command: area area stub Internal router ospf 1 network area 1 area 1 stub << Command: area area stub All routers in the area must be configured as stub including the ABR

52 / / /8 LSA 3 Lo - RouterID /32 LSA 4.1 Area 51 ABR-1 LSA 5 ASBR / /24 Area 0 Lo - RouterID /32 ABR-2.5 Lo - RouterID / /32 Pri 200 Pri 100 LSA /16 LSA 5 LSA 3 Blocked X /24.6 Internal /30 X Blocked Default route to.1 ABR Lo - RouterID /32 Stub Area /24 injected Area 1 Area 1

53 Totally Stubby Areas Cisco proprietary, however the RFC does make some provisions for this as an optional feature. Same considerations as with Stub areas, except: ABR blocks all LSA 3s, except propagating a default route. Default route is injected into totally stubby area by ABR

54 Totally Stubby Areas ABR-2 router ospf 1 network area 0 network area 1 area 1 stub no-summary ^^ Command: area area stub no-summary Creates a totally stubby area. No Type 3 e 4 LSAs Internal router ospf 1 network area 1 area 1 stub ^^ Command: area area stub

55 / / /8 LSA 3 Lo - RouterID /32 LSA 4.1 Area 51 ABR-1 LSA 5 ASBR / /24 Area 0 Lo - RouterID /32 ABR-2.5 Lo - RouterID / /32 Pri 200 Pri 100 LSA /16 LSA 5 LSA 3 Blocked X X /24.6 Internal /30 X Blocked Default route to.1 ABR Lo - RouterID /32 Totally Stubby Area /24 injected Area 1 Area 1

56 Totally Stubby Areas Internal Internal# show ip route Gateway of last resort is to network /16 is variably subnetted, 2 subnets, 2 masks C / /24 is directly connected, FastEthernet0 C /30 is directly connected, Serial /32 is subnetted, 1 subnets C is directly connected, Loopback0 O*IA /00 0 0/0 [110/782] via , :03:09, Serial0 Default route is injected into totally stubby area by ABR for all other networks (inter-area and external routes) Does not receive routes from other areas (Inter-Area) Does not receive routes from External A.S. (External Routes)

57 NSSA (Not So Stubby Area) Relatively new, these standards are based on OSPF enhancement, RFC NSSA allows an area to remain a stub area, but carry external routing information (Type 7 LSAs) from its stubby end, back towards the OSPF backbone. XTERNAL ASBR in NSSA injects external routing information into the backbone and the NSSA area, but rejects external routing information coming from the ABR. The ABR does not inject a default route into the NSSA. This is true for a NSSA Stub, but a default route is injected for a NSSA Totally Stubby area.

58 NSSA Generic Default route via RTG NSSA Backbone Area Area 2 Area 0 RTH RIP RTE LSA 7 RTG ASBR LSA 7 RTF LSA 7 RTD LSA 7 RTB ABR RTC LSA 7s LSA 7 LSA 7 Blocked LSA 5 RTA (Possible ASBR) Configuring NSSA Stub Area Configured for all routers in Area 2: router ospf 1 network area 2 area 2 nssa

59 NSSA (Not So Stubby Area) NSSA Stub and NSSA Totally Stubby There are two types of NSSA: Stub Totally Stubby

60 NSSA Stub and Totally Stubby NSSA stub areas: NSSAs that block type 4 and 5, but allow type 3. To make a stub area into an NSSA, use the following command under the OSPF configuration. This command must be configured on all routers in area 2. router ospf 1 area 2 nssa NSSA Totally Stubby areas: NSSAs that block type 4, 5 and 3, but allow a default. This command must be configured on ABR router in area 2. router ospf 1 router ospf 1 area 2 nssa no-summary

61 NSSA Stub Areas Default route via RTG RTH RIP RTG ASBR LSA 7 RTH routes:n1/n2 RTF RTE LSA 7 NSSA Backbone Area Area 2 Area 0 LSA 7 RTD LSA 3s LSA 4s & LSA 5s X /0 X LSA 7 LSA 5 RTB ABR RTC LSA 7s LSA 7 LSA 7 Blocked RTH routes: E1/E2 RTA (Possible ASBR) Area 2 routers: router ospf 1 network area 2 area 2 nssa

62 NSSA Totally Stubby Areas Default route via RTG RTH RIP RTG ASBR LSA 7 RTH routes: N1/N2 RTF RTE LSA 7 NSSA Backbone Area Area 2 Area 0 LSA 7 RTD LSA 7 LSA 3s X LSA 4s & LSA 5s X /0 RTB ABR RTC LSA 7s LSA 7 LSA 7 Blocked RTB (ABR): router ospf 1 network area 0 network area 2 area 2 nssa no-summary Area 2 routers: router ospf 1 network area 2 area 2 nssa LSA 5 RTH routes: E1/E2 RTA (Possible ASBR)

63 Virtual Links A virtual link has the following two requirements: It must be established between two routers that share a common area and are both ABRs. One of these two routers must be connected to the backbone. should be used only as a temporary fix to an unavoidable topology problem.

64 Virtual Links The command to configure a virtual link is as follows: area <area-id> virtual-link <remote-router-id> RTA(config)#router ospf 1 RTA(config-router)#network area 51 RTA(config-router)#network area 3 RTA(config-router)#area 3 virtual-link RTB(config)#router ospf 1 RTB(config-router)#network area 3 RTB(config-router)#network area 0 RTB(config-router)#area 3 virtual-link

65 Route Summarization Inter-Area Route Summarization - Area Range By default ABRs do not summarize routes between areas. Route summarization is the consolidation of advertised addresses. This feature causes a single summary route to be advertised to other areas by an ABR. In OSPF, an ABR will advertise networks in one area into another area. Router(config-router)# area area-id range network-address subnet-mask area-id - Identifier of the area about which routes are to be summarized. (From area) ABR router ospf 100 area 1 range

66 Route Summarization External Route Summarization - summary-addressaddress When redistributing routes from other protocols into OSPF (later), each route is advertised individually in an external link state advertisement (LSA). However, you can configure the Cisco IOS software to advertise a single route for all the redistributed routes that are covered by a specified network address and mask. Doing so helps decrease the size of the OSPF link state database. Router(config-router)# summary-address address network-address subnet- mask ASBR router ospf 100 summary-address

67 Injecting Default Routes into OSPF There are two ways to generate a default. 1) default-information originate If the ASBR already has the default route (ip route ), you can advertise into the area. 2) default-information originate always If the ASBR doesn't have the route (ip route ), you can add the keyword always to the default-information originate command, and then advertise

68 Redistributing External Routes router ospf 1 redistribute routing-protocol metric-type [1 2] metric-type 1 - A type 1 cost is the addition of the external cost and the internal cost used to reach that route. redistribute rip [metric value] metric-type type 1 ASBR router ospf 1 redistribute rip metric 500 metric-type 1 metric-type 2 - The cost of a type 2 route is always the external cost, irrespective of the interior cost to reach that route. redistribute rip [metric value] metric-type type 2 ASBR router ospf 1 redistribute rip metric 500 metric-type 2

69 Configuring Simple Authentication A router, by default, trusts that routing information received, has come from a router that should be sending it. Rtr(config-if)# ip ospf authentication-key passwd Configured on an interface password = Clear text unless message-digest is used (next) Easily captured using a packet sniffer Passwords do not have to be the same throughout an area, but they must be same between neighbors. After a password is configured, you enable authentication for the area on all participating area routers with: Rtr(config-router)# area area authentication Configured for an OSPF area, in ospf router mode.

70 Configuring Simple Authentication s / /16 RouterA RouterB s / /24 RouterA interface Serial1 ip address ip ospf authentication-key secret! router ospf 10 network area 0 network area 0 area 0 authentication RouterB interface Serial2 ip address ip ospf authentication-key secret! router ospf 10 network area 0 network area 0 area 0 authentication

71 Configuring MD5 Encrypted Authentication Rtr(config-if)# ip ospf message-digest-key key-id md5 password Key-id = 1 to 255, must match on each router to authenticate. md5 = Encryption-type password = encrypted Passwords do not have to be the same throughout an area, but they must be same between neighbors. After a password is configured, you enable authentication for the area on all participating area routers with: Rtr(config-router)# router)# area area authentication [messagedigest] message-digest option must be used if using message-digest-key If optional message-digest is used, a message digest, or hash, of the password is sent.

72 Configuring MD5 Encrypted Authentication s / /16 RouterA RouterB s / /24 RouterA interface Serial1 ip address ip ospf message-digest-key 1 md5 secret! router ospf 10 network area 0 network area 0 area 0 authentication message-digest RouterB interface Serial2 ip address ip ospf message-digest-key 1 md5 secret! router ospf 10 network area 0 network area 0 area 0 authentication message-digest

73 Know your outputs show ip route show ip ospf show ip ospf neighbor show ip ospf border-router show ip database show ip interface

74 show ip route Router# show ip route /16 is variably subnetted, 4 subnets, 3 masks O IA /32 [110/783] via , 00:11:44, FastEthernet0 O /24 [110/782] via , 00:12:29, Serial0 C /30 is directly connected, Serial0 C /24 is directly connected, FastEthernet0 O E /8 [110/20] via , 00:11:44, FastEthernet0 O E /8 [110/782] via , 00:11:44, FastEthernet0 O = OSPF routes within the same area (intra-area routes) 110/number = Administrative Distance/metric (cumulative 10 8 /bandwidth) E2 = Routes outside of the OSPF routing domain, redistributed into OSPF. Default is E2 with a cost of 20 and does not get modified within the OSPF O IA = OSPF routes from another area (inter-area routes) E1 = Routes outside of the OSPF routing domain and get additional cumulative costs added on by each router, just like other OSPF routes.

75 show ip ospf Router#show ip ospf Routing Process "ospf 1" with ID Supports only single TOS(TOS0) routes It is an area border router SPF schedule delay 5 secs, Hold time between two SPFs 10 secs Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs Number of external LSA 3. Checksum Sum 0x97E3 Number of DCbitless external LSA 0 Number of DoNotAge external LSA 0 Number of areas in this router is 2. 2 normal 0 stub 0 nssa External flood list length 0 Area BACKBONE(0) Number of interfaces in this area is 1 Area has no authentication SPF algorithm executed 8 times <text omitted> Area 1 <text omitted>

76 show ip ospf interface Router# show ip ospf interface Ethernet0 is up, line protocol is up Internet Address /24, Area 1 Process ID 1, Router ID , Network Type BROADCAST, Cost: 10 Transmit Delay is 1 sec, State BDR, Priority 1 Designated Router (ID) , Interface address Backup Designated router (ID) , Interface address Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:00 Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor (Designated Router) Suppress hello for 0 neighbor(s) Serial0 is up, line protocol is up Internet Address /24, Area 1 Process ID 1, Router ID , Network Type POINT_TO_POINT, TO Cost: 64 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:04 Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor Suppress hello for 0 neighbor(s)

77 show ip ospf neighbor RouterB#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface FULL/DROTHER 00:00: Ethernet FULL/BDR 00:00: Ethernet WAY/DROTHER 00:00: Ethernet FULL/ - 00:00: Serial0 In this example, we are the DR DROTHER may be in FULL or 2 WAY state, both cases are normal. Usually if there are multiple DROTHERs, they will be in either FULL or 2WAY state but not both.

78 debug ip ospf adj (adjacency) Router# debug ip ospf adj 04:19:46: OSPF: Rcv hello from area 0 from FastEthernet :19:46: OSPF: 2 Way Communication to on FastEthernet0, state 2WAY 04:19:46: OSPF: End of hello processing <text omitted> 04:20:22: OSPF: end of Wait on interface FastEthernet0 04:20:22: OSPF: DR/BDR election on FastEthernet0 04:20:22: 22 OSPF: Elect BDR :20:22: OSPF: Elect DR :20:22: OSPF: Elect BDR :20:22: OSPF: Elect DR :20:22: DR: (Id) BDR: (Id) 04:20:23: OSPF: Rcv DBD from on FastEthernet0 seq 0x2657 opt 0x2 flag 0x7 len 32 mtu 1500 state EXSTART 04:20:23: OSPF: NBR Negotiation Done. We are the SLAVE 04:20:23: OSPF: Send DBD to on FastEthernet0 seq 0x2657 opt 0x2 flag 0 x2 len 92 04:20:23: 23 OSPF: Rcv DBD from on FastEthernet0 th t0 seq 0x2658 opt 0x2 flag 0x3 len 72 mtu 1500 state EXCHANGE <text omitted> 04:20:23: OSPF: Synchronized with on FastEthernet0, state FULL Displays adjacency information including Hello processing, DR/BDR election, authentication, and the Steps to OSPF Operation.

79 debug ip ospf events Router# debug ip ospf events 08:00:56: OSPF: Rcv hello from area 0 from FastEthernet :00:56: OSPF: Mismatched hello parameters from :00:56: Dead R 40 C 20, Hello R 10 C 5 Mask R C Shows much of the same information as debug ip ospf adj in the previous slide including, adjacencies, flooding information, designated router selection, and shortest path first (SPF) calculation. This information is also displayed with debug ip ospf events. R = Received C = Current

80 OSPF Configuration Commands - Review Required Commands: Rtr(config)# router ospf process-id Rtr(config-router)#network address wildcard-mask area area-id Optional Commands: Rtr(config-router)# default-information originate (Send default) Rtr(config-router)# area area authentication (Plain authen.) Rtr(config-router)# area area authentication message-digest g (md5 authen.) Rtr(config)# interface loopback number (Configure lo as RtrID) Rtr(config)# interface type slot/port Rtr(config-if)# ip ospf priority <0-255> (DR/BDR election) Rtr(config-if)# bandwidth kbps (Modify default bandwdth) RTB(config-if)# ip ospf cost cost (Modify inter. cost) Rtr(config-if)# ip ospf hello-interval seconds (Modify Hello) Rtr(config-if)# ip ospf dead-interval seconds (Modify Dead) Rtr(config-if)# ip ospf authentication-key passwd (Plain/md5authen) Rtr(config-if)# ip ospf message-digest-key g key-id md5 password

81 OSPF Show Commands - Review Router# show ip route Router# show ip ospf Router# show ip ospf interface Router# show ip ospf neighbor Router# show ip ospf database Router# debug ip ospf adj Router# debug ip ospf events

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