OSPF is a link state protocol

Transcription

1 OSPF BSCI Module 3 BSCI Module Cisco Systems, Inc. All rights reserved. Cisco Public 1 Terminology 2 1

2 OSPF Overview OSPF does not gather routing table information, but routers and the status of their connections, links. OSPF routers use this information to build a topological data base (link state database), runs the Shortest Path First (SPF), Dijkstra s algorithm, and creates a SPF tree. From that SPF tree, a routing table is created. 3 OSPF is a link state protocol Link: interface on a router Link state: the status of a link between to routers. 4 2

3 Cisco s OSPF s metric is based on cost Cost is an OSPF metric expressed as an 16bit integer, from 1 to Cisco uses a default cost of 10 8 /BW, where BW is the configured bandwidth (bandwidth command) of the interface and 10 8 ( ) as the reference bandwidth. Example: A serial link with bandwidth:128k cost: / = OSPF Areas Review of OSPF area characteristics: Minimizes routing table entries Localizes impact of a topology change within an area Detailed LSA flooding stops at the area boundary Requires a hierarchical network design Transit Area: Regular Area: aka Backbone, Area 0 aka Nonbackbone areas 6 3

4 OSPF Areas Every OSPF router must belong to at least one area. Every OSPF network must have an Area 0 (backbone area). All other Areas should touch Area 0. There are exceptions to this rule Routers in the same area have the same link-state information. Much more on areas at the end of the chapter (OSPF Multiple Areas). 7 OSPF Database OSPF maintains three databases Adjacency Database (show ip ospf neighbor) Link-state Database (show ip ospf database) Forwarding Database (show ip route) 8 4

5 Operation 9 OSPF neighbor relationships OSPF uses 5 different types of packets to communicate. OSPF Type-1 (Hello) OSPF Type-2 (DBD) OSPF Type-3 (LSR) OSPF Type-4 (LSU) OSPF Type-5 (LSAck) 10 5

6 Steps to OSPF Operation 1. Establishing router adjacencies 2. Electing DR and BDR 3. Discovering Routes 4. Choosing Routes 5. Maintaining Routing Information 11 OSPF States OSPF router interfaces can be in one of seven states: Down State Init State Two-way State ExStart State Exchange State Loading State Full Adjacency State 12 6

7 Steps to OSPF Operation with OSPF States 1. Establishing router adjacencies Down State Init State Two-way State 2. Electing DR and BDR ExStart State with DR and BDR Two-way State with all other routers 3. Discovering Routes ExStart State Exchange State Loading State Full State 4. Choosing Routes 5. Maintaining Routing Information Establishing Adjacencies Initially, an OSPF router interface is in the down state. RTB perspective and assuming routers are configured correctly. Trying to start a relationship and wanting to enter the init state RTB begins multicasts OSPF Hello packets ( , AllOSPFRouters), advertising its own Router ID. 14 7

8 1. Establishing Adjacencies Router ID = Highest active IP address (including loopback). Loopback address has the advantage of never going down, thus diminishing the possibility of having to reestablish adjacencies. (more in a moment) Use private ip addresses for loopbacks, so you do not inadvertently advertise a route to a real network that does not exist on your router Establishing Adjacencies RTA and RTC receive Hello packets from RTB and add RTB s Router ID to the Neighbor ID field of the Hello packet its sends back to RTB, at the same time entering the init state. When a router receives its first Hello packet, it enters the init state, meaning the router is ready to take the relationship to the next level. From init state to the two-way state 16 8

9 Steps to OSPF Operation with OSPF States 1. Establishing router adjacencies Down State Init State Two-way State 2. Electing DR and BDR ExStart State with DR and BDR Two-way State with all other routers 3. Discovering Routes ExStart State Exchange State Loading State Full State 4. Choosing Routes 5. Maintaining Routing Information Electing a DR and BDR DR - Designated Router BDR - Backup Designated Router DR s serve as collection points for LSAs A BDR backups the DR. On point-to-point links adjacencies (don t get this confused with being fully adjacent or the full state) are established with all neighbors, because there is only one neighbor. On multi-access networks, OSPF elects a DR and BDR to limit the number of adjacencies. Reduce routing update traffic 18 9

10 2. Electing a DR/BDR Designated Router Router with the highest Router ID is elected the DR. But like other elections, this one can be rigged. The router s priority field can be set to either ensure that it becomes the DR or prevent it from being the DR. The router can be assigned a priority between 0 and 255, with 0 preventing this router from becoming the DR (or BDR) and 255 ensuring at least a tie. (The highest Router ID would break the tie) Electing a DR/BDR All other routers, DRother, establish adjacencies with only the DR and BDR. DRother routers multicast LSAs to only the DR and BDR ( all DR routers) DR sends LSA to all adjacent neighbors ( all OSPF routers) 20 10

11 2. Electing a DR/BDR Backup Designated Router - BDR Listens, but doesn t act. If LSA is sent, BDR sets a timer. If timer expires before it sees the reply from the DR, it becomes the DR and takes over the update process. The process for a new BDR begins Electing a DR/BDR Once a DR is established, a new router that enters the network with a higher priority or router id will NOT become the DR or BDR. (Bug in early IOS 12.0) If DR fails, BDR takes over as DR and selection process for new BDR begins. State of the relationship DRothers enter ExStart state with DR and BDR and two-way state with all other routers 22 11

12 Steps to OSPF Operation with OSPF States 1. Establishing router adjacencies Down State Init State Two-way State 2. Electing DR and BDR ExStart State with DR and BDR Two-way State with all other routers 3. Discovering Routes ExStart State Exchange State Loading State Full State 4. Choosing Routes 5. Maintaining Routing Information Discovering Routes and reaching Full State adjacent OSPF Type-1 (Hello) OSPF Type-1 (Hello) OSPF Type-2 (DBD) OSPF Type-2 (DBD) OSPF Type-5 (LSAck) OSPF Type-3 (LSR) OSPF Type-4 (LSU) OSPF Type-5 (LSAck) 24 12

13 3. Discovering Routes and reaching Full State ExStart State ExStart state - prepare for initial database exchange Purpose of ExStart is to establish a master/slave relationship between the two routers decided by the higher router id. Once the roles are established they enter the exchange state. Exchange State Exchange state - routers exchange one or more Type-2 DBDs (Database Description) packets, which is a summary of the link-state database. Routers compare these DBDs with information in its own database. If the router receives information about a link that is not already in its database, the router requests a complete update from its neighbor. Complete routing information is exchanged in the loading state Discovering Routes and reaching Full State Loading State If the other router has more updated information, this router sends a LSR (Link-State Request) packet requesting more information. Remote router sends the requested information in a LSA Type-4 packet (more on this packet type(s) in next chapter). Router sends LSAck to acknowledge receipt Full State Full state - after all LSRs have been updated. At this point the routers should have identical link-state databases 26 13

14 Steps to OSPF Operation with OSPF States 1. Establishing router adjacencies Down State Init State Two-way State 2. Electing DR and BDR ExStart State with DR and BDR Two-way State with all other routers 3. Discovering Routes ExStart State Exchange State Loading State Full State 4. Choosing Routes 5. Maintaining Routing Information Choosing Routes The router now has a complete link-state database Now the router is ready to create a routing table, but first needs to run the Shortest Path First Algorithm on the link state database, which will create the SPF tree

15 Steps to OSPF Operation with OSPF States 1. Establishing router adjacencies Down State Init State Two-way State 2. Electing DR and BDR ExStart State with DR and BDR Two-way State with all other routers 3. Discovering Routes ExStart State Exchange State Loading State Full State 4. Choosing Routes 5. Maintaining Routing Information 29 Basic OSPF Configuration 30 15

16 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)# ip ospf process-id area area-id [secondaries none] Optional method to enable OSPF explicitly on an interface. 31 Configuring OSPF for Multiple Areas 32 16

17 OSPF router-id Command Router(config-router)# router-id ip-address This command is configured under the router ospf [processid] command. Any unique arbitrary 32-bit value in an IP address format (dotted 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 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 33 Loopback interface Rtr(config)# interface loopback 0 Rtr(config-if)# ip add Very useful in setting Router IDs. Configuring OSPF Router Priority (DR/BDR) Rtr(config)# interface fastethernet 0 Rtr(config-if)# ip ospf priority <0-255> Higher priority becomes DR/BDR Default = 1 0 = Ineligible to become DR/BDR 34 17

18 Why Does the show ip ospf neighbor Command Reveal Neighbors Stuck in 2-Way State? (This is normal in this situation) In the following topology, all routers are running OSPF neighbors over the Ethernet network: Following is sample output of the show ip ospf neighbor command on R7: router-7#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface FULL/BDR 00:00: Ethernet WAY/DROTHER 00:00: Ethernet FULL/DR 00:00: Ethernet WAY/DROTHER 00:00: Ethernet0 router-7# 35 NBMA OSPF Configuration 36 18

19 OSPF over NBMA Topology Modes of Operation RFC 2328-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)# ip ospf network [{broadcast non-broadcast point-tomultipoint [non-broadcast] point-to-point}] This interface command defines OSPF network type. 37 NBMA Non-Broadcast Multi-access Access Networks. Frame Relay X.25 Without broadcasts and multicasts, DR/BDR election is problematic 38 19

20 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 RFC-compliant Point-to-Multipoint Mode One IP subnet. Uses multicast OSPF hello packet to automatically discover neighbors. DR and BDR not required. Router sends additional LSAs with more information about neighboring routers. Typically used in a partialmesh or hub-and-spoke topology. RTB(config-if)#ip ospf network point-to-multipoint RTB(config-router)#network area

21 NBMA Networks and OSPF 41 Cisco s Point-to-Multipoint Non-broadcast mode Cisco extension to RFC-compliant point-to-multipoint mode Must statically define neighbors, like nonbroadcast mode Like point-to-multipoint mode, DR/BDR not elected Used in special cases where neighbors cannot be automatically discovered RTB(config-if)#ip ospf network point-to-multipoint nonbroadcast RTB(config-router)#network area 0 RTB(config-router)#neighbor cost 10 RTB(config-router)#neighbor cost

22 Cisco 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 43 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 point-to-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

23 OSPF over NBMA Topology Summary 45 OSPF Multi-Area 46 23

24 OSPF Multi-Area Areas LSAs Type of areas: Stub Areas Totally Stubby Areas E1 and E2 routes NSSA (Not So Stubby Areas) Virtual Links Route Summarization 47 Issues with large OSPF nets Frequent SPF calculations Large routing table Large link-state table 48 24

25 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, are restricted within an area. 49 OSPF Router Types 50 25

26 OSPF Router Types Internal: Routers with all their interfaces within the same area. Backbone: Routers with at least one interface connected to area 0. ABR: (Area Border Router): Routers with interfaces attached to multiple areas. ASBR: (Autonomous System Boundary Router): Routers that have at least one interface connected to an external internetwork (another autonomous system). 51 LSA types 52 26

27 LSA Types 53 LSA Types 54 27

28 LSA Type 1: Router LSA One router LSA (type 1) for every router in an area: Includes list of directly attached links Identified by the router ID of the originating router Floods within its area only; does not cross ABR Link-state ID depends on link type 55 LSA Type 2: Network LSA Advertised by the DR of the broadcast network Floods within its area only; does not cross ABR Link-state ID is the DR 56 28

29 LSA Type 3: Summary LSA Advertised by the ABR of originating area. Regenerated by subsequent ABRs to flood throughout the autonomous system. By default, routes are not summarized, and type 3 LSA is advertised for every subnet. Link-state ID is the network or subnet advertised in the summary LSA 57 LSA Type 4: Summary LSA Summary (type 4) LSAs are used to advertise an ASBR to all other areas in the autonomous system. They are generated by the ABR of the originating area. They are regenerated by all subsequent ABRs to flood throughout the autonomous system. Link-state ID is the router ID of the ASBR

30 LSA Type 5: External LSA External (type 5) LSAs are used to advertise networks from other autonomous systems. Type 5 LSAs are advertised and owned by the originating ASBR. The Link-state ID is the external network number. 59 E1 vs. E2 External Routes External routes fall under two categories, external type 1 and 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 (E2) route is always the external cost, irrespective of the interior cost to reach that route. A type 1 (E1) cost is the addition of the external cost and the internal cost used to reach that route. Type 2 (E2) is the default! 60 30

31 E1 vs. E2 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-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-type 2 61 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 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> 62 31

32 Area Types Standard Backbone Stub Stub Totally Stubby Area (TSA) Not-so-stubby-area (NSSA) 63 Area Types 64 32

33 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 or 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 stub 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. 65 Stub Areas Receives all routes from within A.S.: Within the local area - LSA 1s and LSA 2s (if appropriate) From other areas (Inter-Area) - LSA 3s and LSA 4s Does not receive routes from External A.S. (External Routes). ABR: LSA 3s and LSA 4s are propagated by the ABR. ABR blocks all LSA 5s. If LSA 5s are not know inside an area, are LSA 4s are necessary?? Default route is injected into stub area by ABR External Routes: Once the ABR gets a packet headed to a default route, it must have a default route, either static or propagated by the ASBR via default information originate (coming!) Configuration: All routers in the area must be configured as stub 66 33

34 Totally Stubby Areas Cisco proprietary Same considerations as with Stub areas: An area could be qualified a stub when there is a single exit point (a single ABR) from that area or 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 stub 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. - This is even more true with Totally Stubby areas 67 Totally Stubby Areas Receives routes from within A.S.: Only from within the local area - LSA 1s and LSA 2s (if appropriate) Does not receive routes from other areas (Inter-Area) - LSA 3s and LSA 4s Does not receive routes from External A.S. (External Routes) ABR: ABR blocks all LSA 5s. ABR blocks all LSA 3s and LSA 4s, except propagating a default route. Default route is injected into totally stubby area by ABR. Configuring: All routers must be configured as stub ABR must be configured as stub no-summary 68 34

35 Multi-area Example RIP 69 Multi-area Example All routes to all areas including LSA 3s (IA) other areas routes from ABRs, LSA 4s (IA to ASBR) reachability to ASBR from ABRs, and LSA 5s (E1/E2) external routes from the ASBR. ABR ASBR RIP 70 35

36 Stub Example ABR LSA 3s (IA routes) via ABR No Type LSA 5s Route to /0 via ABR ASBR 71 Totally Stubby Example no summary ABR Totally Stubby Area No Type 3, 4, or 5 LSAs ASBR Route to /0 via ABR - No more IA routes - Only routes within the area and the default 72 36

37 Propagating Default Routes in NSSAs 73 NSSA Example NSSA Area 2 Backbone Area Area 0 RTH RIP RTE RTG ASBR RTF RTD RTC RTB ABR RTA (Possible ASBR) 74 37

38 Default route via RTG NSSA Area 2 Backbone Area Area 0 RTH RIP RTE LSA 7 RTG ASBR LSA 7 LSA 7 RTF LSA 7 RTD RTC LSA 7 LSA 7 RTB ABR LSA 7s Blocked LSA 5 RTA (Possible ASBR) NSSA allow external routes to be advertised into the OSPF AS while retaining the characteristics of a stub area to the rest of the AS. ASBR RTG will originate Type-7 LSAs to advertise the external destinations. These LSA 7s are flooded through the NSSA but are blocked by the NSSA ABR. The NSSA ABR translates LSA 7s into 5s and flood other areas. 75 LSA Types (con t) Type 7 LSA 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 default N1 or an N2 in the routing table. (Relative to E1 and E2)

39 Default route via RTG NSSA Area 2 Backbone Area Area 0 RTH RIP RTE LSA 7 LSA 3s & /0 RTG ASBR LSA 7 LSA 7 RTF LSA 7 RTD RTC LSA 7 LSA 7 RTB ABR LSA 7s 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 77 NSSA example 78 39

40 Virtual Link Concepts and Configuration 79 Virtual Links 80 40

41 Virtual Links All areas in an OSPF autonomous system must be physically connected to the backbone area (area 0). In some cases where this is not possible, you can use a virtual link to connect to the backbone through a non-backbone area. As mentioned above, you can also use virtual links to connect two parts of a partitioned backbone through a non-backbone area. The area through which you configure the virtual link, known as a transit area, must have full routing information. Must be configured between two ABRs. The transit area cannot be a stub area. 81 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 82 41

42 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 Special Treatment for LSAs on Virtual Links LSAs usually age out after 30 minutes LSAs learned across virtual links have the DoNotAge (DNA) option set Required to prevent excessive flooding over virtual links 84 42

43 Configuring and Verifying a Virtual Link RouterA#sh ip ospf virtual-links Virtual Link OSPF_VL0 to router is up Run as demand circuit DoNotAge LSA allowed. Transit area 1, via interface Serial0/0/1, Cost of using 781 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:07 Adjacency State FULL (Hello suppressed) Index 1/2, retransmission queue length 0, number of retransmission 1 First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0) Last retransmission scan length is 1, maximum is 1 Last retransmission scan time is 0 msec, maximum is 0 msec 85 Route summarization 86 43

44 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. If the network numbers in an area are assigned in a way such that they are contiguous, you can configure the ABR to advertise a summary route that covers all the individual networks within the area that fall into the specified range. 87 RTB is summarizing the range of subnets from to into one range: This is achieved by masking the first three left most bits of 64 using a mask of In the same way, RTC is generating the summary address into the backbone. Note that this summarization was successful because we have two distinct ranges of subnets, and

45 RTB router ospf 100 area 1 range RTC router ospf 100 area 2 range External Route Summarization - summary-address address 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. On the ASBR only (Summarizes external routes before injecting them into the OSPF domain). Router(config-router)# summary-address network-address subnet-mask 90 45

46 RTA router ospf 100 summary-address redistribute bgp 50 metric 1000 subnets (later) RTD router ospf 100 summary-address redistribute bgp 20 metric 1000 subnets (later) 91 OSPF Authentication 92 46

47 OSPF Authentication Types OSPF supports 2 types of authentication: Simple password authentication (plain text) MD5 authentication Router generates and checks each packet and authenticates the source of each update packet it receives Configure a key (password) Note: all participating neighbors must have the same key configured 93 Configuring Simple Password Authentication Router(config-if)# ip ospf authentication-key password Assign a password to be used with neighboring routers. Router(config-if)# ip ospf authentication [message-digest null] Specifies the authentication type for an interface (since IOS 12.0). Router(config-router)# area area-id authentication [message-digest] Specifies the authentication type for an area (was in IOS before 12.0)

48 Example Simple Password Authentication Configuration 95 R2 Configuration for Simple Password Authentication <output omitted> interface Loopback0 ip address <output omitted> interface Serial0/0/1 ip address ip ospf authentication ip ospf authentication-key plainpas <output omitted> router ospf 10 log-adjacency-changes network area 0 network area

49 Configuring OSPF MD5 Authentication Router(config-if)# ip ospf message-digest-key key-id md5 key Assign a key ID and key to be used with neighboring routers. Router(config-if)# ip ospf authentication [message-digest null] Specifies the authentication type for an interface (since IOS 12.0). Router(config-router)# area area-id authentication [message-digest] Specifies the authentication type for an area (was in IOS before 12.0). 97 Example MD5 Authentication Configuration 98 49

50 Q and A