OSPF in Detail. ISP Workshops

Transcription

1 OSPF i Detail ISP Workshops These materials are licesed uder the Creative Commos Attributio-NoCommercial 4.0 Iteratioal licese ( Last updated 29 th September

2 Ackowledgemets p This material origiated from the Cisco ISP/IXP Workshop Programme developed by Philip Smith & Barry Greee p Use of these materials is ecouraged as log as the source is fully ackowledged ad this otice remais i place p Bug fixes ad improvemets are welcomed Please workshop (at) bgp4all.com Philip Smith 2

3 Ageda p Detailed Backgroud about OSPF p OSPF Desig i SP Networks p Addig Networks i OSPF p OSPF i Cisco s IOS 3

4 OSPF: The detail Techical Backgroud 4

5 OSPF p Ope Shortest Path First p Lik state (SPF) techology p Developed by OSPF workig group of IETF (RFC 1247) p OSPFv2 stadard described i RFC2328 p Desiged for: TCP/IP eviromet Fast covergece Variable-legth subet masks Discotiguous subets Icremetal updates Route autheticatio p Rus o IP, Protocol 89 5

6 Lik State Q s Lik State Z Z s Lik State Q Y A B C Q Z X X s Lik State X Topology Iformatio is kept i a Database separate from the Routig Table 6

7 Lik State Routig p Neighbour discovery p Costructig a Lik State Packet (LSP) p Distribute the LSP (Lik State Aoucemet LSA) p Compute routes p O etwork failure New LSPs flooded All routers recompute routig table 7

8 Low Badwidth Utilisatio LSA X R1 LSA p Oly chages propagated p Uses multicast o multi-access broadcast etworks 8

9 Fast Covergece p Detectio Plus LSA/SPF Kow as the Dijkstra Algorithm Alterate Path R2 N1 R1 X R3 N2 Primary Path 9

10 Fast Covergece p Fidig a ew route LSA flooded throughout area Ackowledgemet based LSA Topology database sychroised Each router derives routig table to destiatio etwork N1 R1 X 10

11 OSPF Areas p Area is a group of cotiguous hosts ad etworks Reduces routig traffic p Per area topology database Ivisible outside the area p Backboe area MUST be cotiguous All other areas must be coected to the backboe R1 Area 2 Area 3 Rc Rb Area 0 Backboe Area R8 Rd Area 4 R6 R7 R5 Area 1 Ra R3 R4 R2 11

12 Virtual Liks betwee OSPF Areas p Virtual Lik is used whe it is ot possible to physically coect the area to the backboe p ISPs avoid desigs which require virtual liks Icreases complexity Decreases reliability ad scalability R8 Rc Rd Area 4 Area 0 Backboe Area R7 R5 Area 1 Rb Ra R4 R6 R3 12

13 Classificatio of Routers R1 IR R2 IR ASBR To other AS Area 2 Area 3 R5 Rc Rd ABR/BR R4 Area 0 Rb Ra IR/BR p Iteral Router (IR) p Area Border Router (ABR) p Backboe Router (BR) p Autoomous System Border Router (ASBR) Area 1 R3 13

14 OSPF Route Types R1 IR R2 IR ASBR To other AS Area 2 Area 3 R5 Area 1 Rc Rd R4 R3 ABR/BR Area 0 Rb p Ra p p Itra-area Route All routes iside a area Iter-area Route Routes advertised from oe area to aother by a Area Border Router Exteral Route Routes imported ito OSPF from other protocol or static routes 14

15 Exteral Routes p Prefixes which are redistributed ito OSPF from other protocols p Flooded ualtered throughout the AS Recommedatio: Avoid redistributio!! p OSPF supports two types of exteral metrics Type 1 exteral metrics Type 2 exteral metrics (Cisco IOS default) OSPF R2 Redistribute RIP EIGRP BGP Static Coected etc. 15

16 Exteral Routes p Type 1 exteral metric: metrics are added to the summarised iteral lik cost Cost = 10 R2 to N1 Exteral Cost = 1 R1 Network N1 N1 Type Cost = 8 Next Hop R2 R3 R3 Selected Route to N1 Exteral Cost = 2 16

17 Exteral Routes p Type 2 exteral metric: metrics are compared without addig to the iteral lik cost Cost = 10 R2 to N1 Exteral Cost = 1 R1 Network N1 N1 Type Cost = 8 Next Hop R2 R3 R3 Selected Route to N1 Exteral Cost = 2 17

18 Topology/Lik State Database p A router has a separate LS database for each area to which it belogs p All routers belogig to the same area have idetical database p SPF calculatio is performed separately for each area p LSA floodig is bouded by area p Recommedatio: Limit the umber of areas a router participates i!! 1 to 3 is fie (typical ISP desig) >3 ca overload the CPU depedig o the area topology complexity 18

19 The Hello Protocol p Resposible for establishig ad maitaiig eighbour relatioships p Elects desigated router o multi-access etworks Hello Hello Hello 19

20 The Hello Packet p Cotais: Router priority Hello iterval Router dead iterval Network mask List of eighbours DR ad BDR Optios: E-bit, MC-bit, (see A.2 of RFC2328) Hello Hello Hello 20

21 Desigated Router p There is ONE desigated router per multi-access etwork Geerates etwork lik advertisemets Assists i database sychroizatio Desigated Router Backup Desigated Router Desigated Router Backup Desigated Router 21

22 Desigated Router by Priority p Cofigured priority (per iterface) ISPs cofigure high priority o the routers they wat as DR/BDR p Else determied by highest router ID Router ID is 32 bit iteger Derived from the loopback iterface address, if cofigured, otherwise the highest IP address R1 DR R2 R1 Router ID = R2 Router ID =

23 Neighbourig States p Full Routers are fully adjacet Databases sychroised Relatioship to DR ad BDR DR Full BDR 23

24 Neighbourig States p 2-way Router sees itself i other Hello packets DR selected from eighbours i state 2-way or greater 2-way DR BDR 24

25 Whe to Become Adjacet p Uderlyig etwork is poit to poit p Uderlyig etwork type is virtual lik p The router itself is the desigated router or the backup desigated router p The eighbourig router is the desigated router or the backup desigated router 25

26 LSAs Propagate Alog Adjacecies DR BDR p LSAs ackowledged alog adjacecies 26

27 Broadcast Networks p IP Multicast used for Sedig ad Receivig Updates All routers must accept packets set to AllSPFRouters ( ) All DR ad BDR routers must accept packets set to AllDRouters ( ) p Hello packets set to AllSPFRouters (Uicast o poit-topoit ad virtual liks) 27

28 Routig Protocol Packets p Share a commo protocol header p Routig protocol packets are set with type of service (TOS) of 0 p Five types of OSPF routig protocol packets Hello packet type 1 Database descriptio packet type 2 Lik-state request packet type 3 Lik-state update packet type 4 Lik-state ackowledgemet packet type 5 28

29 Differet Types of LSAs p Six distict type of LSAs Type 1 : Type 2 : Type 3 & 4: Type 5 & 7: Type 6: Type 9, 10 & 11: Router LSA Network LSA Summary LSA Exteral LSA (Type 7 is for NSSA) Group membership LSA Opaque LSA (9: Lik-Local, 10: Area) 29

30 Router LSA (Type 1) p Describes the state ad cost of the router s liks to the area p All of the router s liks i a area must be described i a sigle LSA p Flooded throughout the particular area ad o more p Router idicates whether it is a ASBR, ABR, or ed poit of virtual lik 30

31 Network LSA (Type 2) p Geerated for every trasit broadcast ad NBMA etwork p Describes all the routers attached to the etwork p Oly the desigated router origiates this LSA p Flooded throughout the area ad o more 31

32 Summary LSA (Type 3 ad 4) p Describes the destiatio outside the area but still i the AS p Flooded throughout a sigle area p Origiated by a ABR p Oly iter-area routes are advertised ito the backboe p Type 4 is the iformatio about the ASBR 32

33 Exteral LSA (Type 5 ad 7) p Defies routes to destiatio exteral to the AS p Default route is also set as exteral p Two types of exteral LSA: E1: Cosider the total cost up to the exteral destiatio E2: Cosiders oly the cost of the outgoig iterface to the exteral destiatio p (Type 7 LSAs used to describe exteral LSA for oe specific OSPF area type) 33

34 Iter-Area Route Summarisatio p Prefix or all subets p Prefix or all etworks p Area rage commad R2 Backboe Area 0 With summarisatio Without summarisatio Network 1 Network 1.A 1.B 1.C Next Hop R1 Next Hop R1 R1 R1 (ABR) R1 Area 1 1.A 1.B 1.C 34

35 No Summarisatio p p Specific Lik LSA advertised out of each area Lik state chages propagated out of each area 1.A 1.B 1.C 1.D Area 0 2.A 2.B 2.C 2.D 3.A 3.B 3.C 3.D 1.A 1.B 3.A 3.B 1.C 1.D 2.A 2.B 3.C 3.D 2.C 2.D 35

36 With Summarisatio p p Oly summary LSA advertised out of each area Lik state chages do ot propagate out of the area 1 Area A 1.B 3.A 3.B 1.C 1.D 2.A 2.B 3.C 3.D 2.C 2.D 36

37 No Summarisatio p p Specific Lik LSA advertised i to each area Lik state chages propagated i to each area 2.A 2.B 2.C 2.D 3.A 3.B 3.C 3.D Area 0 1.A 1.B 1.C 1.D 3.A 3.B 3.C 3.D 1.A 1.B 1.C 1.D 2.A 2.B 2.C 2.D 1.A 1.B 3.A 3.B 1.C 1.D 2.A 2.B 3.C 3.D 2.C 2.D 37

38 With Summarisatio p p Oly summary lik LSA advertised i to each area Lik state chages do ot propagate i to each area 2 3 Area A 1.B 3.A 3.B 1.C 1.D 2.A 2.B 3.C 3.D 2.C 2.D 38

39 Types of Areas p Regular p Stub p Totally Stubby p Not-So-Stubby p Oly regular areas are useful for ISPs Other area types hadle redistributio of other routig protocols ito OSPF ISPs do t redistribute aythig ito OSPF p The ext slides describig the differet area types are provided for iformatio oly 39

40 Regular Area (Not a Stub) p From Area 1 s poit of view, summary etworks from other areas are ijected, as are exteral etworks such as X.1 X Area 0 X ASBR X.1 Exteral etworks 1 2 X.1 X.1 1.A 1.B X.1 3.A 3.B 1.C 1.D X.1 2.A 2.B 3.C 3.D 2.C 2.D 40

41 Normal Stub Area p p Summary etworks, default route ijected Commad is area x stub Default 2 3 Area 0 Default 1 3 ASBR X.1 Exteral etworks 1 2 Default X.1 1.A 1.B X.1 3.A 3.B 1.C 1.D X.1 2.A 2.B 3.C 3.D 2.C 2.D 41

42 Totally Stubby Area p p Oly a default route ijected Default path to closest area border router Commad is area x stub o-summary Totally Stubby Area Default Area 0 Default 1 3 ASBR X.1 Exteral etworks 1 2 Default X.1 1.A 1.B X.1 3.A 3.B 1.C 1.D X.1 2.A 2.B 3.C 3.D 2.C 2.D 42

43 Not-So-Stubby Area p p p Capable of importig routes i a limited fashio Type-7 LSA s carry exteral iformatio withi a NSSA NSSA Border routers traslate selected type-7 LSAs ito type-5 exteral etwork LSAs ASBR Not-So- Stubby Area Default Area 0 Default X X.1 Exteral etworks 1 2 Default X.2 X.2 X.1 1.A 1.B X.2 X.1 3.A 3.B X.2 Exteral etworks 1.C 1.D X.1 2.A 2.C 2.D 2.B 3.C 3.D 43

44 ISP Use of Areas p ISP etworks use: Backboe area Regular area p Backboe area No partitioig p Regular area Summarisatio of poit to poit lik addresses used withi areas Loopback addresses allowed out of regular areas without summarisatio (otherwise ibgp wo t work) 44

45 Addressig for Areas Area 0 etwork rage Area 1 etwork rage Area 2 etwork rage Area 3 etwork rage p Assig cotiguous rages of subets per area to facilitate summarisatio 45

46 Summary p Fudametals of Scalable OSPF Network Desig Area hierarchy DR/BDR selectio Cotiguous itra-area addressig Route summarisatio Ifrastructure prefixes oly 46

47 OSPF Desig As applicable to Service Provider Networks 47

48 Service Providers p p p p p SP etworks are divided ito PoPs PoPs are liked by the backboe Trasit routig iformatio is carried via ibgp IGP is oly used to carry the ext hop for BGP Optimal path to the ext hop is critical 48

49 SP Architecture p p Major routig iformatio is ~670K prefixes via BGP Largest kow IGP routig table is ~9 10K POP Area 1/L1 BGP 1 POP Area 2/L1 BGP 1 p Total of 680K p p 10K/680K is 1½% of IGP routes i a ISP etwork A very small factor but has a huge impact o etwork covergece! Area 6/L1 BGP 1 POP IP Backboe Area 5/L1 BGP 1 POP Area0/L2 BGP 1 Area 4/L1 BGP 1 POP Area 3/L1 BGP 1 POP 49

50 SP Architecture p You ca reduce the IGP size from 10K to approx the umber of routers i your etwork p This will brig really fast covergece p Optimise where you must ad summarise where you ca p Stops uecessary flappig RR Access Regioal Core IGP customer customer customer 50

51 OSPF Desig: Addressig p OSPF Desig ad Addressig go together Objective is to keep the Lik State Database lea Create a address hierarchy to match the topology Use separate Address Blocks for loopbacks, etwork ifrastructure, customer iterfaces & customers Customer Address Space PtP Liks Ifrastructure Loopbacks 51

52 OSPF Desig: Addressig p Miimisig the umber of prefixes i OSPF: Number loopbacks out of a cotiguous address block p But do ot summarise these across area boudaries: ibgp peer addresses eed to be i the IGP Use cotiguous address blocks per area for ifrastructure poit-to-poit liks p Use area rage commad o ABR to summarise p With these guidelies: Number of prefixes i area 0 will the be very close to the umber of routers i the etwork It is critically importat that the umber of prefixes ad LSAs i area 0 is kept to the absolute miimum 52

53 OSPF Desig: Areas p Examie physical topology Is it meshed or hub-ad-spoke? p Use areas ad summarisatio This reduces overhead ad LSA couts (but watch ext-hop for ibgp whe summarisig) p Do t bother with the various stub areas No beefits for ISPs, causes problems for ibgp p Push the creatio of a backboe Reduces mesh ad promotes hierarchy 53

54 OSPF Desig: Areas p Oe SPF per area, floodig doe per area Watch out for overloadig ABRs p Avoid exterals i OSPF DO NOT REDISTRIBUTE ito OSPF Exteral LSAs flood through etire etwork p Differet types of areas do differet floodig Normal areas Stub areas Totally stubby (stub o-summary) Not so stubby areas (NSSA) 54

55 OSPF Desig: Areas p Area 0 must be cotiguous Do NOT use virtual liks to joi two Area 0 islads p Traffic betwee two o-zero areas always goes via Area 0 There is o beefit i joiig two o-zero areas together Avoid desigs which have two o-zero areas touchig each other (Typical desig is a area per PoP, with core routers beig ABR to the backboe area 0) 55

56 OSPF Desig: Summary p Thik Redudacy Dual Liks out of each area usig metrics (cost) for traffic egieerig p Too much redudacy Dual liks to backboe i stub areas must be the same cost other wise sub-optimal routig will result Too Much Redudacy i the backboe area without good summarisatio will effect covergece i the Area 0 56

57 OSPF Areas: Migratio p Where to place OSPF Areas? Follow the physical topology! Remember the earlier desig advice p Cofigure area at a time! Start at the outermost edge of the etwork Log ito routers at either ed of a lik ad chage the lik from Area 0 to the chose Area Wait for OSPF to re-establish adjacecies Ad the move oto the ext lik, etc Importat to esure that there is ever a Area 0 islad aywhere i the migratig etwork 57

58 OSPF Areas: Migratio A Area 0 B C E D Area 10 F G p Migrate small parts of the etwork, oe area at a time Remember to itroduce summarisatio where feasible p With careful plaig, the migratio ca be doe with miimal etwork dowtime 58

59 OSPF for Service Providers Cofigurig OSPF & Addig Networks 59

60 OSPF: Cofiguratio p Startig OSPF i Cisco s IOS router ospf 100 Where 100 is the process ID p OSPF process ID is uique to the router Gives possibility of ruig multiple istaces of OSPF o oe router Process ID is ot passed betwee routers i a AS May ISPs cofigure the process ID to be the same as their BGP Autoomous System Number 60

61 OSPF: Establishig Adjacecies p Cisco IOS OSPFv2 automatically tries to establish adjacecies o all defied iterfaces (or subets) p Best practice is to disable this Potetial security risk: sedig OSPF Hellos outside of the autoomous system, ad riskig formig adjacecies with exteral etworks Example: Oly POS4/0 iterface will attempt to form a OSPF adjacecy router ospf 100 passive-iterface default o passive-iterface POS4/0 61

62 OSPF: Addig Networks Optio Oe p Redistributio: Applies to all coected iterfaces o the router but seds etworks as exteral type-2s which are ot summarised p Do NOT do this! Because: router ospf 100 redistribute coected subets Type-2 LSAs flood through etire etwork These LSAs are ot all useful for determiig paths through backboe; they simply take up valuable space 62

63 OSPF: Addig Networks Optio Two p Per lik cofiguratio from IOS 12.4 owards OSPF is cofigured o each iterface (same as IS-IS) Useful for multiple subets per iterface iterface POS 4/0 ip address ip address secodary ip ospf 100 area 0! router ospf 100 passive-iterface default o passive-iterface POS 4/0 63

64 OSPF: Addig Networks Optio Three p Specific etwork statemets Every active iterface with a cofigured IP address eeds a OSPF etwork statemet Iterfaces that will have o OSPF eighbours eed passive-iterface to disable OSPF Hello s p That is: all iterfaces coectig to devices outside the ISP backboe (i.e. customers, peers, etc) router ospf 100 etwork area 51 etwork area 51 passive-iterface Serial 1/0 64

65 OSPF: Addig Networks Optio Four p Network statemets wildcard mask Every active iterface with cofigured IP address covered by wildcard mask used i OSPF etwork statemet Iterfaces covered by wildcard mask but havig o OSPF eighbours eed passive-iterface (or use passive-iterface default ad the activate the iterfaces which will have OSPF eighbours) router ospf 100 etwork area 51 passive-iterface default o passive iterface POS 4/0 65

66 OSPF: Addig Networks Recommedatios p Do t ever use Optio 1 p Use Optio 2 if supported; otherwise: p Optio 3 is fie for core/ifrastructure routers Does t scale too well whe router has a large umber of iterfaces but oly a few with OSPF eighbours solutio is to use Optio 3 with o passive o iterfaces with OSPF eighbours p Optio 4 is preferred for aggregatio routers Or use ibgp ext-hop-self Or eve ip uumbered o exteral poit-to-poit liks 66

67 OSPF: Addig Networks Example Oe (Cisco IOS 12.4) p Aggregatio router with large umber of leased lie customers ad just two liks to the core etwork: iterface loopback 0 ip address ip ospf 100 area 0 iterface POS 0/0 ip address ip ospf 100 area 0 iterface POS 1/0 ip address ip ospf 100 area 0 iterface serial 2/0:0... ip uumbered loopback 0! Customers coect here ^^^^^^^ router ospf 100 passive-iterface default o passive iterface POS 0/0 o passive iterface POS 1/0 67

68 OSPF: Addig Networks Example Oe (Cisco IOS < 12.4) p Aggregatio router with large umber of leased lie customers ad just two liks to the core etwork: iterface loopback 0 ip address iterface POS 0/0 ip address iterface POS 1/0 ip address iterface serial 2/0:0... ip uumbered loopback 0! Customers coect here ^^^^^^^ router ospf 100 etwork area 51 etwork area 51 etwork area 51 passive-iterface default o passive iterface POS 0/0 o passive iterface POS 1/0 68

69 OSPF: Addig Networks Example Two (Cisco IOS 12.4) p Core router with oly liks to other core routers: iterface loopback 0 ip address ip ospf 100 area 0 iterface POS 0/0 ip address ip ospf 100 area 0 iterface POS 1/0 ip address ip ospf 100 area 0 iterface POS 2/0 ip address ip ospf 100 area 0 iterface POS 2/1 ip address ip ospf 100 area 0 router ospf 100 passive iterface loopback 0 69

70 OSPF: Addig Networks Example Two (Cisco IOS < 12.4) p Core router with oly liks to other core routers: iterface loopback 0 ip address iterface POS 0/0 ip address iterface POS 1/0 ip address iterface POS 2/0 ip address iterface POS 2/1 ip address router ospf 100 etwork area 0 etwork area 0 etwork area 0 etwork area 0 etwork area 0 passive iterface loopback 0 70

71 OSPF: Addig Networks Summary p Key Theme whe selectig a techique: Keep the Lik State Database Lea Icreases Stability Reduces the amout of iformatio i the Lik State Advertisemets (LSAs) Speeds Covergece Time 71

72 OSPF i Cisco IOS Useful features for ISPs 72

73 Areas p A area is stored as a 32-bit field: Defied i IPv4 address format (i.e. Area ) Ca also be defied usig sigle decimal value (i.e. Area 0) p reserved for the backboe area Area 3 Area 2 Area 0 Area 1 73

74 Loggig Adjacecy Chages p The router will geerate a log message wheever a OSPF eighbour chages state p Sytax: [o] [ospf] log-adjacecy-chages (OSPF keyword is optioal, depedig o IOS versio) p Example of a typical log message: %OSPF-5-ADJCHG: Process 1, Nbr o Etheret0 from LOADING to FULL, Loadig Doe 74

75 Number of State Chages p The umber of state trasitios is available via SNMP (ospfnbrevets) ad the CLI: show ip ospf eighbor [type umber] [eighbor-id] [detail] Detail (Optioal) Displays all eighbours give i detail (list all eighbours). Whe specified, eighbour state trasitio couters are displayed per iterface or eighbour ID 75

76 State Chages (Cotiued) p To reset OSPF-related statistics, use the followig commad: clear ip ospf couters This will reset eighbour state trasitio couters per iterface or eighbour id: clear ip ospf couters [eighbor [<type umber>] [eighbor-id]] 76

77 Router ID p If the loopback iterface exists ad has a IP address, that is used as the router ID i routig protocols stability! p If the loopback iterface does ot exist, or has o IP address, the router ID is the highest IP address cofigured dager! p OSPF sub commad to maually set the Router ID: router-id <ip address> 77

78 Cost & Referece Badwidth p Badwidth used i Metric calculatio Cost = 10 8 /badwidth Not useful for iterface badwidths > 100 Mbps p Sytax: ospf auto-cost referece-badwidth <referece-bw> p Default referece badwidth still 100 Mbps for backward compatibility p Most ISPs simply choose to develop their ow cost strategy ad apply to each iterface type 78

79 Cost: Example Strategy 100GE 100Gbps cost = 1 40GE/OC768 40Gbps cost = 2 10GE/OC192 10Gbps cost = 5 OC48 2.5Gbps cost = 10 GigabitEtheret 1Gbps cost = 20 OC12 622Mbps cost = 50 OC3 155Mbps cost = 100 FastEtheret 100Mbps cost = 200 Etheret 10Mbps cost = 500 E1 2Mbps cost =

80 Default routes p Origiatig a default route ito OSPF default-iformatio origiate metric <> Will origiate a default route ito OSPF if there is a matchig default route i the Routig Table (RIB) The optioal always keyword will always origiate a default route, eve if there is o existig etry i the RIB 80

81 Clear/Restart p OSPF p p p commads clear If o process ID is give, all OSPF processes o the router are assumed clear ip ospf [pid] redistributio This commad clears redistributio based o OSPF routig process ID clear ip ospf [pid] couters This commad clears couters based o OSPF routig process ID clear ip ospf [pid] process This commad will restart the specified OSPF process. It attempts to keep the old router-id, except i cases where a ew router-id was cofigured or a old user cofigured router-id was removed. Sice this commad ca potetially cause a etwork chur, a user cofirmatio is required before performig ay actio 81

82 Use OSPF Autheticatio p Use autheticatio Too may operators overlook this basic requiremet p Whe usig autheticatio, use the MD5 feature Uder the global OSPF cofiguratio, specify: area <area-id> autheticatio message-digest Uder the iterface cofiguratio, specify: ip ospf message-digest-key 1 md5 <key> p Autheticatio ca be selectively disabled per iterface with: ip ospf autheticatio ull 82

83 Poit to Poit Etheret Liks p For ay broadcast media (like Etheret), OSPF will attempt to elect a desigated ad backup desigated router whe it forms a adjacecy If the iterface is ruig as a poit-to-poit WAN lik, with oly 2 routers o the wire, cofigurig OSPF to operate i "poit-to-poit mode" scales the protocol by reducig the lik failure detectio times Poit-to-poit mode improves covergece times o Etheret etworks because it: p Prevets the electio of a DR/BDR o the lik, p Simplifies the SPF computatios ad reduces the router's memory footprit due to a smaller topology database. iterface fastetheret0/2 ip ospf etwork poit-to-poit 83

84 Tuig OSPF (1) p DR/BDR Selectio ip ospf priority 100 (default 1) This feature should be i use i your OSPF etwork Forcibly set your DR ad BDR per segmet so that they are kow Choose your most powerful, or most idle routers, so that OSPF coverges as fast as possible uder maximum etwork load coditios Try to keep the DR/BDR limited to oe segmet each 84

85 Tuig OSPF (2) p OSPF startup max-metric router-lsa o-startup wait-for-bgp Avoids blackholig traffic o router restart Causes OSPF to aouce its prefixes with highest possible metric util ibgp is up ad ruig Whe ibgp is ruig, OSPF metrics retur to ormal, make the path valid p IS-IS equivalet: set-overload-bit o-startup wait-for-bgp 85

86 Tuig OSPF (3) p Hello/Dead Timers ip ospf hello-iterval 3 (default 10) ip ospf dead-iterval 15 (default is 4x hello) This allows for faster etwork awareess of a failure, ad ca result i faster recovergece, but requires more router CPU ad geerates more overhead p LSA Pacig timers lsa-group-pacig 300 (default 240) Allows groupig ad pacig of LSA updates at cofigured iterval Reduces overall etwork ad router impact 86

87 Tuig OSPF (4) p OSPF Iteral Timers timers spf 2 8 (default is 5 ad 10 respectively) Allows you to adjust SPF characteristics The first umber sets wait time from topology chage to SPF ru The secod is hold-dow betwee SPF rus BE CAREFUL WITH THIS COMMAND; if you re ot sure whe to use it, it meas you do t eed it; default is sufficiet 95% of the time 87

88 Tuig OSPF (5) p LSA filterig/iterface blockig Per iterface: ip ospf database-filter all out p Per eighbor: (o optios) eighbor p database-filter all out (o optios) OSPFs router will flood a LSA out all iterfaces except the receivig oe; LSA filterig ca be useful i cases where such floodig uecessary (i.e., NBMA etworks), where the DR/BDR ca hadle floodig chores area <area-id> filter-list <acl> Filters out specific Type 3 LSAs at ABRs p Improper use ca result i routig loops ad black-holes that ca be very difficult to troubleshoot 88

89 Summary p OSPF has a bewilderig umber of features ad optios p Observe ISP best practices p Keep desig ad cofiguratio simple p Ivestigate tuig optios ad suitability for your ow etwork Do t just tur them o! 89

90 OSPF i Detail ISP Workshops 90