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  1. 1. OSPF PART 1 packetlife.net Protocol Header Attributes 8 16 24 32 Type Link-State Version Type Length Algorithm Dijkstra Router ID Metric Cost (Bandwidth) Area ID AD 110 Checksum Instance ID Reserved Standard RFC 2328, 2740 Data Protocols IP Transport IP/89 Link State Advertisements Authentication Plaintext, MD5Router Link (Type 1)Lists neighboring routers and the cost to each; flooded within an area AllSPF Address Link (Type 2) AllDR Address by a DR; lists all routers on an adjacent segment; flooded Metric Formulawithin an areaNetwork Summary (Type 3) 100,000 Kbps* cost =Generated by an ABR and advertised among areas link speedASBR Summary (Type 4) * modifiable withInjected by an ABR into the backbone to advertise the presence of an ospf auto-cost reference-bandwidthASBR within an areaExternal Link (Type 5) Adjacency StatesGenerated by an ASBR and flooded throughout the AS to advertise a 1 Down 5 Exstartroute external to OSPFNSSA External Link (Type 7) 2 Attempt 6 ExchangeGenerated by an ASBR in a not-so-stubby area; converted into a 3 Init 7 Loadingtype 5 LSA by the ABR when leaving the area 4 2-Way 8 Full Router Types Area Types DR/BDR ElectionInternal Router Standard AreaAll interfaces reside within the Default OSPF area type The DR serves as a common point forsame area all adjacencies on a multiaccess Stub Area segmentBackbone Router External link (type 5) LSAs areA router with an interface in replaced with a default route The BDR also maintains adjacenciesarea 0 (the backbone) with all routers in case the DR fails Totally Stubby AreaArea Border Router (ABR) Type 3, 4, and 5 LSAs are Election does not occur on point-to-Connects two or more areas replaced with a default route point or multipoint linksAS Boundary Router (ASBR) Not So Stubby Area (NSSA) Default priority (0-255) is 1; highestConnects to additional routing A stub area containing an ASBR; priority wins; 0 cannot be electeddomains; typically located in type 5 LSAs are converted to typethe backbone 7 within the area DR preemption will not occur unless the current DR is reset External Route TypesE1 Cost to the advertising ASBR plus the external cost of the route Virtual LinksE2 (Default) Cost of the route as seen by the ASBR Tunnel formed to join two areas across an intermediate Troubleshooting Both end routers must share ashow ip [route | protocols] show ip ospf border-routers common areashow ip ospf interface show ip ospf virtual-links At least one end must reside in area 0show ip ospf neighbor debug ip ospf […] Cannot traverse stub areasby Jeremy Stretch v2.1
  2. 2. OSPF PART 2 packetlife.net Network Types Nonbroadcast Multipoint Multipoint (NBMA) Broadcast Nonbroadcast Broadcast Point-to-Point DR/BDR Elected Yes No No Yes No Neighbor Discovery No Yes No Yes Yes Hello/Dead Timers 30/120 30/120 30/120 10/40 10/40 Defined By RFC 2328 RFC 2328 Cisco Cisco Cisco Supported Topology Full Mesh Any Any Full Mesh Point-to-Point Configuration Example interface Serial0/0 Router A WAN Area 0 Area 9 description WAN Link Backbone Totally Stubby Area ip address ! interface FastEthernet0/0 description Area 0 ip address A ! interface Loopback0 ! Used as router ID ip address C ! B router ospf 100 ! Advertising the WAN cloud to OSPF redistribute static subnets network area 0 ! Area 1 Area 2 ! Static route to the WAN cloud Stub Area Standard Area ip route Router B Router Cinterface Ethernet0/0 interface Ethernet0/0 description Area 0 description Area 9 ip address ip address ip ospf 100 area 0 ip ospf 100 area 9! !interface Ethernet0/1 interface Ethernet0/1 description Area 2 description Area 2 ip address ip address ip ospf 100 area 2 ip ospf 100 area 2 ! Optional MD5 authentication configured ! Optional MD5 authentication configured ip ospf authentication message-digest ip ospf authentication message-digest ip ospf message-digest-key 1 md5 FooBar ip ospf message-digest-key 1 md5 FooBar ! Give B priority in DR election ! Give C second priority (BDR) in election ip ospf priority 100 ip ospf priority 50! !interface Ethernet0/2 ! description Area 1 ! ip address ! ip ospf 100 area 1 !! !interface Loopback0 interface Loopback0 ip address ip address! !router ospf 100 router ospf 100 ! Define area 1 as a stub area ! Define area 9 as a totally stubby area area 1 stub area 9 stub no-summary ! Virtual link from area 0 to area 9 ! Virtual link from area 9 to area 0 area 2 virtual-link area 2 virtual-link Jeremy Stretch v2.1