ROUTE Course OSPF
OSPF FEATURES OSPF is an open standards routing protocol This works by using the Dijkstra algorithm OSPF provides the following features: Minimizes routing update traffic Allows scalability (e.g. RIP is limited to 15 hops) Has unlimited hop count Supports VLSM/CIDR Allows multi-vendor deployment (open standard)
OSPF Tables There are three type of tables Neighbor Topology Routing
Areas Reasons and Features A larger topology database requires more memory on each router. Processing the larger topology database with the SPF algorithm requires more processing power A single interface status change, anywhere in the internetwork forces every router to run SPF again. A router can be a member of more than one area (ABR) All routers in the same area have same topology database When multiple areas exist, there must always be an area 0 (the backbone) to which other areas connect
OSPF Area Autonomous System Border Router (ASBR)
ROUTER ID The Router ID (RID) is an IP address used to identify the router Cisco chooses the Router ID by using the highest IP address of all configured loopback interfaces If no loopback interfaces are configured with addresses, OSPF will choose the highest IP address of all active physical interfaces. You can manually assign the router ID. The RID interface MUST always be up, therefore loopbacks are preferred
OSPF PACKETS Data Field of the OSPF packet contents depend on the OSPF packet type: Hello packet: Contains a list of known neighbors. DBD packet: Contains a summary of the LSDB, which includes all known router IDs and their last sequence number, among several other fields. LSR packet: Contains the type of LSU needed and the router ID of the router that has the needed LSU. LSU packet: Contains the full LSA entries. Multiple LSA entries can fit in one OSPF update packet. LSAck packet: This data field is empty.
OSPF PACKETS Hello: Discovers neighbors and builds adjacencies between them Database Description (DBD): Checks for database synchronization between routers Link-state request (LSR): Requests specific link-state records from another router LSU: Sends specifically requested link-state records LSAck: Acknowledges the other packet types
Hello Packet Information Router ID: 32-bit Hello and dead intervals: must be the same on neighboring routers (must match) Neighbors: list of adjacent routers Area ID: Contains the full LSA entries. Multiple LSA entries can fit in one OSPF update packet (must match) Router Priority: 8-bit DR and BDR IP Addresses Authentication Password: (must match) Stub Area Flag: (must match)
Network Types Point-to-point Broadcast mutliaccess Non-Broadcast Multiaccess (NBMA) The contents of the LSA (excluding the LSA header) have changed OSPF over L2 and L3 MPLS VPN DR and BDR have been selected, any router added to the broadcast network establishes full adjacencies with the DR and BDR only
Neighbor Adjacency States Router A send hello on LAN. All connected routers add A on list of neighbors (init state) All routers received Hello send unicast reply to A with corresp. Info and list on their neighbors inc. A A adds the received neighbors IDs on table (2- way state)
Discovering Network Routes in BC domain After DR and BDR selected: Master-slave relationship higher RID is the Masters (Exstart state) Master & slave exchange DBD packets (Exchange state) DBD is LSA entries header (linkstate type, Addr. Of Advertising router, link cost, Seq. No.)
Discovering Network Routes in BC domain When receive DBD: Router Ack the DBD using LSAck Compare received info with its LSDB, send LSR for newest LSA (Loading state) Other router replies with complete info about requested entry using LSU Router Ack the received LSU Router adds new LSAs into LSDB (Full state) Attempt state for NBMA
Link-State Advertisement LSA LSAs advertised to routers with split-horizon rule Each LSA entry has aging timer in age field of LSA (def. 30min) Router originated the entry resends the LSA with higher seq. no. in LSU to verify link still active LSA discarded when LSA reach its maxage (60 min) LS entry must be refreshed every 30 min
LINK-STATE UPDATE LSU OPERATION
SPF Recalculation SFP triggered when any of the following happen: The LSA s Options field has changed The LSA s LS age is set to maxage The Length field in the LSA header has changed The contents of the LSA (excluding the LSA header) have changed An SPF calculation is performed separately for each area in the topology database.
Adjacency for NBMA Network DR &BDR needs full L2 connectivity with routers in NBMA Several OSPF configuration choices are available for FR network depending on network topology: Nonbroadcast: B/DR are elected, neighbors are manually config Point-to-multipoint: B/DR not req., multicast hello auto neighbors disc. Point-to-multipoint nonbroadcast: B/DR not req, manual neighbors config Broadcast: B/DR are elected, multicast hello auto neighbors disc. Point-to-point: no B/DR, differ. IP subnets Point-to-point needs less configuration, nonbroadcast less traffic overhead R(config-if)#ip ospf network {broadcast non-broadcast point-tomultipoint [non-broadcast] point-to-point}
NBMA Mode Configuration Fully meshed topology DR & BDR are elected Not Fully meshed DR & BDR are manually selected DR & BDR should have full connectivity with all routers LSUs are replicated for each PVC Configure nonbroadcast mode by: Manually configuring neighbors in DR and BDR Define OSPF network type as nonbroadcast neighbor ip-address [priority number] [poll-interval number] [cost number] [database-filter all]
Point-to-multipoint Mode Configuration Not full-mesh topology OSPF treats this mode as several point-to-point links No static neighbor configuration, multicast hello discover neighbors Duplicated LSA packets Hello interval 30 sec, and dead interval 120 sec One IP subnet
Point-to-multipoint Nonbroadcast Mode Configuration Used when no broadcast and multicast OSPF treats this mode as several point-to-point links No automatic neighbor discovery, so must use static neighbor configuration Bandwidth for each neighbor can be defined Hello interval 30 sec, and dead interval 120 sec One IP subnet
OSPF over Frame Relay Subinterface Configuration Point-to-point subinterface: Each VC gets its own subinterface No DR and BDR, automatic neighbor discovery Point-to-multipoint subinterface: Nonbroadcast id the default mode
LSA Types LSA type 1: Router LSA LSA type 2: Network LSA LSA type 3 : Summary LSA LSA type 4: ASBR LSA type 5: AS External LSA LSA type 6: Multicast OSPF LSA (not supported by Cisco) LSA type 7: LSA for NSSA LSA type 8: External Attributes for BGP (Not supp. by Cisco) LSA type 9, 10, 11: Opaque LSA (future upgrades)
LSA Type 1 link type is defined by (1,2,3, or 4) Link ID: what is on the other end of the link which depends on link type Link data: IP address of the link, or subnet mask in case of stub network Type 1 LSA includes the OSPF cost for each link, and whether the router is an ABR or ASBR Link Type 1 2 3 Decription Point-point Transit Network Stub network Link ID Field Contents Neighbor RID DR s interface address IP network/subnet 4 Virtual link Neighbor RID
LSA Type 2 Transit Network has at least 2 directly attached OSPF routers LSA type 2 list all attached routers, DR, and subnet mask DR is responsible for advertising Network LSA Link-state ID is the IP address of DR
LSA Type 3 Advertises networks for an area to the rest of areas Type 3 summary LSA is advertised into the backbone area for every subnet defined in the originating area Manual summarization at ABR should be considered Receiving type 3 LSA does not cause router to run SPF To summarize inter-area, IOS a creates summary route to null 0: area area-id range address mask [advertise not-advertise] [cost cost] Link-state ID is destination network number (summary network)
LSA Type 4 Generated by an ABR only when an ASBR exists within an area It identifies the ASBR and provide a route to it. ASBR send type 1 LSA with external bit(e bit) is set ABR generate type 4 LSA and floods it to backbone area and into their area Link-state ID is ASBR ID
LSA Type 5 Describe a routes networks outside the OSPF AS To reduce flooding of LSAs for external networks, summarization should be considered at ASBR Link-state ID is the external network number To manually summarize external routes: summary-address ip-address mask [not-advertise] [tag tag]
Types of OSPF Routes Route Designator O description OSPF intra-area (router LSA) and network LSA Networks within the area advertised by router and network LSAs O IA O E1 O E2 OSPF interarea (summary LSA) Type 1 external routes Type 2 external routes Networks from outside area and within AS advertised by summary LSAs Networks from outside AS advertised by way of external LSAs.) Networks from outside AS advertised by way of external LSAs. E1: adding external cost to internal cost (when multiple ASBRs, E2 the cost in only the external cost
PROPAGATE DEFAULT ROUTE default-information originate when configured on a OSPF router it becomes an ASBR, the router already has a default route When use always keyword to the command it advertise regardless whether the router already has default route default-information originate [always] [metric metric-value] [metric-type type-value] [route-map map-name] The default metric value for type of interfaces is 1
VIRTUAL LINKS A virtual link allows discontiguous area 0s to be connected, or a disconnected area to be connected to area 0, via a transit area. It cannot go through more than one area, nor through stub areas. LSA on virtual link does not age out (DoNotAge DNA) To configure virtual link: area area-id virtual-link router-id [authentication [message-digest null]] [hello interval seconds] [retransmit-interval seconds] [transmit-delay seconds] [deadinterval seconds] [[authentication-key key] [message-digest-key key-id md5 key]]
SPECIAL AREA TYPES Standard Area: Default area link updates, route summaries, and external routes Backbone Area: area 0, all other areas connected to this area Stub Area: don t accept external routes Totally Stubby Area: don t accept external routes but can send a packet to external using a default route, no ASBR. (Cisco proprietary) Not so-stubby area (NSSA): don t accept external information but instead use default route, but can allow ASBR and use type 7 LSA Totally Stubby NSSA: Allows ASBR and does not accept external routes or summary routes
STUB OR TOTALLY STUBBY AREA There is a single exit point from that area; or if there are multiple exits, one or more ABRs inject a default route into the stub area and suboptimal routing paths are acceptable. All OSPF routers inside the stub area, incl. ABRs, are configured as stub routers before they become neighbors and exchange routing information. Hello packets contains stub area flag The area is not used as a transit area for virtual links, no ASBR is inside the area, and not a backbone area (area0) Type 4 and 5 LSAs not permitted to flood into stub area Type 3, 4 and 5 LSAs are not permitted to flood into totally stubby area
STUB OR TOTALLY STUBBY AREA To configure stub area: area area-id stub area area-id default-cost cost to change the default route cost, configured on ABR To configure totally stubby area: area area-id stub [no-summary] area area-id default-cost cost to change the default route cost configured on internal routers
NOT-SO-STUBBY AREA NSSA ASBR exits on area generate type 7 LSA into area with propagate bit (P) to avoid propagation loop between NSSA and BB. NSSA ABR translates the type 7 LSA to Type 5 LSA ABR sends default route into NSSA instead of external routes Routers in NSSA set N-bit to confirm the support for NSSA. This option checked during neighbor discovery Type 7 LSA described in routing table by O N1 or O N2 To configure the NSSA area Area area-id stub nssa [no-redistribution] [default-information originate] [metric metric] [metric-type value] [no-summary] When use keyword [no-summary] totally stubby NSSA is configured
ROUTE FILTERING No route filtering permitted inside area due to LSDB convergence Route filtering can be applied for: Type 3 LSAs on ABR Type 5 LSAs on ASBR Route redistribution Filtering OSPF routes when adding to IP routing table
TYPE 3 LSA FILTERING No route filtering permitted inside area due to LSDB convergence area number filter-list prefix name in out ip prefix-list {name number} {seq number} {deny permit} netowrk/length [ge value] [le value] Example: ip prefix-list filter-into-area-34 seq 5 deny 10.16.3.0/24 ip prefix-list filter-into-area-34 seq 10 permit 0.0.0.0/0 le 32 router ospf 1 area 34 filter-list prefix filter-into-area-34 in
FILTERING OSPF ROUTES ADDED TO ROUTING TABLE Don t affect LSAs, LSDB flooding process, and SPF calculation It is enabled by using distribute-list in in OSPF subcommand It filter the routes from being added to router s IP routing table Routes match ACL statement with permit are added to routing table while that match deny are filtered interface interfaceno can be used to compare parameters to the route s outgoing interface Example: ip prefix-list filter-1 seq 5 deny 10.16.1.0/24 ip prefix-list filter-1 seq 10 permit 0.0.0.0/0 le 32 router ospf 1 distribute-list prefix filter-1 in
OSPF Authentication By default OSPF do not use authentication Two methods: simple password, MD5 To configure simple authentication: Router(config-if)# ip ospf authentication Router(config-if)# ip ospf authentication-key password Router(config-router)# area area-id authentication To configure MD5 authentication: Router(config-if)# ip ospf authentication [message-digest null] Router(config-if)# ip ospf message-digest-key key-id md5 key You must configure: service password-encryption
PLANNING FOR OSPF IP Addressing Plan IP subnets and addressing plan considering summarization Network Topology Detailed network topology include link types, backup links, stub areas, redistribution OSPF Areas LSDB table size should be considered when dividing networks into areas, ABR and ASBR routers should be identified OSPF routing parameters (times, areas, authentication, RID, ) should be determined