ccnp1: advanced routing v3.0 cisco networking academy program chapter 2 – single area ospf ospf...
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CCNP1: Advanced Routing v3.0
CISCO NETWORKING ACADEMY PROGRAM
Chapter 2 – Single Area OSPF
OSPF
Link State Routing
Single Area and Multiarea OSPF
CCNP1: Advanced Routing
CISCO NETWORKING ACADEMY PROGRAM
Chapter 6 –OSPF
Identify Distance Vector & Link State Routing Characteristics
Periodic updates
Topology changes trigger updates
Updates sent to neighbours
Updates contain changes only
Updates contain entire routing table
Increased memory & processing requirements
Updates consume significant bandwidth
Rapid convergence
Updates sent to all routers Support CIDR/VLSM
Slow convergence
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Chapter 6 –OSPF
Identify Distance Vector & Link State Routing Characteristics
Periodic updates
Topology changes trigger updates
Updates sent to neighbours
Updates contain changes only
Updates contain entire routing table
Increased memory & processing requirements
Updates consume significant bandwidth
Rapid convergence
Updates sent to all routers Support CIDR/VLSM
Slow convergence
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Chapter 6 –OSPF
Summary of Link State Features• Responds quickly to network changes• Use ‘hellos’ to discover and create neighbors• Send updates when a there has been a change
in the network topology • Updates contain changes not whole routing table• Calculates shortest path to each route in the
network from a separate topology table
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Chapter 6 –OSPF
Link State Operation • Routers are aware of directly connected networks known
as ‘links’• Routers send ‘hellos’ to discover neighbors• Routers send Link State Advertisements (LSAs) to other
routers informing them of their links• All routers add Link State Advertisements to their
topological database (topology table)• Shortest Path algorithm calculates best route to each
network• When link states change, LSA update are sent to all
routers which recalculate their routes
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Chapter 6 –OSPF
Topological Database
• Every router advertises directly connected networks via Link State Advertisements
• Every router has it’s own view of the network – it builds a ‘topological database’
• Router A is aware of 2 paths to 192.168.157.0 – this provides redundancy should one of the routers fail
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Chapter 6 –OSPF
Evaluation of Link State Routing
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Chapter 6 –OSPF
Review – Link State & Distance Vector
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Chapter 6 –OSPF
OSPF Overview
• Preferred to RIP on larger networks• Open Standard - IETF RFC 2328• Link State routing protocol• Interior Gateway Protocol for Autonomous
systems• Metric based on bandwidth - Cost• Supports VLSM• OSFP can use ‘areas’ to allow hierarchical
design – Multiarea OSPF
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Chapter 6 –OSPF
OSPF Key WordsAdjacencies database• Directly connected routersTopological Database• Routes to every networkRouting table
– Best path to each network(chosen from topological database)
Designated Router• A router elected by all others to
represent the network area(multi-access networks only)
Area 0• backbone
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Chapter 6 –OSPF
OSPF Packet Types• Type 1 – Hello
– Establishes and Maintains adjacency info w/ neighbors• Type 2 – Database description packet (DBD)
– Describes the contents of an OSPF router’s link-state database
• Type 3 – Link State Request– Requests specific pieces of a routers link-state database
• Type 4 – Link State Update (LSUs)– Transports link-state advertisements (LSAs) to neighbor
routers• Type 5 – Link-state acknowledgement (LSAKs)
– Acknowledges receipt of a neighbor’s LSA
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Chapter 6 –OSPF
OSPF Hello Protocol
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Chapter 6 –OSPF
Designated Router/Backup DR
• All LSA sent to DR/BDR instead of to every single router
• Reduces overhead of LSA updates
• Standard on multi-access networks
• DR is single point of failure – solution is BDR
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Chapter 6 –OSPF
DR/BDR
• Once a DR is established, a new router with a higher priority or router ID will NOT become the DR or BDR.
• If DR fails, BDR takes over as DR and selection process for new BDR begins.
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Chapter 6 –OSPF
Backup Designated Router
• 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.
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Chapter 6 –OSPF
DR/BDR selection• To suit the topology used the network
administrator will want to choose DR/BDR• DR/BDR election based on OSPF priority• Highest priority=DR• 2nd highest priority=BDR• Priority of 0 = DROTHER (ensures will not be DR)• Default priority = 1Router(config-if)#ip ospf priority number
Router#show ip ospf interface type number
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Chapter 6 –OSPF
OSPF Loopback Address
• For OSPF to function there must always be an active interface
• Physical interfaces e.g. serial/Ethernet may not always be active – routing would fail
• Configure virtual “loopback” interface as solution• Subnet mask will always be 255.255.255.255
Router(config)#interface loopback number
Router(config-if)#ip address ip-address subnet-mask
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Chapter 6 –OSPF
Drothers – All Other OSPF Routers
• All other routers, DROTHER, establish adjacencies with DR and BDR only.
• LSAs are multicast to DR and BDR only– (224.0.0.6 - all DR routers)
• DR sends LSA to all adjacent neighbors – (224.0.0.5 - all OSPF routers)
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Chapter 6 –OSPF
OSPF basic commands
Router(config)#router ospf process-id
Router(config-router)#network address wildcard-mask area area-id
EXAMPLE
Router(config)#router ospf 2
Router(config-router)#network 172.16.10.0 0.0.0.255 area 2
NOTES• process-id can be a value between 0 and 65,535• Wildcard mask NOT subnet mask used with network command
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Chapter 6 –OSPF
Basic OSPF Configuration
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Chapter 6 –OSPF
Configuring OSPF Authentication within a Single Area
Rtr(config)# router ospf process-id
Rtr(config-router)#network address wildcard-mask area area-id
Rtr(config-router)# area area authentication [message-digest]
Rtr(config)# interface type slot/port
Rtr(config-if)# ip ospf priority <0-255>
RTB(config-if)# ip ospf cost cost
Rtr(config-if)# ip ospf hello-interval seconds
Rtr(config-if)# ip ospf dead-interval seconds
Rtr(config-if)# ip ospf message-digest-key key-id md5 [encryption-type] password
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Chapter 6 –OSPF
Steps to OSPF Operation
1. Establishing router adjacencies
2. Electing DR and BDR
3. Discovering Routes
4. Choosing Routes
5. Maintaining Routing Information
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Chapter 6 –OSPF
Router States
• down• init• 2 way• exstart• exchange• loading• full
Good neighbors, no LSA sharing.
Sharing route via LSAs.
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Chapter 6 –OSPF
Reaching 2-Way
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Chapter 6 –OSPF
Electing a DR/BDR
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Chapter 6 –OSPF
1.Establishing Adjacencies
An OSPF router tries to form an adjacency with at least one neighbor for each IP network it’s connected to.
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Chapter 6 –OSPF
2.Electing a DR and BDR• On point-to-point links adjacencies 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
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Chapter 6 –OSPF
3.Discovering Routes
• EXSTART state - prepare for initial database exchange of Database Description Packets (DBDs)
• master/slave relationship decided (higher router id)• EXCHANGE state - routers exchange one or more
DBDs (Database Description) packets, which is a summary of the link-state database– send LSAcks to verify– compares DBD with its own database
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Chapter 6 –OSPF
3.Discovering Routes• LOADING STATE - When a slave router receives
a DBD it:– Acknowledges receipt of the DBD by sending an LSAck– Compares the information it received with the
information it has by checking the LSA sequence number
– If the DBD has a more up-to-date link-state entry, the slave router sends a link state request (LSR) to the master
– The master responds with a link state update (LSU).
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Chapter 6 –OSPF
3.Discovering Routes
• When all LSRs have been satisfied for a given router, the adjacent routers are considered to be synchronized and in a FULL STATE.
• At this point all routers within the network should have identical link-state databases.
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Chapter 6 –OSPF
4.Choosing Routes• OSPF bases routing metrics on cost.• Cisco routers, cost = 108/BW• BW is the configured bandwidth for an interface
and may be changed using the ip ospf cost command. The bandwidth command can also be used to change the bandwidth metric on an interface.
• ip ospf cost is used when converting the metric between routers from different vendors.
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Chapter 6 –OSPF
Choosing Routes
• SPF, Shortest Path First calculations use the Dijkstra algorithm, placing itself as the root and creating a “tree diagram” of the network
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Chapter 6 –OSPF
5.Maintaining Routing Info• Flooding process• When there is a state change in one of the connected
links, as soon as the router learns of this change it floods it to all adjacent neighbors (224.0.0.6 - all DR/BDRs).
• Only the changed link information is flooded, not the entire database.
• DR sends LSU (LSA) to others on the network, area (224.0.0.5 all SPF routers)
• Router which receives LSU updates links-state database, computes the SPF algorithm, and generates a new routing table
• LSAs are aged at 30 minutes and flooded every 30 minutes
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Chapter 6 –OSPF
Additional Configuration
Network administrators can also configure:
• LSA update authentication• OSPF Priority at the interface• ‘Hello’ & ‘Dead’ interval timers• Default route to routers outside the
area/autonomous system
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Chapter 6 –OSPF
Configuring OSPF Loopback Address and Router Priority
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Chapter 6 –OSPF
Setting OSPF Priority
The priorities can be set to any value from 0 to 255. A value of 0 prevents that router from being elected. A router with the highest OSPF priority will win the election for DR.
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Chapter 6 –OSPF
Modifying OSPF Cost Metric
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Chapter 6 –OSPF
Configuring OSPF Authentication
The encryption setting of 7 is Cisco proprietary andwill not work properly.
Use 5 for MD5 hashinstead.
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Chapter 6 –OSPF
Configuring timers
Rtr(config-if)# ip ospf hello-interval secondsRtr(config-if)# ip ospf dead-interval seconds
• For OSPF routers to be able to exchange information, the must have the same hello intervals and dead intervals.
• By default, the hello interval is 4 times the dead interval, so the a router has four chances to send a hello packet being declared dead. (not required)
Defaults• On broadcast networks hello interval = 10 seconds, dead
interval 40 seconds.• On non-broadcast networks hello interval = 30 seconds, dead
interval 120 seconds.
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Chapter 6 –OSPF
Configuring OSPF Timers Example
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Chapter 6 –OSPF
OSPF - Propagating a Default Route
Default routes are used if the destination network is not in the routing table.The border router (Router B) is the default router.Router B must use the command ‘default-information originate’ to propagate default information to the rest of the OSPF network.
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Chapter 6 –OSPF
OSPF Default Router Example
Router(config)# int s0/0Router(config-if)# ip address 172.16.16.2 255.255.255.252Router(config-if)# no shutRouter(config-if)# exitRouter(config)# ip route 0.0.0.0 0.0.0.0 172.16.16.1Router(config)# router ospf 1Router(config-router)# network 192.168.1.0 0.0.0.3 area 0Router(config-router)# network 192.168.1.128 0.0.0.63 area 0Router(config-router)# default-information originate
Notice the 172. network is not included in the OSPF configuration because youare not running OSPF to the Internet provider.
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Chapter 6 –OSPF
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
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Chapter 6 –OSPF
OSPF uses Areas
• Hierarchical routing enables you to separate large internetworks (autonomous system) 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.
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Chapter 6 –OSPF
OSPF Areas
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Chapter 6 –OSPF
OSPF Router Types
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Chapter 6 –OSPF
OSPF Router TypesInternalInternal: Routers with all their interfaces within the same
area BackboneBackbone: Routers with at least one interface connected
to area 0 ASBRASBR: (Autonomous System Boundary Router): Routers
that have at least one interface connected to an external internetwork (another autonomous system)
ABRABR: (Area Border Router): Routers with interfaces
attached to multiple areas.
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Chapter 6 –OSPF
Area Types
• Standard• Backbone• Stub
– Stub– Totally Stubby Area (TSA)– Not-so-stubby-area (NSSA)
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Chapter 6 –OSPF
Area Types
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Chapter 6 –OSPF
Stub, Totally Stubby and Not-so-stubby
• A stub area does not accept information about routes external to the AS
• A totally stubby area, which is a Cisco specific feature, blocks external Type 5 LSAs and summary, Type 3 and Type 4, LSAs from entering the area.
• An NSSA does not allow Type 5 LSAs but does allow Type 7 LSAs, which can carry external routing information and be flooded throughout the NSSA.
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Chapter 6 –OSPF
Area Types
• Key difference between area types:– How they handle external routes (E1 and E2).– External routes are injected into OSPF by ASBR– Type 1 (E1): cost metric increments as route is
passed through OSPF domain– Type 2 (E2): cost metric remains the same as route is
passed through OSPF domain
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Chapter 6 –OSPF
LSA Types
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Chapter 6 –OSPF
LSA Types (con’t)
• Type 6 MOSPF (Multicast OSPF) Not supported by Cisco. – MOSPF enhances OSPF by letting routers use their
link-state databases to build multicast distribution trees for the forwarding of multicast traffic.
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Chapter 6 –OSPF
LSA Types (con’t)
• 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).
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Chapter 6 –OSPF
Multi-area Example
ABR
ASBR
RIP
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Chapter 6 –OSPF
Stub Example
ABR
ASBR
No Type 5 LSAs
Route to 0.0.0.0/0 via RTB
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Chapter 6 –OSPF
Totally Stubby Example
ABR
ASBR
No Type 3, 4, or 5 LSAs
Route to 0.0.0.0/0 via RTB
(no more IA routes)
Totally Stubby Area
no summary
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Chapter 6 –OSPF
NSSA Example
ABR
ASBR
No Type 5 LSAs
Type 7 okay
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Chapter 6 –OSPF
NSSA Example
ABR
ASBR
No Type 3, 4 or 5 LSAs
Type 7 okay
no summary
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Chapter 6 –OSPF
Configuring Summary Routes
• Interarea route summarization(at ABR)
Router(config-router)# area [area-id] range [address] [mask]
Router(config-router)# area 1 range 192.168.16.0 255.255.252.0
• External route summarization(at ASBR)
Router(config-router)# summary-address [address] [mask]
Router(config-router)# summary-address 200.9.0.0 255.255.0.0
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Chapter 6 –OSPF
Virtual Links
• A virtual link has the following two requirements:
– It must be established between two routers that share a common area.
– One of these two routers must be connected to the backbone.
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Chapter 6 –OSPF
Virtual Links
RTA(config)#router ospf 1
RTA(config-router)#network 192.168.0.0 0.0.0.3 area 51
RTA(config-router)#network 192.168.1.0 0.0.0.3 area 3
RTA(config-router)#area 3 virtual-link 10.0.0.1
...
RTB(config)#router ospf 1
RTB(config-router)#network 192.168.1.0 0.0.0.3 area 3
RTB(config-router)#network 192.168.2.0 0.0.0.3 area 0
RTB(config-router)#area 3 virtual-link 10.0.0.2
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Chapter 6 –OSPF
Why Virtual Link?
• Temporary fix when two existing OSPF networks merge (company merger, etc.)
• Backup path
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Chapter 6 –OSPF
Common OSPF Configuration Issues
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Chapter 6 –OSPF
OSPF Interface InformationRtr# show ip ospf interfaceEthernet0 is up, line protocol is up Internet Address 206.202.2.1/24, Area 1 Process ID 1, Router ID 1.2.202.206, Network Type BROADCAST, Cost: 10 Transmit Delay is 1 sec, State BDR, Priority 1 Designated Router (ID) 2.2.202.206, Interface address 206.202.2.2 Backup Designated router (ID) 1.2.202.206, Interface address 206.202.2.1 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 2.2.202.206 (Designated Router) Suppress hello for 0 neighbor(s)Serial0 is up, line protocol is up Internet Address 206.202.1.2/24, Area 1 Process ID 1, Router ID 1.2.202.206, Network Type POINT_TO_POINT, 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 2.0.202.206 Suppress hello for 0 neighbor(s)
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Chapter 6 –OSPF
Verifying OSPF Configuration
• show ip protocol• show ip route• show ip ospf interface• shop ip ospf• show ip ospf neighbor detail• show ip ospf database
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The debug and clear Commands for OSPF Verification
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Chapter 6 –OSPF
NBMA
• Non-Broadcast Multi-access Access Networks.– Frame Relay– X.25
• Without broadcasts and multicasts, DR/BDR election is problematic
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Chapter 6 –OSPF
Network TypesCisco routers can treat NBMA interfaces using any of the following:Non-Broadcast• OSPF is aware that multicast packets cannot be sent over the
interface and sends OSPF packets directly to neighbors using unicast addresses.
• DR and BDR are not elected by default• DR represent the NBMA cloud as a transit network, using
network LSAs• Suitable only for when the VCs are fully meshed
Broadcast• OSPF treats the interface as belonging to a broadcast segment,
thus using multicasts to send OSPF packets.• DR and BDR are elected• Suitable only for when the VCs are fully meshed.
NBMA Networks and OSPF
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Chapter 6 –OSPF
Network TypesCisco routers can treat NBMA interfaces using any of the following:
Point-to-multipoint• OSPF treats the interface as a placeholder for a set of point-to-point
adjacencies.• No DR/BDR is elected• Very much like point-to-point interfaces, except that every router
announces a host route to its own IP address.
Point-to-point• OSPF treats the interface as a set of point-to-point adjacencies• No DR/BDR is elected.
NBMA Point-to-Point and Multipoint Networks
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Chapter 6 –OSPF
NBMA Solution
• Full meshFull mesh
•Full Mesh network is on one subnet
The OSPF neighbor command tells a router about the IP addresses of its neighbors so that it can exchange routing information without multicasts.
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Chapter 6 –OSPF
NBMA Solution• Point-to-pointPoint-to-point uses subinterfaces
• Point-to-point networks are all on different subnets
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Chapter 6 –OSPF
NMBA Solution
• Point-to-multipointPoint-to-multipoint
•Point-to-Multipoint network is on one subnet
The broadcast keyword permits the router to send broadcasts by way of the specified DLCI to the mapped neighbor or neighbors.