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OPEN SHORTEST PATH FIRST (OSPF)
Presented byBEHZAD ZAHID Operations Engineer-Core
To Understand the OSPF as Dynamic Routing Protocol
Basic Configuration of OSPF on Cisco Routers and Troubleshooting
OBJECTIVE
Prerequisite OSI Model Basics of Routing OSPF
◦ Understanding of OSPF◦ Neighbor Establishment Stages◦ LSP Types and its Propagation
Configuring OSPF on Cisco Routers Trouble Shooting OSPF
OVERVIEW
Understanding of OSI Model◦ Transformation from Data to Bits◦ Significance of Each Layer
Understanding of Routing and Routing table
PREREQUISITES
Routing is a way to get one packet from one destination to the next
A router creates or maintains a table of the available routes.
ROUTING
Path Determination
WHY WE NEED ROUTING
Static routing is simply the process of manually entering routes into a device's routing table.
Static routing is the simplest form of routing, but it is a manual process.
Disadvantage:◦ Not scalable
STATIC ROUTING
Routing device (the router) dynamically learn network destinations and how to get to them and also advertise those destinations to other routers.
Dynamic routing is assisted by routing protocols.
DYNAMIC ROUTING
Exterior Gateway Protocol◦ Border Gateway Protocol
For Large Internetworks Interior Gateway Protocol
◦ Distance Vector RIP
◦ Link State OSPF ISIS
◦ Hybrid EIGRP
TYPES ROUTING PROTOCOL
Open Shortest Path First (OSPF) is a routing protocol for Internet Protocol (IP) networks.
It uses a link state routing algorithm and falls into the group of interior routing protocols(IGP), operating within a single autonomous system (AS).
OSPF
Area◦ A contiguous grouping of routers and router
interfaces.◦ Normal Areas:
Summary LSAs from other areas are injected External LSAs are injected
◦ Stub Areas: Summary LSAs from other areas are injected The default route is injected as a summary route External LSAs are not injected.
OSPF AREA DESIGN AND TERMS
◦ Totally Stubby Areas: No summary LSAs are allowed No external LSAs are allowed The default route is injected as a summary route.
◦ Not-So-Stubby Areas: Type 7 LSAs carry external information within an
NSSA Type 7 LSAs are converted into Type 5 LSAs at the
NSSA ABR No external LSA are allowed, Summary LSAs are
injected
OSPF AREA DESIGN AND TERMS
OSPF AREA DESIGN AND TERMS
Types of Routers in OSPF◦ Internal routers: Routers that have all their
interfaces in the same area and have identical LSDBs.
◦ Backbone routers: Routers having at least one interface connected to area 0.
◦ ABRs: Routers that have interfaces attached to multiple areas.
◦ ASBRs: Routers that have at least one interface attached to an external internetwork (another AS), such as a non‐OSPF network.
OSPF AREA DESIGN AND TERMS
OSPF AREA DESIGN AND TERMS
◦ Designated Router On Multiaccess data links , an ospf
router elected by the routers on that data link to perform special functions (e.g generation of LSAs representing the subnet)
◦ Backup Designated Router A router on Multiaccess data link that
becomes prepared to take over for the DR, should the DR fail.
OSPF AREA DESIGN AND TERMS
OSPF Packet Types:◦ Hello Message
To form neighbor-ship◦ Data Base Descriptor
Has LSA headers (LSID and Seq No)◦ Link State Request
I need complete info of this Network◦ Link State Advertisement
Info of asked Network◦ Link State Update
Set of LSAs◦ Link State Acknowledgement
OSPF AREA DESIGN AND TERMS
OSPF Cost◦ Cost is used to find best router◦ OSPF Metric :
Cost = 100/ BW in MBPS
T1 65E1 48Ethernet 10Fast Ethernet 1Gigabit Ethernet 1
OSPF AREA DESIGN AND TERMS
Hello Interval◦ Interval at which a router sends OSPF Hello
messages on an interface. (10ms for Broadcast) Dear Interval
◦ Timer use to determine neighboring router has failed. (40ms for Broadcast)
Update Destination Address◦ Normally sent to 224.0.0.5◦ All designated Routers 224.0.0.6
OSPF FEATURE SUMMARY
Neighbor Discovery and Forming Neighbor Relationship◦ Sending Hellos Message
Exchanging Topology Database◦ Exchanging DBDs, LSRs, LSUs
Route Computation◦ SPF
OSPF PROCESS
Router ID (must be unique 1.1.1.1 ) Hello and Dead Timers * (10ms and 40ms) Network Mask * Area ID * (e.g Area 1) List of neighbors Router Priority DR/BDR IP address Authentication Password *
OSPF HELLO MESSAGE
Determine Your Router ID◦ Hard coded◦ Highest loopback IP◦ Highest Interface IP
Add interfaces to the Link State Database (dictated by Network Command)
NEIGHBOR FORMING
Exchange hello Messages
NEIGHBOR FORMING
Topology Exchange◦ Master/Slave Relation
NEIGHBOR FORMING
NEIGHBOR STATES
OSPF LSA Types:◦ LSA Type 1 : Router LSA◦ LSA Type 2 : Network LSA (DR Generated)◦ LSA Type 3 : ABR Summary (ABR Generated)◦ LSA Type 4 : ASBR Location (ABR Generated)◦ LSA Type 5 : External LSA (ASBR Generated)◦ LSA Type 7 : NSSA External LSA
LSA TYPES AND FLOODING
Router LSA◦ Each router creates its own type 1 LSA to
represent itself for each area to which it connects.◦ To flood the LSA, the originating router sends it to
its neighbor inside the same area, who in turn to their neighbors.
◦ It lists : Router interface subnet mask and interface ospf cost
(stub network- no DR Elected Interface) IP address of DR (Transit Network- DR elected
interface) Neighbors Router ID (no DR elected interface)
TYPE 1 LSA
◦ OSPF identifies Type 1 LSA using a 32bit LSID.◦ Each router uses its own OSPF router ID value as
the LSID for Type 1 LSA.
TYPE 1 LSA
R5’s type 1 LSA will list these three interfaces as links, plus it will refer to the two working neighbors.
TYPE 1 AND TYPE 3 LSA
LSA FLOODING
LSA FLOODING
SPF requires that the LSDB model the topology with nodes (routers) and connections between nodes (Links).
OSPF can model all the topology inside a single area using TYPE 1 and TYPE 2 LSAs.
MAKING TOPOLOGY
Analyze the LSDB to find all possible routes to reach the subnet
For each possible route, add the ospf interface cost for all outgoing interfaces in that route
Pick the route with the lowest cost
CHOOSING THE BEST OSPF ROUTE
INTRA-AREA COST
INTER-AREA COST
router ospf process-id Configure an OSPF routing process and enter
router configuration mode.
router-id [X.X.X.X] To use a fixed router ID, use the above command in
router configuration mode.
network [ip-address ][wildcard-mask] area [area-id]
To define the interfaces on which OSPF runs and to define the area ID for those interfaces, use the network area command in router configuration mode.
CONFIGURING OSPF
show ip ospf [process-id] To display general information about OSPF
routing processes
show ip ospf neighbor To display OSPF-neighbor information.
TROUBLESHOOTING OSPF
show ip ospf interface [interface-type interface-number]
To display OSPF-related interface information.
show ip ospf [process-id [area-id]] database To display lists of information related to
the OSPF database for a specific router
TROUBLESHOOTING OSPF
Routing OSPF as Dynamic Routing Protocol
◦ Neighbor Forming◦ LSAs Flooding◦ Finding Best Routes
Configuring and Troubleshooting OSPF
SUMMARY
CCNP Route 642-902 Certification Guide CCNA Exploration Companion Guide
◦ Network Fundamentals
REFERENCES
Recommended