Modules 1 - 2: OSPF Concepts and Configuration
link-state update packet
- advertises new information - used to reply to LSRs - contains different types of LSAs
database description packet
- checks for DB synchronization between routers by compare local topology to that sent by another router - helps maintain and improve LSDB, creating a more accurate SPF tree
hello packet
- discovers neighbours and establish adjacencies - contains information to elect DR & BDR
two-way state
-Communication between the two routers is bidirectional. -On multiaccess links, the routers elect a DR and a BDR.
Init state
-Hello packets containing RouterIDs are received from the neighbor.
Loading state
-LSRs and LSUs are used to gain additional route information. -Routes are processed using the SPF algorithm.
By default, what is the OSPF cost for any link with a bandwidth of 100 Mb/s or greater?
1
single-area OSPF
All the routers are in the backbone area
What is the recommended Cisco best practice for configuring an OSPF-enabled router so that each router can be easily identified when troubleshooting routing issues?
Configure a value using the router-id command.
Link-State Update (LSU) packets
Contains the different types of link-state advertisements
adjacency database
Creates the neighbor table Unique per router
Forwarding Database
Creates the routing table
link-state database
Creates the topology table Routers in the same area have identical LSDBs
After modifying the router ID on an OSPF router, what is the preferred method to make the new router ID effective?
HQ# clear ip ospf process
Which three OSPF states are involved when two routers are forming an adjacency?
Init Two-way Down
In an OSPFv2 configuration, what is the effect of entering the command network 192.168.1.1 0.0.0.0 area 0?
It tells the router which interface to turn on for the OSPF routing process.
full state
LSDBs are identical, routers have converged
Down state
No Hello packets received = Down. Router sends Hello packets
Exstart state
On point-to-point networks, the two routers decide which router will initiate the DBD packet exchange and decide upon the initial DBD packet sequence number.
3. Building the topology table
Router builds a link-state database based on received LSAs
2. Exchanging link-state advertisements
Router floods link-state and cost information about each directly connected link
5. Choose best route & add to routing table
Router inserts the calculated best paths into the routing table
1. Establish Neighbor Adjacencies
Router sends Hello packets out all of the OSPF-enabled interfaces
What is a benefit of multiarea OSPF routing?
Topology changes in one area do not cause SPF recalculations in other areas.
What is the format of the router ID on an OSPF-enabled router?
a 32-bit number formatted like an IPv4 address
Shortest path first
algorithm used by OSPF
What will an OSPF router prefer to use first as a router ID?
any IP address that is configured using the router-id command
Database Description (DBD) packets
contains an abbreviated list of the LSDB of a sending router and is used by receiving routers to check against the local LSDB
Hello Packets
discovers neighbors and builds adjacencies between them
Which step in the link-state routing process is described by a router running an algorithm to determine the best path to each destination?
executing the SPF algorithm
What indicates to a link-state router that a neighbor is unreachable?
if the router no longer receives hello packets
link-state request packet
query another router for additional information
Which command will a network engineer issue to verify the configured hello and dead timer intervals on a point-to-point WAN link between two routers that are running OSPFv2?
show ip ospf interface serial 0/0/0
What command would be used to determine if a routing protocol-initiated relationship had been made with an adjacent router?
show ip ospf neighbor
Which command is used to verify that OSPF is enabled and also provides a list of the networks that are being advertised by the network?
show ip protocols
A network technician issues the following commands when configuring a router: R1(config)# router ospf 11 R1(config-router)# network 10.10.10.0 0.0.0.255 area 0 What does the number 11 represent?
the OSPF process ID on R1
What is used to facilitate hierarchical routing in OSPF?
the use of multiple areas
What is the default router priority value for all Cisco OSPF routers?
1
Link State Routing Process
1. Establish neighbour adjacencies 2. Exchange LSAs 3. Build LSDB/topology table 4. Execute SPF algorithm 5. Choose best route
OSPF topology table?
A link-state database that represents the network topology. When converged, all routers in an area have identical topology tables. Can be viewed via the show ip ospf database command.
Exchange state
Routers exchange DBD packets. If additional router information is required then transition to Loading; otherwise, transition to the Full state.
To establish a neighbor adjacency two OSPF routers will exchange hello packets. Which two values in the hello packets must match on both routers?
dead interval hello interval
When an OSPF network is converged and no network topology change has been detected by a router, how often will LSU packets be sent to neighboring routers?
every 30 minutes
Which step does an OSPF-enabled router take immediately after establishing an adjacency with another router?
exchanges link-state advertisements
Which OSPF data structure is identical on all OSPF routers that share the same area?
link-state database
What does an OSPF area contain?
routers with the same link-state information in their LSDBs
What is the reason for a network engineer to alter the default reference bandwidth parameter when configuring OSPF?
to more accurately reflect the cost of links greater than 100 Mb/s
What are the two purposes of an OSPF router ID?
to uniquely identify the router within the OSPF domain to facilitate router participation in the election of the designated router
When are DR and BDR elections required?
when the routers are interconnected over a common Ethernet network
