Single Area OSPFv2 Concepts
Two-Way State
-In this state, communication between the two routers is bidirectional. -On multiaccess links, the routers elect a DR and a BDR. -Transition to ExStart state.
LSA Types
1-11
Link-State Operation
1. Establish Neighbor Adjacencies 2. Exchange Link-State Advertisements 3. Build the Link State Database 4. Execute the SPF Algorithm 5. Choose the Best Route
What is the correct order in the steps for Link-State operation?
1. Establish Neighbor Adjacencies 2. Exchange Link-State Advertisements 3. Build the Topology Table 4. Execute the SPF Algorithm 5. Choose the Best Route
The OSPF hello timer has been set to 15 seconds on a router in a point-to-point network. By default, what is the dead interval on this router?
60 seconds. The dead interval is calculated as 4 times the hello interval.
SPF tree
A digest of the SPF algorithm, best routes are offered to the IP routing table.
What is an OSPF area?
A group of routers that share the same link-state info in their LSDBs.
What is a link
A link is interface on a router, a link is also a network segment that connects two routers, or a stub network such as an Ethernet LAN that is connected to a single router.
OSPFv2
A version of OSPF protocol that supports IPv4 only
LSA Type 6
Acknowledges the other packet types
Which of the following OSPF components is associated with the neighbor table?
Adjacency database
Exchange Link-State Advertisements
After adjacencies are established, routers then exchange link-state advertisements (LSAs). Each router is flooded with LSAs. LSAs contain the state and cost of each directly connected link.
Building the Link Database
After the routers receive their LSAs, the OSPF-enabled routers build the topology table (LSDB) based on the received LSAs. This database eventually holds all the information about the topology of the area.
Single Area OSPF
All routers are in one area. Best practice is to use area 0.
Which statement is correct about multiarea OSPF?
Arranging routers into areas partitions a large autonomous system in order to lighten the load on routers.
Why was OSPF developed?
As an alternative for the distance vector Routing Information protocol. OSPF is a link-state routing protocol.
LSA Type 5
Autonomous System External LSAs
Database Description (DBD)
Checks for database synchronization between routers
The Need for A DR (1 of 2)
Creation of multiple adjacencies - Ethernet networks could potentially interconnect many OSPF routers over a common link.
Synchronizing Databases
Decide the First Router, Exchange DBDs, Send an LSR
LSA Type 7
Defined for Not-So-Stubby Areas
Which of the following OSPF components is responsible for computing the cost of each route?
Dijkstra's algorithm
What are hello packets used for?
Discover OSPF neighbors and establish neighbor adjacencies. Advertise parameters on which two routers must agree to become neighbors. Elect the DR and BDR on multiaccess network like Ethernet.
Hello pakcet
Discovers neighbors and builds adjacencies between them.
During this OSPF state, no Hello packets are received?
Down State
What are the steps to establish neighbor adjacencies?
Down State, Init State, Two Way State, and Elect the DR and BDR
What are the operational states of an OSPF network?
Down State, Init State, Two-Way State, ExStart State, Exchange State, Loading State, Full State
Exchange the DBDs (exchange state)
During this time the router exchanges Database Description packet between each other. R2 sends a DBD packet to R1 and when R1 receives a DBD, it acknowledges the receipt of using LSAck. R1 sends DBD packets to R2, R2 acknowledges R1 R2 - DBD packet - R1 R1 - Receives with LsAck- R2. R1 - DBD Packet - R2 R2 - Receives with LsAck - R1
Why do we need multiarea OSPF?
Every time a new router receives information about the topology change. New devices addition etc. It has to rerun the SPF algorithm, create a new tree, update the routing table, this can be really intensive with the CPU. Therefore arranging the routers into different areas is effective.
During this OSPF state on point-to-point networks, the routers decide which router initiates the exchange of DBD packets
ExStart State
During this OSPF state, router send each other DBD packets
Exchange State
The Need for a DR (2 of 2)
Extensive flooding of LSA - Link-state routers flood their LSAs any time OSPF is initialized, or when there is a change in the topology. This becomes excessive.
LSA Type 8
External Attributes LSA for Border Gateway Protocol (BGP)
OSPF advantages:
Faster convergences, scales to larger network implementation. Divided the routing domain into distinct area that help control routing update traffic.
Which of the following OSPF components is associated with the routing table?
Forwarding database
At which OSPF state are neighbor routers converged and able to exchange routing updates?
Full
During this OSPF state, routers have converged link-state databases.
Full State
Types of OSPF pakcets
Hello packet, DBD packet, LSR packet, LSU packet, LSAck packet.
Init State
Hello packets are received from the neighbor. They contain the Router ID of the sending router.
Routing Message
Hello packets, Database Description packet, Link-state request pkt, link-state update packet, link-state acknowledgement packet.
What is the order of packet types used by an OSPF router to establish convergence?
Hello, DBD, LSR, LSU, LSAck
What order does the election go for BR and BDR?
Highest Router ID, Highest IP address, Highest Loopback address.
Two-Way State
In this state, communication between the two routers is bidirectional. On multiaccess links, the routers elect a DR and a BDR.
An OSPF router enters this state when it has received a Hello packet from a neighbor, containing the sending Router ID
Init State
LSA Type 9
Intra-Area Prefix LSAs
OSPF ipv4 security vs OSPF ipv6 security
Ipv4 ospf uses md5 authentication between two neighboring OSPF routers. Ipv6 ospfv3 does not include any authentication capabilities of its own but uses IPsec
What is a feature of the OSPF routing protocol?
It scales well in both small and large networks
How long is a Router ID, and why do we need one?
It's about 32 bits and the reason for a Router ID is because we need to identify them uniquely.
Loading State
LSR and LSU are used to gain additional route information. Routes are processed using the SPF algorithm.
Which OSPF packet contains the different types of link-state advertisements?
LSU
Which of the following OSPF components is associated with the topology table?
Link-State database
During this OSPF state, routes are processed during the SPF algorithm
Loading State
LSA Type 2
Network LSAs
Down State
No Hello packets are received and they are donw.
Establishing Neighbor Adjacencies
OSPF enabled router sends out hello packets to see if other routers have activated OSPF routers.
Multiarea OSPF
OSPF is implemented using multiple area, in a hierarchical fashion. All areas must connect to the backbone area (area 0). Router interconnecting the areas are referred to as Area Border Routers (ABR)
A router is participating in an OSPFv2 domain. What will always happen if the dead interval expires before the router receives a hello pakcet from an adjacent DROTHEE OSPF router?
OSPF will remove that neighbor from the router link-state database.
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. 1. DBD(R2), LSAck(R1 acknowledges R2), DBD (R1 gives R2 information), LSAck(R2 acknowledges the information), LSR, LSU, LSAck.
LSA Type 10, 11
Opaque
Down State to Init State
R1 sends a hello packet to another router. And discover OSPF neighbor to develop adjacencies with.
The Init State
R2 receives the hello packet from R1 and adds the information from the hello packet with RouterID to its neighbor list. R2 then sends it's hello packet containing it's own routerID of R1 and R2, and it's neighbor interfaces.
Link-State Request (LSR)
Request specific link-state records from router to router. Basically ask for more information from the database description, by sending a LSR.
RIP vs OSPF
Rip is only local knowledge, while OSPF is more about the whole global network.
Which of the following is used with the Hello Packet to uniquely identify the originating router?
Router ID
LSA Type 1
Router LSAs
Exchange state
Routers exchange DBD packets. If additional router information is required then transition to loading; otherwise transition to the full state.
Execute the SPF Algorithm
Routers then execute the SPF algorithm. The gears in the figure for this step are used to indicate the execution of the SPF algorithm
SPF algorithm
SPF algorithm creates a SPF tree by placing each router at the root of tree and calculating the shortest path to each node.
Choose the Best Route
SPF tree is built, the best paths to each network are offered to the IP routing table. Routing distances are based on entries.
How can OSPF can be implemented?
Single Area OSPF, Multiarea OSPF
Advantages of OSPF
Smaller routing tables, reduced link state update overhead(costs), reduced frequency of SPF calculations.
Link state
State of link, it contains information about network prefix, prefix length and cost.
LSA Type 3 or 4
Summary LSAs
Full state
The link-state database of the router is fully synchronized.
Algorithm
The router builds a topology table using results of calculations based on the Dijkstra shortest path first (SPF) algorithm.
Which three statement describe features of the OSPF topology table?
The table can be viewed via the show ip ospf database command, it is a link-state database that represents the network topology, when converged, all routers in an area have identical topology tables.
Decide First Router (ExStart State)
The two routers decide which router will send the DBD packets first. The router with the higher router ID will be the first router to send DBD packets during the Exchange state.
What happens immediately after two OSPF routers have exchanged hello packets and have formed a neighbor adjacency?
They exchange abbreviated lists of their LSDBs
Adjacency Database
This creates the neighbor table
Forwarding database
This creates the routing table
Link-State databse (LSDB)
This creates the topology table
Link-State Update(LSU) packet
This is used to respond to link state requests and announce new information. Contains different types of Link state acknowledgement.
What is a function of OSPF hello packets?
To discover neighbors and build adjacencies between them
What are two reasons for creating an OSPF network with multiple areas?
To reduce use of memory and processor resources, reduce SPF calculation
Two Way State vs Full State
Two way state is when the routers are only configured, and that the routers are not concerned with the topology. The BR and BDR gives the router the updates. Full state means that the routers are synchronized.
During the OSPF state on multiaccess network, the routers select a Designated Router (DR) and a Backup Designated Router (BDR)
Two-Way State
Which of the following OSPF packets is responsible for establishing and maintaining adjacency with other OSPF routers?
Type 1: Hello packet
Which of the following OSPF concepts contains an abbreviated list of the LSDB of the sending router?
Type 2: DBD packet
Which of the following OSPF packets is used by routers to request more information?
Type 3: LSR pacckets
Which of the following OSPF packets is used by routers to annouce new information?
Type 4: LSU packet
Which of the following OSPF packets is used to confirm receipt of an LSA?
Type 5: LSAck packet
Send an LSR (Loading State)
Updates (LSUs) are sent only to neighbors in the following conditions When a change is perceived (incremental updates) Every 30 minutes R1 - LSR (I need info about information about the 172.16.6.0 network) - R2 R2 - LSU ( Here is the entry for the 172.16.6.0/24) - R1 R1 - LSack ( Thanks for information) - R2.
OSPFv3
Version 3 of the OSPF routing protocol. It is used to support both IPv4 and IPv6 unicast address families.
Link-State Acknowledgement (LSAck)
What an LSU is received, the router sends an LSAck to confirm receipt of the LSU. The LSAck data field is empty.
Distance vector (RIP) vs Link state routing protocol (OSPF)
While RIP does not require that much bandwidth and can rapidly deploy compared to OSPF. Remember OSPF uses link-state routing packets to flood the traffic and give information to others.
What is used to create the OSPF neighbor table?
adjacency database
What is identical on all OSPF routers within a single area?
link-state database
