OSPF: A Comprehensive Guide
For which IP networks is OSPF version 2 designed for?
The current version 2 (RFC 2328) is designed for IPv4.
What are the major advantages of OSPF compared to distance vector protocols?
The major advantages of OSPF as well as all link-state protocols are: fast convergence, scalability to larger networks, less susceptibility to bad routing info.
What are the main features of OSPF?
- Use of areas: reduce the impact on memory and CPU, contains the flow of routing protocol traffic, and make feasible the construction of hierarchical topologies;
- Fully classless: eliminates discontinuous subnets problems, support VLSM, and supernetting for efficient address management.;
- A dimensionless arbitrary metric.;
- Equal-cost load balancing for more efficient use of multiple paths.;
- Use of multicast reserved addresses for non-OSPF speaking devices.;
- Support of authentication for more secure routing.;
- Use of route tagging for the tracking of external routes.;
In which data structure are the LSAs recorded?
Each router receiving an LSA from a neighbor records the LSA in its Link-State Database. Due to the flooding process, all routers within an area will build identical link-state databases.
By which mean is a Router uniquely identified within the OSPF domain? How is such identity derived?
Each router has a Router ID, which is an IP address. Cisco routers derive their IDs by the following means: from a manual configuration (router-id command), or the numerically highest IP address of any of its loopback interfaces (in the absence of manual configuration), or the numerically highest IP address of its physical interfaces (if no loopback interfaces). In the former case, the interface from which the Router ID is taken does not have to be running OSPF.
What are the advantages of using loopback interfaces in OSPF?
Using addresses associated with loopback interfaces is more stable than any physical interface. It is active when the router boots up, and only fails if the entire router fails. In addition, the network administrator has the flexibility to assign recognizable addresses as the Router IDs.
What are the aims of the Hello Protocol?
- Neighbor discovery;
- Advertisement of several parameters on which two routers must agree before the can become neighbors.;
- Neighbor maintenance: Hello packets act as keepalive between neighbors;
- Bi-directional communication insurance;
- Election of Designated Routers (DRs) and Backup Designed Routers (BDRs) on broadcast and Nonbroadcast Multiaccess networks (NBMA).
What is the periodicity of Hello Packets?
Hello packets are periodically sent out each OSPF-enabled interface. The periodicity is defined by the HelloInterval (10 second by default) and it is configured on per interface basis.
When does an OSPF router declare its neighbor as dead?
If a router has not received a Hello packet from a neighbor within a period of time known as RouterDeadInterval (40 seconds), it declares the neighbor down. This interval could be set by the (ip ospf dead-interval command)
What is the information enclosed in a Hello Packet?
A Hello packet contains: the Router ID of the originator, the Area ID of the originating router interface, the address mask of the originating interface, the authentication type, the HelloInterval, the RouterDeadInterval, the Router Priority, the DR and BDR, five flags for optional capabilities, and Router IDs of the originating router’s neighbors.
What are the types of networks that OSPF define?
OSPF defines five network types: Point-to-Point networks (T1, DS-3, and SONET links), broadcast networks (Ethernet, Token Ring, and FDDI), Nonbroadcast Multiaccess networks (NBMA, such as X.25, Frame Relay, and ATM), Point-to-multipoint networks, and Virtual Links.
How are OSPF packets sent in broadcast networks, NBMA networks, and Point-to-Multipoint networks?
OSPF packets are multicasted in broadcast neworks (DR and BDR use 224.0.0.5 and all other routers use 224.0.0.6 for link-state update and link-state acknowledgment) and unicasted in NBMA and Point-to-Multipoint.
What is Router Priority?
Each multi-access interface of each router has a Router Priority (integer ranging from 0 to 255). The default priority for Cisco Routers is 1 (could be changed with ip ospf priority command). Routers with a priority of 0 are eligible to become DR or BDR.
To which values are the DR and the BDR set when an interface first becomes active?
The DR and the BDR are set to 0.0.0.0
Can a Router that has already declared itself a candidate to be DR be elected to be BDR?
No. For fault tolerance issue, these routers should be different.
In which scenario does a BDR become a DR?
If there is no active DR, the BDR is promoted to DR and a new election is held for the BDR.
Is it possible that a Router with a higher priority that becomes active after a DR and BDR have been elected could replace them? Why?
No. Once the election is effective, the DR and BDR will listen to the multicast address 224.0.0.6 and not to 224.0.0.5
What does Process ID refer to in OSPF?
Cisco Routers are capable of running multiple OSPF processes on a single interface and use Process ID to distinguish them.
What is the bandwidth of the network interface if the displayed Cost of a Cisco Router’s interface is 781?
Each interface has a Cost defined by the rule 108/Bandwidth. Therefore for this value, the correspondent bandwidth is 128K. The cost can be changed by (ip ospf cost)
For which purpose is the Cisco command auto-cost reference-bandwidth used for?
This command is used for modern media with bandwidths higher than 100M such as Gigabit Ethernet.
What are the different phases required for the establishment of adjacency?
- Neighbor Discovery;
- Bi-directional communication: established when two neighbors list each other’s Router ID in their Hello Packets).;
- Database Synchronisation: using Database Description, Link State Request, Link State Update, Link State Acknowledgment messages.;
- Full Adjacency: maintained through the exchange of Hello packets.
How are LSAs acknowledged?
The LSA may be acknowledged either explicitly (Link-State Acknowledgment), or implicitly (reception of a Link-State Update that contains the same instance of the LSA).
How is the transmission of DD packets (Database Description) controlled between two adjacent routers?
One router is elected as Master, the other as Slave. Only the Master controls the synchronisation process and ensures that only one DD packet is outstanding at a time
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When does the synchronisation process between two adjacent routers end? The Master knows that the synchronisation process is complete when it has sent all the DD packets necessary to fully describe its link-state database and has received a DD packet with the M-bit set to 0.
What are the LSA fields used to ensure that a router has an up-to-date copy of the LSA in its database? Each LSA contains three values that are used to ensure that the most recent copy of the LSA exists in every database: sequence number, checksum, and age. What is an area in OSPF? An area is a logical grouping of OSPF routers and links that effectively divide OSPF domain into-subdomains. Each area is identified by an Area ID that may be expressed either as a decimal number of in dotted decimal (both supported by Cisco)
What does Area 0.0.0.0 refer to? Area ID 0 or (0.0.0.0) is reserved for the backbone.
What is a Partitioned Area? A partitioned area is an area in which a link failure causes on part of the area to become isolated. Traffic between partitioned areas goes through the Backbone.
What are the main classes of OSPF routers? All OSPF routers can be classified as: Internal Router, a Backbone Router, an Area Border Router (ABR), or an Autonomous System Boundary Router (ASBR).
What is a virtual link in OSPF? For which purposes it is used? A virtual link is a link to the backbone through non-backbone area. It is used either to link an area to the backbone through a non-backbone area, or to reconnects a partitioned backbone through a non-backbone area. In both cases, the virtual link is not associated with a particular physical link. It is a tunnel trough which packets may be routed on the optimal path from one endpoint to another.
What are the different types of LSA? OSPF uses 11 types of LSA : Router LSA, Network LSA, Network Summary LSA, ABSR Summary LSA, AS External LSA, Group Membership LSA, NSSA External LSA, External Attributes LSA, and Opaque LSA (with three sub-types: link-local scope, area-local-scope, and AS scope).
Define what are Opaque LSAs? Opaque LSAs are a class of LSAs that consist of a standard LSA header followed by application-specific information (RFC 2370). The information field can be used directly by OSPF or indirectly by other applications to distribute information through the OSPF domain.
What is the main area of application of Opaque LSAs? Opaque LSAs have been used to add various extensions to OSPF, such as traffic engineering parameters for Multiprotocol Label Switching Networks (MPLS)
What is the Cisco command that displays at one the number of LSAs in link-state database by area and by LSA type? The command is show ip ospf database database-summary.
What are the main types of OSPF areas? There three main areas: Stub Area (into which AS External are not flooded, virtual links cannot be configured within, nor transit, a stub area), Totally Stubby Area, Not-So-Stubby Area.
What are the different types of Paths in OSPF? Each route to a network destination is also classified as one of the four path types: Intra-area paths, Inter-area paths, Type 1 External paths (E1), and Type 2 External paths (E2).
What kind of authentication does OSPF use? simple passwords / MD5 cryptographic checksums.