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:star:Module 1: Single-Area OSPFv2 Concepts :star:, Link-State Operation:…
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Link-State Operation: To maintain routing information, OSPF routers complete a generic link-state routing process to reach a state of convergence. The following are the link-state routing steps that are completed by a router:
Establish Neighbor Adjacencies
Exchange Link-State Advertisements
Build the Link State Database
Execute the SPF Algorithm
Choose the Best Route
Exchange Link-State Advertisements: After adjacencies are established, routers then exchange link-state advertisements (LSAs). LSAs contain the state and cost of each directly connected link.
Establish Neighbor Adjacencies:
OSPF-enabled routers must recognize each other on the network before they can share information. An OSPF-enabled router sends Hello packets out all OSPF-enabled interfaces to determine if neighbors are present on those links.
Build the Link State Database: After LSAs are received, 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.
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.
Choose the Best Route: After the SPF tree is built, the best paths to each network are offered to the IP routing table. The route will be inserted into the routing table unless there is a route source to the same network with a lower administrative distance, such as a static route
Multiarea OSPF: With multiarea OSPF, one large routing domain can be divided into smaller areas, to support hierarchical routing. Routing still occurs between the areas (interarea routing), while many of the processor intensive routing operations.
The hierarchical-topology design options with multiarea OSPF can offer the following advantages.
Smaller routing tables - Tables are smaller because there are fewer routing table entries. This is because network addresses can be summarized between areas. Route summarization is not enabled by default.
Reduced link-state update overhead - Designing multiarea OSPF with smaller areas minimizes processing and memory requirements.
Reduced frequency of SPF calculations - Multiarea OSPF localize the impact of a topology change within an area.
All routing protocols share similar components. They all use routing protocol messages to exchange route information.
Routing Protocol Messages : Routers running OSPF exchange messages to convey routing information using five types of packets. These packets, as shown in the figure, are as follows:
Hello packet
Database description packet
Link-state request packet
Link-state update packet
Link-state acknowledgment packet
Data Structures: OSPF messages are used to create and maintain three OSPF databases, as follows:
Adjacency database - This creates the neighbor table.
Link-state database (LSDB) - This creates the topology table.
Forwarding database - This creates the routing table.
OSPF is a link-state routing protocol that uses the concept of areas. A network administrator can divide the routing domain into distinct areas that help control routing update traffic. A link is an interface on a router.
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Single-Area and Multiarea OSPF: To make OSPF more efficient and scalable, OSPF supports hierarchical routing using areas. An OSPF area is a group of routers that share the same link-state information in their LSDBs. OSPF can be implemented in one of two ways, as follows:
Single-Area OSPF - All routers are in one area. Best practice is to use area 0.
Multiarea OSPF - OSPF is implemented using multiple areas, in a hierarchical fashion. All areas must connect to the backbone area (area 0). Routers interconnecting the areas are referred to as Area Border Routers (ABRs).
1.1.6 OSPFv3: OSPFv3 is the OSPFv2 equivalent for exchanging IPv6 prefixes. Similar to its IPv4 counterpart, OSPFv3 exchanges routing information to populate the IPv6 routing table with remote prefixes
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