Information Technology Reference
In-Depth Information
This chapter covers the link-state routing protocol OSPF. OSPF is an Interior Gateway Pro-
tocol (IGP) used within an autonomous system. OSPF is a popular standards-based proto-
col used in enterprises. OSPFv2 is used for IPv4 networks, and OSPFv3 is used for IPv6
networks. IS-IS is another link-state routing protocol, but it is not covered here because it
is no longer a CCDA exam topic.
The “BGP” section covers the characteristics and design of BGP. eBGP exchanges routes
between autonomous systems. eBGP is commonly used between enterprises and their
service providers.
The section “Route Manipulation” covers how you use policy-based routing (PBR) to
change packets' destination addresses based on policies. This section also covers route sum-
marization, filtering, and redistribution of route information between routing protocols.
The section “IP Multicast Review” covers multicast protocols such as Internet Group
Management Protocol (IGMP), Cisco Group Management Protocol (CGMP), and Protocol
Independent Multicast (PIM).
RFC 2328 defines OSPFv2, a link-state routing protocol that uses Dijkstra's shortest path
first (SPF) algorithm to calculate paths to destinations. OSPFv2 is used in IPv4 networks.
OSPF was created for its use in large networks where RIP failed. OSPF improved the
speed of convergence, provided for the use of variable-length subnet masks (VLSM), and
improved the path calculation.
In OSPF, each router sends link-state advertisements (LSA) about itself and its links to all
other routers in the area. Note that it does not send routing tables but link-state informa-
tion about its interfaces. Then, each router individually calculates the best routes to the
destination by running the SPF algorithm. Each OSPF router in an area maintains an iden-
tical database describing the area's topology. The routing table at each router is individu-
ally constructed using the local copy of this database to construct a shortest-path tree.
OSPFv2 is a classless routing protocol that permits the use of VLSMs. With Cisco routers,
OSPF also supports equal-cost multipath load balancing and neighbor authentication. OSPF
uses multicast addresses to communicate between routers. OSPF uses IP protocol 89.
This section covers OSPF theory and design concepts. It discusses OSPF LSAs, area
types, and router types. OSPF uses a two-layer hierarchy with a backbone area at the top
and all other areas below. Routers send LSAs informing other routers of the status of their
interfaces. The use of LSAs and the characteristics of OSPF areas are important concepts
to understand for the exam.
OSPFv2 Metric
