CCNP Route Study Guide: OSPF LSA Type 3 aka Summary

Welcome back to my CCNP Route Study Guide blog series! We've come to the OSPF LSA Type 3 LSAs, aka Summary LSAs. It's in the name, a summary LSA holds summary information of an area - which is then passed by ABRs to another area. A few things to know, though. A summary LSA does not hold topological information, it is simply, as the name suggests, a summary of the network area - in this case, a list of subnets that exist on a certain area. Type 1 and Type 2 LSAs are exchanged within an area Read more [...]

OSPF Type 2 LSA: Network

OSPF bases the need to use a Type 2 Network LSA on the existence of a DR or designated router on a LAN segment. When a link has no DR present, that link is considered a connection to a stub network. But when there is a DR elected, OSPF assumes that it is a multi-access network and thus creates a Type 2 Network LSA for that segment. Designated Router What is this DR, anyway? We will know by looking at what functions it does, which includes: Create Type 2 Network LSAs and flood it to other routers Read more [...]

OSPF Type 1 LSA: Router

A router LSA is generated by an OSPF router in an area, for an area. An ABR (with more than one area) has one Type 1 LSA for each areas it is attached to. Type 1 LSAs are used by the routers in an OSPF area to identify each other. When a router creates its Type 1 LSA, it then floods it to the other routers in the area by sending it to its neighbors and the neighbors to their neighbors. An area’s LSDB has a Type 1 LSA for each router in the area. Other than the RID of the router, a Type 1 LSA also Read more [...]

OSPF Link State Database and LSAs

OSPF Routers know the exact topology of the area in which it connects to. This is thanks to the information it receives from other routers via LSAs (Link State Advertisements) that the router uses as guides to creating its network diagram, from its point of view. Like pieces of a puzzle, LSAs are used by routers to examine the network, trace a diagram and complete a network map starting from its location to the ends of the area. There are different kinds of LSAs and knowing them will help us understand Read more [...]

OSPF Neighbors And Adjacencies (WANs)

The same basic requirements for LAN neighborships are required for OSPF WAN neighborships. However, two factors affect how OSPF operates on WAN links. These are the OSPF Network Type and the WAN service used. We are going to dig into both and find out how OSPF works on WANs. OSPF Network Types OSPF uses Network Types as interface configuration settings, to select the OSPF features that will be allowed or enabled on an interface. For example, a certain network type, called broadcast, defines Read more [...]