OSPF PART 3

Difference between AS and Area concept

Area concept is a feature of OSPF. It is limited only with OSPF. We cannot use it with other routing protocol.
AS is an independent concept originally defined in RFC 1771. We can use it with any routing protocols which understand its concept.

 

AS

An AS is a group of networks running under a single administrative control. This could be our company or a branch of company. Just like Subnetting AS is also used to break a large network in smaller networks.
AS creates a boundary for routing protocol which allow us to control how far routing information should be propagated. Beside this we can also filter the routing information before sharing it with other AS system. These features enhance security and scalability of overall network.
Basically AS concept was developed for large networks. Routing protocols which were developed for small networks such as RIP do not understand the concept of AS systems.


Public AS Numbers: - We only need to use public numbers if we are going to connect our AS with Internet backbone through the BGP routes from Internet. IANA (Numbers Authority) controls the public AS numbers.

Private AS Numbers: - Private AS numbers are used to break our internal network into the smaller networks. We can use any valid AS number in our network unless we connect it with public network. In above example we used two AS numbers 10 and 20 to divide our company network.

IMPORTANT POINT

1 IN EIGRP TWO AS CAN NOT COMMUNICATE EACH OTHER BY DEFAULT WE HAVE TO DO REDISTRIBUTION
2 IN OSPF TWO AS CAN COMMUNICATE EACH OTHER WITHOUT ANY PROBLEM OR REDISTRIBUTION

MULTI AREA OSPF :-

OSPF areas

OSPF uses the concept of areas. An area is a logical grouping of contiguous networks and routers. All routers in the same area have the same topology table, but they don’t know about routers in the other areas. The main benefits of creating areas is that the size of the topology and the routing table on a router is reduced, less time is required to run the SFP algorithm and routing updates are also reduced.  

Each area in the OSPF network has to connect to the backbone area (area 0). All router inside an area must have the same area ID to become OSPF neighbors. A router that has interfaces in more than one area (area 0 and area 1, for example) is called Area Border Router (ABR). A router that connects an OSPF network to other routing domains 

OSPF Multi Area LAB