CCNA Articles


Total available count: 11
Subject - Networking
Subsubject - CCNA

About Link State routing protocol (OSPF)

Link State routing protocols:

This protocol focuses on neighbor relationships rather than broadcasts. The best thing is that after initial routing table is exchanged, routers send small event based updates. Currently two link-State protocols are used namely OSPF and IS-IS (IS­IS is used in OSI protocol).

Advantages:

•  Much faster to converge
•  No routing loops (routers know everything)
•  Forces you to design your network correctly 

Disadvantages:

Demand on router resources
Requires a solid network design
Technical complexity

Talking about Link State Routing Protocol OSPF(Open Shortest Path First) is the most popular routing protocol in the world.

Route summarization: It is all about making routing tables look smaller, the larger the routing table more inefficient the router becomes. Route summarization scrimps the connected routes and then allows the router to advertise the routing table to its neighbor.

Open Shortest Path First (OSPF) AREA DESIGN AND TERMS:

  All areas must be connected to Area 0
•  All routers in n area must have the same topology table
•  Goal: Localize update within an area
  Requires a hierarchical design

An area in OSPF is a group of routers that all have the same routing information. Inside an area there are internal routers, the routers between the areas connecting them are Area Border Routers, an ABR is the only router which can summarize. In an area routers share the same subnet. An ASBR is a router which connects the OSPF network to a different network, adding to ABR ASBR is another router which can do Route Summarization.

Understanding OSPF neighbor relationship:

Hello messages are sent on chosen interfaces

Once every 10 second on broadcast or point to point networks 

Once every 30 seconds in NBMA networks.

This hello message contains all sorts of information viz.

• Router ID
• Hello and Dead timers
• Network mask
• Area ID
• Neighbors
• Router priority
• DR/BDR IP address
• Authentication password

If the underlined information does not match then the routers will not be able to form neighbor relationship.

Adjacencies:

Adjacency is the next step after the neighboring process. Adjacent routers are routers that go beyond the simple Hello exchange and proceed into the database exchange process. In order to minimize the amount of information exchanged on a particular segment, OSPF elects one router to be a Designated Router (DR), and one router to be a backup designated router (BDR), on each multi-access segment. The BDR is selected as a backup mechanism if DR goes down. The idea behind this is that routers have a central point of contact for information exchange. Instead of each router exchanging updates with every other router on the segment, every router exchanges information with the DR and BDR. The DR and BDR relay the information to everybody else.

OSPF network hierarchy:

As mentioned earlier OSPf is a hierarchical routing protocol. It enables better administration and smaller routing tables due to segmentation of entire network into smaller areas. OSPF consists of a backbone (area 0) network that links all other smaller areas within the hierarchy. The following are theImportant components of an OSPF network:

Areas
Area Border Routers
Backbone Areas
AS Boundary Routers
Stub Areas: Stub areas are areas that do not propagate AS external advertisements. By not propagating AS external advertisements, the size of the topological databse is reduced on the internal routerts of a stub area. This in turn reduces the processing power and the memory requirements of the internal routers.
• No-to-Stubby Areas: An OSPF stub area has no external routes in it. A NSSA allows external routes to be flooded within the area. These routes are then leaked into other areas. This is useful when you have a non-OSPF router conncted to an ASBR of a NSSA. The routes are imported, and flooded throughout the area. However, external routes from other areas still do not enter the NSSA.
• Totally Stubby Areas: Only default summary route is allowed in Totally Stubby Area.
• Transit Areas: Transit areas are used to pass traffic from an adjacent area to the backbone. The trffic does not originate in, nor is it destined for, the transit area.

Link-State Advertisements:

It is important to know different Link State Advertisements (LSA) offered by OSPF protocol. 

Type 1 : router link advertisements generated by each router for each area it belongs to. Type 1 LSA's are flooded to a single area only.

Type 2: Network link advertisements generated by designated routers giving the set of routers attached to a particular network. Type 2 are flooded to the area that contains the network.




Next 5 article(s)

1
Types, Advantages and Functions of Switch
2
Types of NAT (Network Address Translation)
3
Router ,Parts of Router and Access method
4
Implementation of Standard Access List and Extended Access Lists
5
About EIGRP (Enhanced Interior Gateway Routing Protocol) and Access-Lists

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