Orhan Ergun 3 Comments

The difference between Load balancing and Load Sharing

It is important to understand the difference between load balancing and load sharing.

Routing protocols after calculating the routes from their databases , they automatically put equal cost routes into the routing table. Only exception to this behaviour is BGP.   Read more

Orhan Ergun 3 Comments

Route Redistribution Best Practices

You need route redistribution for many reasons.

In this post,the drivers for the route redistribution but more importantly the best practices for applying route redistribution will be explained in great detail. Read more

Orhan Ergun 5 Comments


Why you want to use particular first hop redundancy protocol. In this video Orhan Ergun is explaining Cisco specific HSRP ,GLBP and industry standard VRRP protocols.

What are the design considerations ?.

Orhan Ergun 5 Comments

Spanning Tree CST , PVST+ , RSTP and MST

In this video Orhan Ergun explains all the spanning tree modes and compare them from the design point of view. If you need scalability use MST , if you want fast convergence and flexibility use RSTP and so on. If you want me to share more spanning tree videos or explain specific technology please comment below.


Orhan Ergun 5 Comments

OSPF Area Border Router

OSPF ABR ( Area Border Router ) is the router that non backbone area routers use it as exit from their area. In the OSPF RFC it is stated that if one router has two interfaces on the two area it can be considered as an ABR.

But in the implementation , in order to have an ABR , router needs to have at least one interface is connected to area 0 which is backbone area.



In the figure, R3 is the backbone router but not an ABR since it has only connection to the Area 0. In order to be an ABR , router needs to have one interface in Area 0 and other at least one interface in different area.

R2 and R6 has a connection to both Area1 and Area 0 thus they are ABR. Also R4 is an ABR since it has a connection to both Area 0 and Area 2.

ABR aggregates topology information from one area to another area. What does it mean ?

In the figure although R3 seems only one router in the backbone area, let’s consider we have other routers in the area 0 as well. Their connections to each other and metric informations is not sent to other areas by the ABR.

Instead , ABR only sends the reachability information. The routers in different areas only know that they can reach each other but with the information provided by the ABRs.

Thats why , if you design multi area OSPF network , you can’t have end to end visibility. A router can have topology information about its own area only,