Networking with Cisco

Multi-area OSPF networks are widely used, in this article I am going to show some of the logic behind multi-area OSPF Networks. I will write a series of all 2 (yey!) posts about multiple area OSPF the next couple of weeks. Stay tuned in!

Single areas can be considered subsets of a larger autonomous system.

What are the benefits of splitting networks up in multiple areas?

You can solve situations like

1. Every time a route flaps, it initiates shortest-path-first algorithm calculations on all routers in that area.
   This causes high CPU utilization that could be used for something more productive.
2. The routing table is getting too large and equipment that can handle less IPv4 routes will have trouble operating.
3. The Link-state Topology table (we will get back to this) is getting unmanageable.

Terms and definitions
There are some terms and definitions that you should know:

1. Backbone area / Transit area / area 0
   This refers to the area with area id 0, which can be a group of routers acting as the main path for traffic between OSPF areas.
2. ABR
   Area Border Router, technically – a router that is connected to area 0 and at least another area, and therefore maintains two link-state databases are considered ABRs.
3. ASBR
   Autonomous System Border Router is a router that are between the OSPF network and another routing protocol network, for example BGP or IGRP.
4. IR
   Internal router, this type have all its interfaces connected to a single area.

You should be familiar with terms like LSU, LSA and the different types.

This IMPORTANT rule applies to multiarea OSPF networks:
All areas needs to be connected to area 0, if it is impossible to physically connect an area directly to area 0, you can utilize a virtual-link to create a logical path for the traffic from this area to reach the backbone area.

Different area types

1. Standard/normal area
   A default route (0/0) is generally not generated by routers in a normal area, but it can be forced with this command under router ospf

   Router( config-router)# default-information originate always

   Normal areas (like in single area setups) can receive external route information, link updates and route summaries.

2. Stub area
   While stub areas can’t receive external routes, they can receive inter-area routes, intra-area routes and default routes.

3. Totally stubby area
   This area does not receive summary routes from other areas in the network, and it does not receive external routes. To reach networks outside the area it will always use the default route (0/0)

Not So Stubby Area (NSSA)
This is a stubby area which can receive a part of external routes from outside the AS.
The LSA it can receive is Type 7 LSA.

4. Backbone area
   ..or “transit area” always has the area id 0, every other area must have a link to area 0. Either physically or via a logical ‘virtual-link’.

That was the area types, these are defined under the router ospf configuration.
So, every 30 minute all the OSPF routers floods the area with so called LSU (Link state updates) just to make sure that every router in that area agree about the link state database. These LSUs are received by the other routers and flooded across the area until all the routers agree about the current link-state database.

Network events and LSA flooding
When an event happens, for example an interface goes down; the router will send a LSA and a LSU packet to 224.0.0.6 – the multicast address for the BR and BDR – which in turn will flood this packet out on all their active interfaces on the multicast address 224.0.0.5 – which is the multicast address that all routers should listen on, and they will then do the same until the network agrees about the topology and is so called ‘converged’.

In my next post I will cover the configuration and route summarization and LSA types.

Have a nice OSPF Sunday!