This is Part 2 of the Router
OSPF Serieswhich will cover topics about OSPF Areasand OSPF
LSA’s. Before continuing on I recommend reading Part 1 of the Router OSPF Series which
gives an introduction on the topic.
For the ICND2 and CCNA exam you will only
need to configure a single area OSPF implementation, but you will still be
required to know many different things about a full multi-area OSPF network.
OSPF is a link-state routing protocol that
stores a complete topology map of the network it is in. In order for OSPF to
know where the shortest routes are to each network it has to calculate it off
of the topology map using the Dijkstra algorithm every time there is a change
in the network. This process can take up a lot of CPU cycles and slow down the
routing process if there are many routers. In order for OSPF to be able to
scale well in larger network situations with a lot of routers it uses Areas to
logically break up the network.
OSPF
Router Types
Backbone Router – Every OSPF implementation must have a backbone area. The
backbone area will always be area 0. A Backbone router is any router that has
an interface connected to area 0.
Area Border Router (ABR) – Are routers that have on interface in area 0 and another
interface in a different area. ABR’s sit between the two areas. All ABR’s are
backbone routers because they have an interface connected to area 0.
Autonomous System Boundary Router (ASBR) – Share information with other routers that are not running
OSPF.
Internal Router – Are routers that are not area border routers. They simply
are routers running OSPF, but are not passing information between areas.
Here is an example of a network with
multiple OSPF areas so that you can visually see the different OSPF router
roles amongst the areas.
Types of OSPF Areas
Backbone Area – This area is also known as the standard area. Every OSPF
implementation has to have one. Backbone areas accept all types of LSA’s.
Stub Area –
Will only receive summary LSA’s. Routing LSA’s are NOT allowed.
Totally Stubby Area – Absolutely no LSA’s are allowed.
Not So Stubby Area (NSSA) – This is a stub area that has a ASBR router that receives
information from another routing protocol other than OSPF and passes it into
the OSPF network.
OSPF
LSA Types
There are seven different types of
Link-State advertisements that OSPF routers pass around to check up on each
other. When you type “show ip route” on a Cisco router to see the routing
table, ospf entries will show up with the letter “o” in front. There are also
many different variations that will show up in the routing table depending on
the LSA type.
LSA Type 1: “O”
— Intra-Area — Passed around inside an area
LSA Type 2: “O
1A — Inter-area — Pass through an area
LSA Type 3: Summary
LSA by an ABR
LSA Type 4: Summary
LSA to an ASBR
LSA Type 5: “O
E1″ or “O E2″ — From an ASBR about external links
LSA Type 6: This
is a Groupe Membership Link LSA that is sent out by multicast OSPF Routers
LSA Type 7: “O
N1″ or “O N2″ — These are NSSA external routes from an ASBR
Well, I Think that is quite a bit of
OSPF information for today. Tomorrow I’m actually going to show you how to
configure OSPF since we still haven’t gone over that yet.
More
OSPF Reviews:
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