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Cisco CCNP / BSCI Certification: Introduction To ISIS Terminology |
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When you're studying to pass the BSCI exam and earn your
CCNP certification, you're going to be introduced to ISIS.
ISIS and OSPF are both link-state protocols, but ISIS works
quite differently from OSPF. You must master these details
in order to earn your CCNP.
One of the major differences between OSPF and ISIS will be
evident to you when you first begin your BSCI exam studies,
and that is the terminology. ISIS uses terms that no other
protocol you've studied to date uses, and learning these new
terms is the first step to BSCI and CCNP exam success.
First off, what does "IS" stand for in "ISIS"?
It stands for "Intermediate System", which sounds
like a group of routers. As opposed to Autonomous Systems,
which are logical groups of routers, an Intermediate System
is simply a single router. That's it.
You'll also become familiar with End Systems, referred to
in ISIS as an "ES". The End System is simply an
end host.
ISIS and OSPF both use the concept of areas, but ISIS takes
a different approach to this concept. ISIS routers use three
different types of routing levels, according to the area a
router has been placed in. Level 2 routers are connected only
to the backbone and serve as a transit device between non-backbone
areas. Level 1 routers are totally internal to a non-backbone
area.
ISIS uses both Level-1 and Level-2 Hellos, meaning that the
two types of routers just mentioned cannot form an adjacency.
Luckily for us, there is a middle ground, and that is the
Level 1-2 router. These routers connect non-backbone areas
to backbone areas. L1-L2 routers keep two separate routing
tables, one for L1 routing and another for L2 routing. This
is the default setting for a Cisco router, and L1-L2 routers
can form adjacencies with both L1 and L2 routers.
Part of the challenge of learning ISIS is getting used to
the differences between ISIS and OSPF. Keep studying the terminology,
master one concept at a time, and soon you'll be a master
of ISIS and a CCNP to boot!
About the Author:
Article by Chris Bryant.
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