Introduction To ISIS Terminology Cisco CCNP / BSCI Certification

Introduction To ISIS Terminology Cisco CCNP / BSCI Certification
One of the major differences between OSPF & ISIS will be evident to you when you first begin your BSCI exam studies, & that is the terminology. ISIS uses terms that no other protocol you have studied to date uses, & learning these old terms is the first step to BSCI & CCNP exam success.

When you're studying to pass the BSCI exam & earn your CCNP certification, you're going to be introduced to ISIS. ISIS & OSPF are both link-state protocols, but ISIS works differently from OSPF. You must master these details in order to earn your CCNP.

You'll also become familiar with End Systems, referred to in ISIS as an "ES". The End System is basically an end host.

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 basically a single router. that is it.

ISIS uses both Level-1 & Level-2 Hellos, meaning that the two types of routers just mentioned cannot form an adjacency. Luckily for us, there is a middle ground, & that is the Level 1-2 router. These routers connect non-backbone areas to backbone areas. L1-L2 routers keep two separate routing tables, two for L1 routing & another for L2 routing. This is the default setting for a Cisco router, & L1-L2 routers can form adjacencies with both L1 & L2 routers.

ISIS & OSPF both use the concept of areas, but ISIS takes a different approach to this concept. ISIS routers use five different types of routing levels, according to the area a router has been placed in. Level 2 routers are connected only to the backbone & serve as a transit gizmo between non-backbone areas. Level 1 routers are totally internal to a non-backbone area.

Part of the challenge of learning ISIS is getting used to the differences between ISIS & OSPF. Keep studying the terminology, master two concept at a time, & soon you'll be a master of ISIS & a CCNP to boot!