Cisco CCNP / BSCI Certification: Introduction To ISIS Terminology

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!

Cisco CCNP / BSCI Exam Tutorial: 10 ISIS Details You Must Know!

Earning your CCNP certification and passing the BSCI exam depends on knowing the details of many Cisco technologies, ISIS chief among them. To help you prepare for exam success, here's a list of ISIS terminology and basic concepts that will help you pass this tough exam. Enjoy!

ISIS Terms:

Domain: section of the network under common administrative control

Area: logical segment of the network composed of contiguous routers and their data links

Intermediate System: A router.

End System: A host device.

The four levels of ISIS routing:


Level 0: ES-IS routing in the same subnet.

Level 1: IS-IS routing in the same area.

Level 2: IS-IS routing in the same domain.

Level 3: Inter-domain routing performed by InterDomain Routing Protocol (IDRP).

ISIS Adjacency Possibilities:


L1: Can form adjacency with any L1 in the same area and any L1/L2 in the same area.

L2: Can form adjacency with any L2 in any area, and with an L1/L2 in any area.

L1/L2: Can form adjacency with any L1 in the same area, L1/L2 in any area, and L2 in any area.

A router interface’s SNPA (Subnetwork Point Of Attachment) is its highest DLCI number if it’s on a Frame network, and its MAC address if the interface is on an Ethernet segment.

ISIS Hello Types:

ESH: ES Hello – Sent by End Systems to discover a router.

ISH: IS Hello – Send by Intermediate Systems to announce their presence. End Systems listen for these.

IIH: IS-to-IS Hello – Send by one IS to be heard by another IS. These hellos makes IS-IS adjacencies possible.

Best of luck on your CCNP exams!

Cisco CCNA / CCNP Certification Tutorial: Frame Relay End-To-End Keepalives

One of the first things you learned about Frame is that the LMI also serves as a keepalive, or a heartbeat - and if three consecutive LMIs are missed, the line protocol goes down. There's a limitation to LMI as a keepalive, though. The LMI is exchanged only between the DTE and the closest DCE. The LMI is therefore a local keepalive that does not reflect any possible issues on the remote end of the virtual circuit.

Taking the LMI concept to the next logical level, Frame Relay End-To-End Keepalives (FREEK, one of the least-heard Cisco acronyms for some reason) are used to verify that endpoint-to-endpoint communications are functioning properly.

What you have to keep in mind about FREEK is that each and every PVC needs two separate keepalive processes. Remember, with a PVC, there's no guarantee that the path taking through the frame relay cloud to get from R1 to R2 is going to be the same path taken to go back from R2 to R1. One process will be used to send requests for information and handle the responses to these requests; this is the send side. When the send side transmits a keepalive request, a response is expected in a certain number of seconds. If one is not received, an error event is noted. If enough error events are recorded, the VC's keepalive status is marked as down.

The process that responds to the other side's requests is the receive side.

This being Cisco, we've got to have some modes, right? FREEK has four operational modes.

Bidirectional mode enables both the send and receive process enabled on the router, meaning that the router will send requests and process responses (send side) and will also respond to remote requests for information (receive side).

Request mode enables only the send process. The router will send requests and process responses to those requests, but will not answer requests from other routers.

Reply mode enables only the receive process. The router will respond to requests from other routers but will initiate no requests of its own.

Finally, passive reply mode allows the router to respond to requests, but no timers are set and no events are tracked.

Frame Relay End-To-End Keepalive defaults:

Two send or receive errors must be registered in order for the VC to be considered down.

The event window size is three. The event window is the number of events considered by the router when determining the status of the VC. Therefore, using the defaults, two send or receive errors would have to be received within the event window of three events for the VC to be considered down.

The timer mentioned earlier - the amount of time a router waits for a response - is set to 10 seconds

Working with Frame Relay end-to-end keepalives is just one Frame skill you’ll need to pass the CCNP exams – and I wouldn’t be surprised to see them on a CCIE exam. Know the details and you’re on your way to Cisco certification exam success!