CCNP Certification BSCI Exam Training: ISIS Hello Types

By Chris Bryant, CCIE #12933

A major difference between link-state protocols ISIS and OSPF is that with OSPF, one hello packet is pretty much link another. While hello packets serve as keepalives for both ISIS and OSPF, ISIS actually features three different hello types...and you better know them before taking your CCNP BSCI exam! :)

An ES Hello (ESH) is send by all End Systems, and all IS devices listen for this Hello.  This is how a router (IS) discovers a host (ES).

An IS Hello (ISH) announces the presence of an IS.  An IS Hello is sent by all IS devices, and End Systems listen for these hellos.

Finally, an IS-to-IS Hello (IIH) is used by an IS to discover other ISes and to form adjacencies with them. 

An interesting side note:  A router will send an IIH to another router on the link to form or maintain an adjacency, but it will still send an ISH as well in case there are end systems located on that segment.

ISIS takes some getting used to when you first start working with it, since it's unlike any other protocol you're studied. Once you get the fundamentals down, though, you'll be comfortable with in in production networks and in the CCNP exam room!

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com