Cisco CCNP Certification / BSCI Exam Tutorial: ISIS Hellos And Adjacencies
In my last ISIS tutorial, I mentioned that while ISIS and OSPF are both link state protocols, their actual operation differs greatly. To pass the BSCI exam and earn your CCNP, you’ll need to know these differences! Today, we’ll take a look at ISIS Hello types and the adjacency types that form through the use of these Hellos.
Hello packets have been mentioned several times with ISIS, and with good reason. Hello packets are the heartbeat of OSPF and ISIS when heartbeats are no longer heard from a neighbor, that adjacency will be dropped. A major difference between OSPF and ISIS is that OSPF has one type of Hello packet, where ISIS actually has three!
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.
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...