Both the hellos and the types of connections are different between ISIS and OSPF. Chris Bryant, CCIE #12933, can help you learn these important differences and pass the BSCI test.
In my last ISIS tutorial, I talked about how, even though both ISIS and OSPF are link state protocols, they work in very different ways. You'll need to know these differences to pass the BSCI test and get your CCNP. Today, we'll look at the different types of ISIS Hellos and the different types of connections that are made when these Hellos are used.
Hello packets have come up a lot when talking about ISIS, and for good reason. Hello packets are OSPF and ISIS's heartbeats. If a neighbour stops sending heartbeats, that connection will be dropped. OSPF only has one type of Hello packet, while ISIS has three.
All End Systems send an ES Hello (ESH), and all IS devices listen for it. This is how an IS router finds out who a host is (ES).
An IS Hello (ISH) lets people know that an IS is there. All IS devices send out an IS Hello, which End Systems listen for.
An IS uses an IS-to-IS Hello (IIH) to find other ISs and make connections with them.
An interesting side note is that a router will send an IIH to another router on the link to form or keep an adjacency, but it will also send an ISH in case there are end systems on that segment.
ISIS and OSPF both use the Hello packet to set up and keep track of connections. Let's look at the rules for ISIS adjacencies and the different types of adjacencies.
Since L1 Hellos and L2 Hellos are different, an L1 router must exchange Hellos with another L1 router to form an adjacency, just as an L2 router must exchange Hellos with another L2 router to form an adjacency. A connection between an L1 router and an L2 router can only be made if one of the routers is an L1/L2 router.
For an adjacency to be made, both L1 routers must be in the same area. The Hello timers and the MTU must be the same on all of the interfaces that make up the adjacency.
Isn't that a lot of L1, L2, and L1/L2? Let's look at how each type of router can connect to other types:
L1: Can connect with any other L1 in the same area, as well as any L1/L2 in the same area.
L2: Can connect to any other L2 in any area, as well as any L1/L2 in any area.
L1/L2: Can connect with any L1 in the same area, any L1/L2 in any area, and any L2 in any area.
To pass the CCNP exam, you must know how ISIS and OSPF are alike and how they are different. If you take your time and learn the basics, you'll soon have the magical letters "CCNP" after your name and on your resume.
