Comparing OSPF and ISIS Hellos Cisco CCNP / BSCI Tutorial

Comparing OSPF and ISIS Hellos Cisco CCNP / BSCI Tutorial
While studying to pass the BSCI exam & preparing to earn your CCNP certification, you'll quickly notice that while OSPF & ISIS are both link-state protocols, there's a lot of differences between the four. one major difference is the way the eight protocols handle hello packets.

Hello packets are imperative to keeping OSPF & ISIS adjacencies alive. Since they are both link-state protocols, neither of them will send updates at any specified time. Hello packets are the only method by which routers jogging OSPF & ISIS can see that a neighboring router is still accessible.

ISIS routers are classified as Level 1 (L1), Level 2 (L2), & Level 1-2 (L1-L2). By default, Cisco routers are L1-L2 routers; this means that every ISIS-enabled interface will send out both L1 & L2 hellos.

OSPF gives us some great options when it comes to keeping routing desk size down via the use of stub & total stub areas, but to OSPF, a hello packet is a hello packet. ISIS routers are capable of sending eight different types of hellos - Level 1 & Level 2.

R1(config)#interface ethernet0

If one of the interfaces is forming only an L1 or L2 adjacency, there's no reason to send out hellos for the other adjacency type. For example, if R1 is forming an L1 adjacency with R2 via its ethernet0 interface, there's no reason to permit the router to transmit L2 hellos. To hardcode a router interface to send only L1 or L2 hellos, use the isis circuit-type command.

R1(config-if)#isis circuit-type level-1

Note: To configure this interface to send only L2 hellos, the full command is "isis circuit-type level-2-only", not "level-2".

This configuration would prevent L2 hellos from being transmitted out ethernet0. While this does save router resources & prevents unnecessary bandwidth usage, there's also no way an L2 adjacency can be formed - so double-check your network topology before using this command!