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Cisco CCNP / BSCI Tutorial: Comparing OSPF and ISIS Hellos |
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While studying to pass the BSCI exam and preparing to earn
your CCNP certification, you'll quickly notice that while
OSPF and ISIS are both link-state protocols, there are a lot
of differences between the two. One major difference is the
way the two protocols handle hello packets.
Hello packets are imperative to keeping OSPF and 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 running OSPF and ISIS
can see that a neighboring router is still available.
OSPF gives us some great options when it comes to keeping
routing table size down via the use of stub and total stub
areas, but to OSPF, a hello packet is a hello packet. ISIS
routers are capable of sending two different types of hellos
- Level 1 and Level 2.
ISIS routers are classified as Level 1 (L1), Level 2 (L2),
and 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 and L2 hellos.
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 is no reason to allow 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)#interface ethernet0
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 just "level-2".
This configuration would prevent L2 hellos from being transmitted
out ethernet0. While this does save router resources and prevents
unnecessary bandwidth usage, there is also no way an L2 adjacency
can be formed - so double-check your network topology before
using this command!
About the Author:
Article by Chris Bryant.
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