Which three statements are correct about the differences in IS-IS and OSPF? (Choose
three.)
A.
IS-IS is more CPU-intensive than is OSPF.
B.
For greater fine tuning there are more IS-IS timers.
C.
IS-IS LSP contains TLV fields and OSPF LSU contains the LSAs.
D.
New additions to the protocol are easily implemented in OSPF but not with IS-IS.
E.
OSPF has more area types than does IS-IS.
Explanation:
The configuration of OSPF is based on a central backbone, Area 0, with all other areas
being physically attached to Area 0. Because of this, certain design constraints will inevitably
exist. A good, consistent IP addressing structure is necessary when this type of hierarchical
model is used. It is used to summarize addresses into the backbone, and reduce the amount
of information that is carried in the backbone and advertised across the network. In
comparison, IS-IS also has a hierarchy with Level 1 and Level 2 routers. With IS-IS, the area
borders lie on the links. However, significantly fewer link-state PDUs (LSPs) are used.
Therefore, many more routers, up to 1000, can reside in a single areA. This capability makes
IS-IS more scalable than OSPF. IS-IS allows a more flexible approach to extending thebackbone. Adding Level 2 routers can extend the backbone. This process is less complex
than with OSPF.
With regard to CPU use and the processing of routing updates, IS-IS is more efficient. Not
only are there fewer LSPs to process, as compared to OSPF LSAs, but also the mechanism
by which IS-IS installs and withdraws prefixes is less intensive.
Both OSPF and IS-IS are link-state protocols and therefore provide fast convergence. The
convergence time depends on a number of factors, such as timers, number of nodes, and
types of routers. Based on the default timers, IS-IS will detect a failure quicker than OSPF
and therefore should converge more rapidly. If there are many neighbors and adjacencies to
consider, the convergence time depends on the processing power of the router. IS-IS is
typically less CPU intensive than OSPF.
The timers in IS-IS allow more tuning than OSPF. There are more timers to adjust, and
therefore finer granularity can be achieved. By tuning the timers, convergence time can be
significantly decreased. However, this speed may be at the expense of stability, so a
compromise may have to be made. A network engineer should understand the implications
of adjusting these timers.