Which three characteristics apply to IS-IS but not to OSPF? (Choose three.)
A.
uses a default IOS metric of 10 on each interface
B.
uses an on-demand circuit to reduce the hello and LSA flooding across switched WAN
links, such as ISDN
C.
uses a DIS and a backup DIS to present the psuedo-node on the LAN
D.
encapsulates PDUs directly into a data-link frame
E.
uses stubby areas to improve network scalability
F.
runs PRC (Partial Route Calculations) to calculate IP reachability information
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 the
backbone. 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