A network includes a RIPv2 domain, an EIGRP domain, and an OSPF domain. Each pair of
routing domains has multiple routers redistributing routes between the pair of domains. The
design requires that the redistribution configuration avoid matching based on prefix/length
because of the trouble in maintaining such configurations.
Which one of the following tools can be used in all three routing domains to attempt to
prevent domain loops? (This book uses the term domain loop to refer to the long routes that
might be chosen for routes when redistribution exists–for example, a route may forward
packets from the EIGRP domain, to the OSPF domain, back to EIGRP, and then to subnet X
in the RIP domain.)

A.
Setting route tags

B.
Setting the default administrative distance differently for internal and external routes

C.
Setting administrative distance differently per route

D.
Setting metrics much higher for all external routes than for all internal routes

Explanation:
All the answers list reasonable options in some cases, but the only feature listed that is
useful with all three routing protocols is the route tag featurE. RIPv2 does not support the
concept of differentiating between internal and external routes, so the two answers that
suggest setting administrative distance (AD) based on the route type (internal or external)
could not be used in all three routing domains, as required by the question. All three routing
protocols support setting route tags and setting the AD per route; however, because RIPv2
cannot match based on the route type (internal/external), the option to set the route tags is
the only option that applies to all three routing domains.