You need to configure two Company routers for high availability. Which protocol enables a
group of routers to form a single virtual router and use the real IP address of a router as the
gateway address?

A.
HSRP

B.
IRDP

C.
Proxy ARP

D.
GLBP

E.
VRRP

F.
None of the other alternatives apply

Explanation:
The correct answer is VRRP whereby either a virtual or physical address can be chosen as
the gateway address. If the physcial address of R2 is the gateway address, then R2 will
become the gateway. If R2 goes down, R3 (or R4 etc) will become the gateway and will
assume the IP address which, to it, will be a virtual one as none of its interfaces are
configured with that address. In this scenario. R2, R3 and R4 form one virtual router
whereby R1’s physical addres is used as the gateway address. HSRP does not use physical
addresses for the gateway at all.
Incorrect answer:
IRDP is not correct as it does not form a single virtual router. With IRDP, two possible
gateways advertise themselves to the downstream router as potential gateways, one will be
configured with a higher preference (lower IRDP preference number) than the other and so
its physical interface will be elected as the gateway address. If this router becomes
unavailable, the other router’s physical address will become the gateway address. No virtual
routers or addresses are created. For example, suppose R1 was configured for IRDP
discovery and was connected to a segment with routers R2 and R3. If R2 was configured
with irdp preference 20 and R3 was configured as irdp preference 30, R1 would select R2 as
its gateway. If R2 was unavailable, R3 would be selected. In this instance, R2 and R3 do not
communicate with each other and do not form any kind of virtal router.