2) What is routing & routed protocol?
A routed protocol is a Network Layer protocol that is used to move traffic between networks. IP, IPX, and AppleTalk are all examples of routed protocols. Routed protocols allow a host on one network to communicate with a host on another, with routers forwarding traffic between the source and destination networks.
Routing protocol serve a different purpose. Instead of being used to send data between source and destination hosts, a routing protocol is used by routers to exchange routing information with one another. For example, if we want our routers to dynamically “learn” about networks from one another, we configure them with a common routing protocol such as RIP or IGRP. Routers use routing protocols to exchange information about the networks they are aware of. In other words, routing protocols allow routers to “talk” to one another.
3) What is AD ?
Administrative distance is the first criterion that a router uses to determine which routing protocol to use if two protocols provide route information for the same destination. Administrative distance is a measure of the trustworthiness of the source of the routing information. Administrative distance has only local significance, and is not advertised in routing updates.
Note: The smaller the administrative distance value, the more reliable the protocol
OR
Administrative distance is the measure used by Cisco routers to select the best path when there are two or more different routes to the same destination from two different routing protocols. Administrative distance defines the reliability of a routing protocol. Each routing protocol is prioritized in order of most to least reliable using an administrative distance value. A lower numerical value is preferred, e.g. an OSPF route with an administrative distance of 110 will be chosen over a RIP route with an administrative distance of 120
Directly connected - 0
Static with next hop- 1
IGRP Summary - 5
External BGP 20
EIGRP 90
IGRP 100
OSPF 110
RIP 120
External EIGRP 170
IBGP 200
Unknown
4) How router select route to destination if it has multiple path?
If multiple path available then it will select with longest prefix first.
If same prefix then it will consider lowest administrative distance.
If prefix & AD also same then it will go with lowest metric.
If all three are same then it will do load balancing as per routing protocol allowed.
