1) Administrative Distance (AD): Administrative Distance can be defined as a numerical value which tells a router how believable or trustworthy a routing protocol is incase if there are multiple routing protocols running in a router. Always the lesser AD value is preferred more. Let's say a router is running RIP (AD = 120) & EIGRP (AD = 90-internal; 170-external. Please check below for the various AD values). So, if that router sees that it has learnt about a route to network X.X.X.X/XX (X=some numerical integer) from both RIP & EIGRP, then it will place the route in its routing table, which it has learnt from EIGRP. Please note that AD value of EIGRP is less then that of RIP.
Please check the AD values (here RIP & EIGRP) in the below screenshots inside the red colored rectangles. RIP routes are denoted by 'R' and EIGRP internal and external routes are denoted by 'D' & 'D EX' respectively. The values on the left side within the bracket are the AD values.
2) Routing Metric: Metric is a value which is used to influence the routes when a router learns about more than one route to a destination network from the same routing protocol. If multiple routers advertises routes to a destination address to a router, then the route with the lowest metric is placed in the routing table of that router. Suppose, a router RA learns about a network at router RD, from two routers RB and RC. Let's say the metric to reach RD from RB (RA-->RB-->RD) is 40 and from RC (RA-->RC-->RD) is 50. Since, 40 is a lower metric, so RA will prefer the route through RB (remember, lower cost is preferred). Hence, route through RB will be placed in the routing table of RA.
Metrics used by routing protocols:
a) RIP uses hop count as its metric i.e. it will place those routes in its routing table, which has the lowest number of hop counts towards the destination address. Hop count means the number of hops or the number of routers, that a router has to cross in order to reach its destination address.
b) EIGRP uses a composite vector metric of bandwith, load, delay, reliability and MTU. By default, among these, EIGRP uses bandwith and delay only and the metric is referred to as cost. Further details, I will discuss in the EIGRP section. This is just to give a basic idea about what is routing metric and what routing protocol uses what metrics.
c) OSPF uses cost as its metric. The cost is associated with the bandwith of each outgoing interface of a router running OSPF. But cost calculation is done in a different way which will be discussed in the OSPF section.
3) Routing Table: A Routing Table typically consists of the best possible routes to a destination address. The routing table is populated by the algorithm that is specific to each of the routing protocol. For eg. Eigrp uses DUAL algorithm and OSPF uses Dijkstra's algorithm. (No need to worry about the algorithms used by the routing protocols at this stage. It will be discussed in the Routing Protocols section). A routing table basically consists of these fields: the destination address, the exit interface, the next hop address, the cost(read as metric), Administrative distance (AD) among other fields. Whenever a router receives a packet at any of its interface it will examine the destination ip address in the packet and check its routing table to see whether the destination address matches with any of the ip address in its routing table. If it matches then it will send the packet to the next hop ip address for the matched destination address. And if it doesn't, then it will drop the packet untill and unless a default route is present in that router to forward the packet to that default route.
c) OSPF uses cost as its metric. The cost is associated with the bandwith of each outgoing interface of a router running OSPF. But cost calculation is done in a different way which will be discussed in the OSPF section.
3) Routing Table: A Routing Table typically consists of the best possible routes to a destination address. The routing table is populated by the algorithm that is specific to each of the routing protocol. For eg. Eigrp uses DUAL algorithm and OSPF uses Dijkstra's algorithm. (No need to worry about the algorithms used by the routing protocols at this stage. It will be discussed in the Routing Protocols section). A routing table basically consists of these fields: the destination address, the exit interface, the next hop address, the cost(read as metric), Administrative distance (AD) among other fields. Whenever a router receives a packet at any of its interface it will examine the destination ip address in the packet and check its routing table to see whether the destination address matches with any of the ip address in its routing table. If it matches then it will send the packet to the next hop ip address for the matched destination address. And if it doesn't, then it will drop the packet untill and unless a default route is present in that router to forward the packet to that default route.
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