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Most routing protocols have metric structures and algorithms that are not compatible with other protocols. In a network where multiple routing protocols are present, the exchange of route information and the capability to select the best path across the multiple protocols are critical.

Administrative distance is the feature used by routers to select the best path when there are two or more different routes to the same destination from different routing protocols. Administrative distance defines the reliability of a routing protocol. Each routing protocol is prioritized in order of most to least reliable (believable) using an administrative distance value.

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Before we dive deep into the selection process of route for best path, I want to start from very basics first.

Suppose we have two PCs the they are directly connected with each other.

Two PCs Connected

The network starts to grow and we have more PCs.

PCs Connected with Switch1

Now we need a switch to connect them, but still they are in a single network. Network continuously growing and they also need to access the Internet or they need to connect to the other networks as well.

PCs Connected with Router


To establish the communication between the networks, we require router. The router maintains the routing table, and places the routes or networks in the routing table.

Processes Involved In Building And Maintaining The Routing Table

There are three processes involved in building and maintaining the routing table, particularly in a Cisco router:

1. Routing protocol
2. Routing table
3. Packet Forwarding decision

Routing protocol

Router builds routing table by collecting the information about the network or networks, which can be connected directly or remotely. Who will provide the information about these networks? For the directly connected networks router need not any routing protocol, it enters these network into routing table by its own. The networks which are not directly connected, router needs to learn by manually or with the help of routing protocols.

Building the Routing Table

The main considerations while building the routing table are:
Administrative distance (AD)− This is the measure of trustworthiness of the source of the route. If a router learns about a destination from more than one routing protocol, administrative distance is compared and the preference is given to the routes with lower administrative distance. In other words, it is the believability of the source of the route.

Metrics − This is a measure used by the routing protocol to calculate the best path to a given destination, if it learns multiple paths to the same destination. Each routing protocol uses a different metric.

Prefix length

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With the help of example, we will observe how these parameters are affecting in the building the routing table and packet forwarding decisions.

Route Selection in Same Prefix Scenario1 1

In this scenario on R1-R5, configured RIP, OSPF, and EIGRP simultaneously. Let us see how the router will work in this situation. Configure a Loopback interface (Loopback 1, with IP address 1.1.1.1/24) on each router, advertise this loopback on every router and in each routing protocol and also define a static route on R1 for the same.

Routing protocol configurations and Routing tables:

We will observe only the loopback route (1.1.1.1)

Observation 1:

In the output of the R1, we can see network 1.1.1.0/24 is advertised in EIGRP, OSPF, RIP, and static route. R1 will install the directly connected 1.1.1.0/24 and 1.1.1.1/32 in its routing table, reason is that AD of the directly connected route is lowest which is 0.

R1 Routing table
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R2 is receiving the network 1.1.1.1 via EIGRP as 1.1.1.0/24 and via OSPF as 1.1.1.1/32. For R2 1.1.1.0/24 and 1.1.1.1/32 are the different routes, so it includes both are advertised by two different routing protocols.

R2 Routing table


R3 installed the directly connected 1.1.1.0/24 and 1.1.1.1/32 in its routing table.

R3 Routing table

R4 installed the directly connected 1.1.1.0/24 and 1.1.1.1/32 in its routing table.

R4 Routing table

R5 installed the directly connected 1.1.1.0/24 and 1.1.1.1/32 in its routing table.

R5 Routing table

Observation 2:

In the next steps, get ready to see the magic. R1 placed the 1.1.1.1/32 and 1.1.1.0/24 directly connected routes in the routing table. On R1 we also have configured static route for 1.1.1.0/24, turn off the loopback 1 on R1. Now R1 will give the entry to the 1.1.1.0/24 route learn by static route, due to the AD is lower than the any other routing protocol.

R1 Routing table static Route

Observation 3:

If we remove the static route on R1, it will place the 1.1.1.1/32 longest prefix length route in the routing table, which is learn by OSPF.

READ MORE:
http://luminisindia.com/it-networking-blog/156-how-does-route-selection-works-in-cisco-routers