How Does Route Selection Work in Cisco Routers?

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.

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.

The network starts to grow and we have more PCs.

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.

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

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

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.

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 and are advertised by two different routing protocols.

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

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

R5 installed the directly connected 1.1.1.0/24 and 1.1.1.1/32 in its 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 learned by static route, due to the AD is lower than the any other routing protocol.

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 learned by OSPF.

Observation 4:

After disabling the EIGRP, then the routes advertised by OSPF will place in the routing table, because the OSPF AD is 110 and RIP's AD is 120.

Observation 5:

If we will disable OSPF, then the router will advertise the route in routing table, which are learned by RIP.

So, we can say that when a router receives a route with the same prefix from different routing protocols, the selection of the route is dependent on Administrative Distance of the routing protocol and lower AD will be preferred.

Prefix length

Let's look at another scenario to see how the router handles another situation: varying prefix lengths. Assume, again, that each router has running EIGRP, OSPF, and RIP configured 1.1.1.0 on respective router as per given below:

• R3--EIGRP (internal): 1.1.1.0/26
• R4--RIP: 1.1.1.0/24
• R5--EIGRP: 1.1.1.0/19

Q. Which of these routes will be installed in the routing table?

Since EIGRP internal routes have the best administrative distance, it's tempting to assume that the first one will be installed. However, each of these routes have a different prefix length (subnet mask), they're considered different destinations, and they will all be installed in the routing table.

Let's see how the forwarding engine uses the information from the routing table to make forwarding decisions.

Making Forwarding Decisions

Let's look at the three routes we just installed in the routing table, and see how they look on the router.

R2# show ip route

If a packet arrives on a router interface destined for 1.1.1.1, which route would the router choose?

It depends on the prefix length, or the number of bits set in the subnet mask. Longer prefixes are always preferred over shorter ones when forwarding a packet.

In this case, a packet destined to 1.1.1.1 is directed toward 23.1.1.1, because 1.1.1.1 falls within the 1.1.1.0/26 network (1.1.1.0 to 1.1.1.63). It also falls within the other two routes available, but the 1.1.1.0/26 has the longest prefix within the routing table (26 bits verses 24 or 19 bits).

Likewise, if a packet destined for 1.1.1.100 arrives on one of the router's interfaces, it's forwarded to 24.1.1.1, because 1.1.1.100 doesn't fall within 1.1.1.0/26 (1.1.1.0 through 1.1.1.63), but it does fall within the 1.1.1.0/24 destination (1.1.1.0 through 1.1.1.255). Again, it also falls into the range covered by 1.1.1.0/19, but 1.1.1.0/24 has a longer prefix length.

Beyond the scope of those two, rest of the traffic will go to 1.1.1.0/19, and is forwarded towards 25.1.1.1

Route selection can also be influenced by changing in matric and it also varies as per the protocol. 

What do you think about this explanation?

Thanks.