When would we use more than 1 dynamic routing protocol?

Question

I am learning about different dynamic routing protocols and I've learned that each dynamic routing protocol has an AD assigned to it which indicates the trustworthiness of the learned route from that protocol.

However, AD is only used if the routes to the same destination were learned through different routing protocols.

In what case would we use more than 1 dynamic routing protocol? Wouldn't it be enough to just use one, for example OSPF for the entire network, for example?

Answer 1

The administrative distance isn't about "trustworthiness", it's about preference - a combination of cost, delay, bandwidth, resilience, loss rate.

External routing and internal routing generally differ in the trust you put in the sources, so you use BGP as EGP and an IGP like OSPF internally.

In a grown network there may be regions with different architectures, so you may have to integrate other routing protocols, at least for some time.

Trustworthiness (=absence of security threats) plays an important role in connecting different security zones by firewalls.

Answer 2

Keep it simple stupid. The KISS principle. You only want to use what is necessary to achieve the desired outcome, anything more is called overengineering which leads to downtime and security vulnerabilities.

Dynamic routing protocols are not always necessary even in multisite networks. In practice, you will sometimes see two dynamic routing protocols running concurrently when peering with other networks. This can happen when companies merge, or when there are multiple Internet links into a company, for example.

You want to avoid dynamic routing protocols if you can. Keep it simple stupid. There is a time and place to use them. Use wisely.

Finally, it's important to note that Administrative Distance takes a back seat to the most specific route. The most specific route wins everytime.

Answer 3

As others have mentioned, you would use an IGP(like ospf, eigrp, rip, isis) for internal routing within your organization and an EGP(the only one in use is BGP) for connecting between autonomous systems(like if you wanted to connect to a carrier PE for MPLS transport to another site). But there are other scenarios where you might use multiple routing protocols, especially when talking about provider networks. Like in the example of connecting to a provider network if you have two CEs that run eBGP to the provider, they could run iBGP between them.