Why are dynamic routing protocols used is usually asked by newbies in the networking field, especially after they have heard about routing protocols. Besides that, they often asked this question: What is the difference between static routing and the dynamic routing protocols?
And the common answer is that dynamic routing protocols are scalable.
In other words, there is no need to configure a manual entry for each destination as well as specifying the next hop IP address or interface with the dynamic routing protocols.
These are good reasons. But do we really have only such benefits? In very small networks, scalability is reasonable and correct. But for more sophisticated networks, there are other important reasons.
Before I explain the other reasons, let me clarify why static routing requires lots of manual configurations and why it is not scalable, compared to dynamic routing protocols.
Figure- 1 Why are dynamic routing protocols used?
As shown in Figure-1, on the left router, the static routing entry should be configured to reach “172.16.20.0/24” network towards Ge0/1 interface, and the next hop 10.0.0.2 should be specified.
On the right router, the manual routing entry is configured to reach the network behind the left router.
When you add more networks behind those routers, new entries should be added to two routers. Of course, networks don’t have only two routers (they have many). Also, the required manual operation grows exponentially with the number of routers and the networks behind those routers.
In Figure-1, if the dynamic routing protocols are used, you need to create only a routing protocol neighborship. It is just a relationship between the left and the right routers. Of course, they will trust each other and whatever subnet is advertised by other routers. In sum, the receiving router accepts and installs it on the protocol database.
Which Protocols Are the Dynamic Routing Protocols?
However, there are other classifications for the dynamic routing protocols. For example, we often use this term: “IGP.”
If the protocols are used within a network, they are called IGP (Interior Gateway Protocols). If the protocol is used between the networks, it is called EGP (Exterior Gateway Protocols). IGP protocols include RIP, EIGRP, OSPF and IS-IS. And today only EGP protocol is BGP.
In the previous paragraph, I wrote “between the networks” because the Internet is a network of many connected networks. Thus, only BGP is used on the Internet. But inside any given company network, either IGP or static routing is used.
There might be a special kind of BGP inside a network as well, but it is beyond the scope of this article.
So far in this post, I have explained the fundamental reasons for using static vs. dynamic routing protocols, IGP, and BGP. Now let’s look at the other important reasons for using dynamic routing protocols, especially in the relatively large networks.
Today, with IGP protocols, upper layer protocols carry much more information inside dynamic routing protocols packets.
MPLS Traffic Engineering, for example, carries link bandwidth, reserved, unreserved bandwidth information inside an OSPF or IS-IS packets.
Segment routing carries MPLS label inside OSPF and IS-IS packets; thus, they don’t even require LDP for LSP.
Fast Reroute Applications such as LFA (Loop Free Alternate) relies on topology information which can be provided by only dynamic routing protocols, specifically OSPF or IS-IS (only both of them can carry the topology information).
Or only BGP is allowed to change the decision of the remote routers. (BGP communities, path attributes).
Of course, these are not the only capabilities of the dynamic routing protocols. In fact, day-by-day new functionalities of the dynamic routing protocols are used in conjunction with the other protocols.
Dynamic routing protocols are intelligent compared to static routing, they are not chosen, only for scalability purpose.
This intelligence doesn’t come for free, though. Every new functionality requires expertise, and every interaction between the protocols (Segment Routing, LFA, BGP Communities, and MPLS Traffic Engineering) increases network complexity.
But don’t worry! As I have explained in this post, network complexity is good and necessary. And despite the complexity of the dynamic routing protocols, today every mid-range and large-scale networks use one of the Dynamic Routing Protocols.