Introduction to Route Preference in Juniper and JNCIA

In the world of networking, routers learn routes from various sources like OPF, BGP, and static configuration. But how does a router decide which route is better when it learns it from multiple sources? This is where route preference comes into play. In this article, we will explore the concept of route preference and its importance in root selection.

Understanding Route Preference

Route preference, also known as administrative distance, determines the level of trust placed in a routing protocol by a router. Different routing protocols have different levels of trust associated with them. Juniper refers to this as route preference. The lower the route preference value, the more trusted the routing protocol.

Let’s take an example to understand this better. Suppose we have a static route with a preference value of 5, and an SPF route with a preference value of 10. In this case, the static route will be more preferred and trusted than the SPF route. Similarly, an SPF route will be more preferred than an IS-IS route.

Here are some common route preference values:

Please note that the image above shows only a summary of the values. You can find the complete table with all the values here.

Manipulating Route Preference

One of the benefits of route preference is the ability to manipulate it to achieve the desired behavior. In the Cisco world, this is known as a floating static route. It involves assigning a static route with a different route preference than normal, making it less preferred than other routing protocols.

However, it’s important to remember that the preference of a local or direct route cannot be changed. If a routing protocol fails, all its routes are removed, and the floating static route becomes the next best option.

To manipulate the default settings, a qualified next hop can be configured instead of a regular next hop. By doing this, the route preference can be changed. The full configuration would look something like this:

routing-options {
  qualified-next-hop {
    192.168.1.1/32 {
      preference 50;
    }
  }
}

In the routing table, both routes would be valid. However, in the forwarding table, the route with the best preference would be selected. If the selected route becomes invalid, the less preferred route would be used instead.

It’s worth mentioning that the default values of static routes can also be changed under the routing options hierarchy. For example, the default metric used to choose between two routes with the same route preference can be modified. But keep in mind that configuring a qualified next hop will override these settings.

FAQs

Here are some frequently asked questions about route preference:

1. Q: What is route preference?
   A: Route preference, also known as administrative distance, determines the level of trust placed in a routing protocol by a router.

2. Q: How can route preference be manipulated?
   A: Route preference can be manipulated by assigning a different preference value to a static route, making it less preferred than other routing protocols.

3. Q: Can the default values of static routes be changed?
   A: Yes, the default values of static routes can be changed under the routing options hierarchy.

Conclusion

Understanding route preference is essential for network engineers. It helps routers make informed decisions about the best routes to choose. By manipulating route preference, engineers can customize routing behaviors to suit their specific requirements. Remember to consult the complete route preference table for further reference.

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