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Route Precedence — How does a Router choose its preferred path?

Whenever possible, Routers should have multiple paths (or routes) to get to a target network — this is a key component for network resiliency. Which means Routers routinely compare routes to choose one preferred path among multiple that may exist. The Route Precedence process is accomplished by comparing three different attributes.

This video steps through the three step process sequentially, and also shows you what a Router does when all the attributes are identical.

The three attributes for Route Precedence are the Route Specificity, the Administrative Distance, and the Metric. If multiple routes exist and all three of these attributes are identical, the Router will load balance across the available paths.

The topology involves R1 with four possible paths (through R2, R3, R4, and R5) to get to the 9.9.9.128/25 network behind R6. Each path is sequentially enabled to show you exactly what R1 is comparing to choose one best path among the multiple that will exist.

Load Balancing is the first scenario illustrated in the video. Initially, the OSPF paths from R1 to 9.9.9.128/25 through R5 and R4 are identical, and R1 will load balance its packets across both links.

Then, we’ll tweak the Metric on the link between R5 and R6 and show you that R1 still knows of both the paths through R4 and R5, but chooses the path with the lowest Metric.

Following that, we’ll enable the advertisement from R3 via EIGRP and show you how R1 compares the Administrative Distance to choose between an EIGRP path or an OSPF path.

And finally, we’ll enable the advertisement from R2 via RIP to illustrate what occurs when the Router is faced with delivering a packet which matches multiple routes of different specificity — in this case, a /25 route compared to a /24 route.

The video finishes with two Bonus Questions, based upon an additional scenario of adding a Static Route to R1 for the 9.9.9.0 /24 network through R5. The bonus questions are:

  • How will R1’s Routing Table change?
  • What path will R1 choose to reach 9.9.9.129?

I’ll answer the questions officially in another video.

This video explains the knowledge necessary for these CCNA exam objectives:

3.2 Determine how a router makes a forwarding decision by default
3.2.a Longest match
3.2.b Administrative distance
3.2.c Routing protocol metric

Comments

  1. Jon Pinkley says

    Ed,

    Thanks for the coverage of this important topic.

    The video and the text here are not consistent for the description of the static route added for the bonus question, and it is a significant difference.

    The text here has this:
    “The video finishes with two Bonus Questions, based upon an additional scenario of adding a Static Route to R1 for the 9.9.9.128/25 network through R5.”

    The video has this:
    R1(config)# ip route 9.9.9.0 255.255.255.0 10.1.5.5

    i.e. 9.9.9.0/24, and I think the /24 is what you were meaning to reinforce.

    The two static routes will have different effects.

    Third bonus question. If you first add the /24 static route, how does this change effect round trip routing? I.e. what does extended ping with record route display?

    Forth bonus question. If you now add an additional 9.9.9.128/25 route, how does this change effect round trip routing? I.e. what does extended ping with record route display?

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