1

I am sure this has been asked SOMEWHERE but I can't find it anywhere.

Question is simple: Will R1 chose the EIGRP route to R4 over the OSPF route, even though the OSPF route is clearly the better route?

Thank you!!![EIGRP vs OSPF routes

3

You cannot compare metrics between different routing protocols.

To solve that problem, we have Administrative Distance, which gives you a relative trustworthiness of a particular routing protocol, as defined by the router vendor. In particular, Cisco defined the AD of EIGRP to be lower (more trustworthy) than that of OSPF.

It is possible to change the AD.


Without changing any defaults, the same routes as learned by EIGRP and OSPF will install the EIGRP routes in the routing table because the AD for EIGRP is lower.

3
  • Thank you! So in a real world scenario, assuming I do not own/manage anything accept R1, how would I know the different link speeds? Assuming I didn't have this picture I drew. Would I do some sort of speed test between R1-R4 on the EIGRP route, then do it again on the OSPF route and then change the AD of the OSPF route to be lower?
    – Ryan
    Feb 10 '18 at 21:29
  • In the real world, you need to either control the entire network (questions about networks that you do not directly control are off-topic here), or you need to trust what is handed to you by the administrators of the other networks. For example, the Internet is composed of different ASes communicating with each other, and you do not have any control over the other ASes. You can trust what the administrators of the other ASes send to you, or you can come to some business arrangement with them.
    – Ron Maupin
    Feb 10 '18 at 21:32
  • Excellent point, thank you. My last question would be "how can I fix the entire world of networks to avoid all these situations." (just kidding). Thank you again for your quick responses! I take CCENT in 2 days so I'm just filling in some small gaps at this point.
    – Ryan
    Feb 10 '18 at 21:35
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When two routing protocols are configured in router routing towards same destination . Router is designed to chosee routing protocol with least administrative distance (AD) value .

In this scenario OSPF & EIGRP is configured . So router picks up EIGRP because it's AD value is 90 compared to OSPF AD value 110 .

Upon chossing EIGRP protocal .Further to choose best path metric is calculated among available paths .

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  • Does your answer provide new information? Did you read the other answers before posting yours?
    – manish ma
    Jan 3 at 22:14
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A router uses an order in which the best route to the destination is chosen. The order is:

  1. Prefix-length
  2. Administrative distance (To check routes are received from different routing protocols.)
  3. Metric (If routes are received from the same routing protocol)

Let apply this order one by one to your scenario. Let us assume that R4 is advertising same route via both the routing protocols. Hence, prefix length is equal. The second step is checking the administrative distance(AD) which tells the router how "trustworthy" a protocol is. The administrative distance of EIGRP is 90 and OSPF is 110 (in Cisco), hence EIGRP is preferred (Lower AD). The metric will never be compared since the AD already decided which route should be preferred. You can manually configure administrative distance value to help the situation.

In Cisco,

router eigrp 100
distance eigrp 120 170

(First value is for internal EIGRP and second is for external EIGRP)

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  • 2
    Your order should be 1 - 3 - 2. The router compares AD after prefix length. If AD is the same, then it compares metric.
    – Ron Trunk
    Feb 11 '18 at 2:30

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