Original subtitles

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However, what happens when the subnet masks are different?

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Let’s expand on our previous example

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in this example, we have 4 routers

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router 4 is learning about network 10.1.1.0/27

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from router 1 via RIP version 2

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router 4 is learning about network 10.1.0.0/16 from router 2 via BGP

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and router 4 is learning about network 10.0.0.0/8 from router 3 via OSPF.

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Notice the administrative distances RIP is 120

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BGP is 200 in this example, OSPF is 110

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which way will router 4 send traffic if you ping 10.1.1.1 on router 4.

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Will it go to router 1? Because of the routes received via RIP

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or will it let go to router 2 or will it go to router 3?

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Notice OSPF has a lower administrative distance than RIP

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and RIP has a lower administrative distance than BGP.

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Which path is chosen by router 4?

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The important lesson here is that the length of the route

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takes precedence over administrative distance.

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In other words, administrative distance is only used

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if multiple routing protocols are trying to put

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the same route into the routing table.

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Router 4 is learning about network 10.1.1.0/27

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10.1.0.0/16 and 10.0.0.0/8

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the router will see this as 3 separate and distinct routes.

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the 3 routes will be put into the routing table as they are seen as separate routes

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but out of the 3 of them, network 10.1.1.0/27 is chosen as it has the longest match.

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So, in other words, the ping from router 4 will go via router 1

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because it’s the best match or longest match in the routing table

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even though RIP has a high administrative distance than OSPF

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longest match routes take precedence over administrative distance.