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I'm a little confused. I know how the best route makes it to the routing table, but what process does the router follow to choose a path to forward a packet if, for example, you have something like this:

O 10.0.1.0/22 [110/1] via 192.168.1.1 ...
D 10.0.1.0/23 [90/2172416] via 192.168.1.1...
S 10.0.1.0/24 [1/0] via 192.168.1.1..
R 10.0.1.0/25 [120/1] via 192.168.1.1...

Does the router choose the longest match regardless of the AD of the routing protocols?

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Routers route packets by what is in the routing table, and identical routes will have the one with the best (lowest) AD make it into the routing table, but the other identical routes will not. Then it is checked for the longest match with the routes in the routing table.

AD determines what makes it into the routing table, and the logest match in the routing table determines which route from the routing table is used.

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  • Comments are not for extended discussion; this conversation has been moved to chat. – Ron Maupin Aug 23 '20 at 22:58
  • whoever wrote that is to general and not precise enough, they equal they FIB to the CEF table which is not true, but anyway too detailed for this question, carry on – Matt Douhan Aug 23 '20 at 23:01
  • Read the official Cisco explanation, and you will see it is the same thing as what he wrote. "The routing table, also known as the Routing Information Base (RIB), is built from information obtained from dynamic routing protocols, directly connected and static routes. The FIB is built directly from the routing table and contains the next-hop IP address for each destination IP in the network. It keeps a mirror image of the forwarding information contained in the IP routing table." – Ron Maupin Aug 23 '20 at 23:10
  • carry on, this is getting to detailed for this forum, to much mixing of vocabulary etc here in order to have such detailed discussion – Matt Douhan Aug 23 '20 at 23:11
  • You are the one complaining about the answer. You are free to add your own, conflicting answer. The question was tagged with the Cisco tag, and I gave the appropriate Cisco answer. Other vendors may have different interpretations or architectures, but my answer is correct for the question. – Ron Maupin Aug 23 '20 at 23:14
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if multiple route entries are present in router for same destination with different routing protocol means with administrative distance value (AD) value . Lowest the AD value have more priroity to choose route entry

Layers 3 devices chooses routing protocols considering AD value (administrative distance) lesser the AD value have more priority for chossen routing protocol by layer3 devices when more than one routing protocol is configured

Default Adminstrative value of routing protocols

Directly connected route ==> 0

Static routing protocol ==> 1

RIP ===============>>120

EIGRP ==============>>90

OSPF =≠=============>> 110

Lesser the AD value more priority to choose routing protocol by layer3 devices

For example

If layer3 devices is configured with RIP and OSPF routing protocols advertising same route to same destination . Layer 3 devices use OSPF routing protocols advertise route details for passing traffic because OSPF routing protocols AD value is less than RIP

We even have feasible to changes default AD values of routing protocols according to our requirements

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