Which route take precedence

[libra_ali786]

Which route take precedence , please expalin with detail.

[Seb Rupik]

Hi there,

It will go via the R4. Although RIP has the higher AD (120) it is offering the most specific match (longest prefix /32) .

https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/8651-21.html

Your diagram does not detail how R5 receives its routes for R1 (the ICMP source), so there is a good chance the ICMP reply will not return via R4 and may choose an asymmetric path.

cheers,

Seb.

[libra_ali786]

As per my concern about route selection is if first condition meet the selection, then Administrative distance should be selectedt? Why it will go to check second "Metrics " and third "Prefix length" Option.

Building the Routing Table

The main considerations while building the routing table are:

• Administrative distance - This is the measure of trustworthiness of the source of the route. If a router learns about a destination from more than one routing protocol, administrative distance is compared and the preference is given to the routes with lower administrative distance. In other words, it is the believability of the source of the route.

• Metrics - This is a measure used by the routing protocol to calculate the best path to a given destination, if it learns multiple paths to the same destination. Each routing protocol uses a different metric.

• Prefix length

[Dennis Mink · Accepted solution]

what you are quoting is a slection process within the same dynamic routing protocol. so first the lowest admin distance is chosen hen there is a tie. so if you have a staic route to a certain destination through a static route and one through ospf for instance, then the static route will be preferred. that is, if and only if the advertise the same subnet. so if you have 192.168.10.0/24 in a static route and 192.168.10.10/32 through ospf and you send traffic to 192.168.10.10, the the ospf route is more exact and thus chosen.

[Seb Rupik · Accepted solution]

That list is misleading as it is not in the order which is used by the route selection process. IN the same document, take a look at the section titled "Making forwarding decisions". There is an example there.

Another explanation which is routing protocol independent: 

http://packetlife.net/blog/2010/aug/16/route-preference/

cheers,

Seb.

[libra_ali786]

Thanks for the reply . Because of the route selction process was misleading. That is the reason of confusion.

Know my doubt is cleared about the route selction process.

Thanks & Regards

Ali

[paul driver]

Hello

In this scenario 3 routes will be installed in R1 (one from either routing process) and the router will choose the best path which WILL be the longer specified most specific route ( RIP /32), then (OSPF /24) then lastly (EIGRP /8)