What will the router choose, better route or AD?

cisco_kidd

Hey guys,

Confused about one thing, if I had a router configured with OSPF and EIGRP, and a destination route on EIGRP with a /24, however a better route over a /30 via OSPF, which would the router choose? Does the router choose the better route regardless of Administrative distance or does it choose the lower AD?

deth1k

Longest match always wins

cisco_kidd

deth1k wrote: »

Longest match always wins

Thanks deth1k,

So you're saying:

• EIGRP (internal): 192.168.32.0/26
• RIP: 192.168.32.0/29
• OSPF: 192.168.32.0/19
  
  RIP would be the most preferred route?

pham0329

Yes.

Suppose you have a connected route to 192.168.0.0/24, and a RIP route to 192.168.0.128/25. If a packet comes in for 192.168.0.129, it will be routed via the RIP, even though the connected route has an AD of 0.

Forsaken_GA

cisco_kidd wrote: »

Thanks deth1k,

So you're saying:

• EIGRP (internal): 192.168.32.0/26
• RIP: 192.168.32.0/29
• OSPF: 192.168.32.0/19
  
  RIP would be the most preferred route?

RIght. All three routes would be installed in the routing table, since they're different prefix lenghts. AD is only used when prefix lengths are the same.

pham0329

Forsaken_GA wrote: »

RIght. All three routes would be installed in the routing table, since they're different prefix lenghts. AD is only used when prefix lengths are the same.

and it's from different sources! If it's from the same source/routing protocol, the metric will be used to determine the route.

cisco_kidd

Excellent, thanks for the clarification!! Longest match always wins, need to remember that.

johnwest43

Just remember that the order in which you see information in the routing table is the order in which the router uses to decide which route is preferred.
1: Subnet mask, most specific wins.
2: AD, used if subnet mask is equal on two or more routes
3:Metric, used if route is learned from 2 or more routers using the same routing protocol.

Subnet Mask----AD-- Metric
D 10.0.0.64/30 [90 / 130816] via 10.0.0.34,

cisco_trooper

Yep, the most specific route will always win. AD won't matter unless the routes have the same subnet mask.

johnwest summed it up pretty well.