Dumb - and somewhat basic- routing question

[tbissett]

The scenario:
A router receives (not advertises) the following routes:
10.1.1.0/24 via BGP
10.0.0.0/8 via EIGRP

The question:
Assuming admin distances, weights, etc. are at their defaults, will traffic desinted for 10.1.1.x use the BGP route because of the longer subnet mask (more specific route), or will it prefer the EIGRP route because of EIGRP has a lower default admin distance than BGP?

 

[octavian10]

BGP is an Exterior Gateway Protocol which in short means it only exchanges routing information between Autonomous Systems (AS). If contacting the 10.1.1.x network which is either in the same or diffrent (AS) as your ISP you will be using BGP to exchange routing info.

"I hear and I forget. I see and I remember. I do and I understand."
- Confucius (551 BC - 479)

 

[jneiberger]

Do not confuse routing protocol operations with route lookup operations. BGP, EIGRP, and other routing protocols simply influence how routes get installed into the routing table. Individual route metrics determine what happens when the same routing protocol learns the same route from multiple sources. Administrative Distance comes into play when the router has learned the same exact route from two different routing protocols.

The route lookup process begins when a packet arrives at an interface to be routed. At this point, the principle of longest match applies. In your case, a route to 10.1.1.0/24 is more specific than a route for 10.0.0.0/8 (both of which would appear in the routing table.) The longest match applies, so the packet would follow the route for 10.1.1.0.

As an aside, the command "no ip classless" or "ip classless" affects the way route lookups occur. It's best to have "ip classless" configured. Otherwise, route lookups behave in a really unintuitive fashion.

HTH,
John