Auto-Summary question

tech671

A router using 3 routing protocols RIP, EIGRP, and OSPF. All routing protocols are configured properly and auto-summary is applied on Eigrp. The serial0 interface conncected to WAN. Which protocols will it use?

a. Rip
b. Eigrp
c. OSPF
d. routing table will not be updated.

I am a bit confuse, please explain. I choose Eigrp has the lowest AD but if auto-summary is on would the routing table not update and automatically select OSPF or answer will be D ???

rakem

the router will always use the protocol with the lowest AD.

the auto summary command will not affect which protocol gets used, it may affect if the routers can communicate between each other, because depending on the network design you may need a no auto-summary command for EIGRP to work properly, so in the case of your question D could be correct but it depends on design.

agustinchernitsky

Rip has an AD of 120
EIGRP of 90 and 170 (for external)
and OSPF of 110

so... EIGRP would do it

tech671

Thank you guys for the clarification. This site is a big help for self studying.

PCHoldmann

It would be EIGRP. Unless...

The other protocol produced a longer match. With auto-summary on, network 10.10.10.0/24 would be advertised as 10.0.0.0/8, as it would be with RIP. OSPF, however, would use the 10.10.10.0/24. In this case, both the /8 and /24 routes would be installed in the routing table, with the /24 route being prefered for the 10.10.10.0/24 network, or any subnets. The AD only comes into play with routes of the same length match.

Peter