ICND 2 question: classful bounderies and auto-summary

2URGSE

There are several routers connected to a main router that's in the middle (Star topology, Router E is in the middle)

All routers are running EIGRP.

Routers A and B have the following LAN subnets:

Router A: 172.16.1.0 /24

Router B: 172.16.2.0 /24


At the end they advise that the no auto-summary command should be issued on both of these routers.

I feel I sort of understand what's going on here, but need some direction.

From what I understand, if both of these routers advertise a summarized route, Router E (the one in the middle) may not know to which router to send the packets (A or

MrBrian

Pretty much. With auto-summary, routers A and B will both tell router E that they have the network 172.16.0.0! This is because auto-summary auto summarizes the routers more specific networks, and advertises them to their classful boundary. So it'll probably depend on what advertisement router E heard from more recent that will dictate to what router it will send packets to for this network.

Things might or might not work depending.. let's say router E thinks to reach 172.16.0.0 it sends packets to router A.. well if a packet for 172.16.1.x comes in, it's good to go.. but if a packet with 172.16.2.x comes in, then they're not gonna reach the target.

MrBrian

Come to think of it, if routers A and B both advertise the 10 network, I think router E will install both paths to the 10 network, and load-balance on them.

2URGSE

Thanks Brian!