CCNA3 Case Study enquiry.

hansch

Hi guys. I'm just wondering why an ethernet connection is used between the 2 routers (in Admin LAN) instead of a serial link. Also, why two routers are required? The case study is version 3.1. Thanks in advance.

EDIT: Diagram included.

sprkymrk

There could be several reasons for the ethernet link between the routers. It's not uncommon on a LAN to do that. Many times serial links are used on WAN links, and fiber or CAT5 on a LAN.

If you notice, one serial link goes to the ISP and one to another building - both probably are some distance away. The border router itself appears to be in the admin building, so an ethernet connection is capable of managing the distance to the other router in the same building.

The two routers could be due to different models. One looks like it's the border router and may be built for speed with minimal ACL usage, and the next one downline (in the admin building) may be using some heavy filtering/ACL's for security serving as a firewall.

hansch

Thanks. it makes sense now after you explained.

lwwarner

Part of the case study excercise involves influencing the OSPF DR election. OSPF doesn't perform a DR election on ptp links, so without the ethernet segment between those two routers there would be no DR election for you to play with.

Humper

Here's a scenario for you:

At work we have a site with the main router (in the office) that's connected to an ISP (serial link). The router plugs into the Cisco 2950 switch at the site. There is another router that plugs into the switch via ethernet cable. This router provides ISDN to another office (and router) about a KM down the road.

Just one of the many reasons to use ethernet