Refer to the exhibit. The customer uses Gigabit Ethernet on all router interfaces and gets point-to- point Layer 2 VPN connections from their Telco. This network runs IS-IS with default parameters…

Decreasing the metric on the B-D circuit lowers the cost of that segment on the high-speed path toward router H, making it the preferred route for traffic originating from the A side.

The D-F circuit is not on the direct high-speed path between router A and router H in the described topology, so lowering its metric would not direct traffic onto the correct high-speed links.

Decreasing the metric on the F-G circuit completes the lower-cost high-speed path in the reverse direction, ensuring bidirectional traffic uses the intended high-speed links between router A and router H.

The C-F circuit represents an alternate path that bypasses the desired high-speed route; lowering its metric could attract traffic away from the intended high-speed links rather than toward them.