User PCs, R1, and R2 share subnet 10.1.99.0/24. Each router is configured with a default route that points at the other router. When a PC sends traffic to 10.1.99.1 (R1’s address), R2 believes R1 is the next hop and forwards the frame to R1; R1’s default route points back to R2. Because 10.1.99.1 belongs to the local subnet, neither router will ARP for it and both will repeatedly forward to the other, creating a Layer-3 routing loop.

B – All users are affected; the problem is not limited to User A.

C – R2 does not drop the traffic; it keeps forwarding it to R1.

D – R1 forwards (then receives again) the traffic; it does not silently discard all packets.

1. Juniper Networks TechLibrary, “Understanding and Preventing Static-Route Loops”, Section ‘Mutual Default Routes’, Example 2 (2022-12).

2. University of Colorado (Coursera) “Computer Communications”, Week 4 notes, p. 5-6, “Loops created by reciprocal static routes”.