Hello Wang,
actually distance vector routing protocols implement several features to try to avoid routing loops.
A routing loop for network A happens when two routers (or more) think each other to be the next hop to reach the destination.
counter measures include:
split horizon
count to infinity
poison reverse
triggered updates
Imagine three routers connected in a triangle each advertising their connected lan subnet.
If netA on RA fails in absence of safety measures a routing loop will form.
RA will simply stop to advertise netA.
RC will think that RB is the next-hop for netA with metric 2 hop counts
Hope to help
Giuseppe
