Exercise 3: One-Directional Traffic Across a VPC Peering Connection — Possible Solution
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The likely cause, per the chapter's own warn-box: a route to the
peering connection was configured on only ONE of the two VPCs' route
tables, not both.

Reasoning:

VPC peering itself establishes the underlying network CONNECTION
between the two VPCs, but per this chapter, that connection alone
isn't sufficient for traffic to actually flow -- each VPC's own route
table also needs an explicit route pointing traffic destined for the
OTHER VPC's IP range toward the peering connection. This has to be
configured independently on BOTH sides, because each VPC's routing
decisions are governed entirely by its own route table.

If VPC A's route table has a route to VPC B via the peering
connection, but VPC B's route table was never updated with a
corresponding route back to VPC A, then:

- Traffic originating in VPC A, destined for VPC B, is routed
  correctly by VPC A's route table and successfully reaches VPC B.
- Traffic originating in VPC B, destined for VPC A, has no route
  telling VPC B's route table to send it via the peering connection at
  all -- it simply isn't routed there, and never reaches VPC A.

This produces exactly the reported symptom: traffic flows in one
direction (the side with the correctly configured route) but not the
other (the side missing it) -- not a general connectivity failure, but
a specifically asymmetric, one-sided routing gap.

The fix: add the missing route to the peering connection on the VPC
that currently lacks it, so both route tables have a matching,
symmetric route to each other.

WHY THIS WORKS AS AN ANSWER
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This directly applies the chapter's own warn-box -- "a route existing
in one direction but not verified for the return path... both VPCs
need a route pointing at the peering connection, not just the one that
initiated it" -- and traces through exactly why an asymmetric route
configuration produces an asymmetric traffic-flow symptom, rather than
just citing the rule without showing the underlying mechanism.