Let's analyze the code step by step to identify the potential CWE-787 (Out-of-bounds Write) vulnerability:

1. Input analysis:
   - The function takes several parameters, including `nexthop_mac[]`, which is the destination buffer for the memcpy operation.
   - `neigh` is a pointer to a `struct neighbour` obtained from a lookup operation.

2. Control flow:
   - The code reaches line 20 only if `neigh` is not NULL (line 19).
   - There are no explicit checks on the size or validity of `nexthop_mac[]` or `neigh->ha`.

3. Vulnerable operation:
   - The memcpy operation on line 20 copies ETH_ALEN bytes from `neigh->ha` to `nexthop_mac`.
   - ETH_ALEN is typically defined as 6 bytes (48 bits) for Ethernet addresses.

4. Potential vulnerability:
   - The code assumes that `nexthop_mac[]` is large enough to hold ETH_ALEN bytes.
   - There's no explicit check to ensure that `nexthop_mac[]` is at least ETH_ALEN bytes long.
   - If `nexthop_mac[]` is smaller than ETH_ALEN, this would result in an out-of-bounds write.

5. CWE-787 confirmation:
   - This scenario fits CWE-787 (Out-of-bounds Write) because it potentially writes data beyond the end of the intended buffer.
   - The vulnerability occurs because the code doesn't validate the size of the destination buffer before performing the memcpy operation.

Conclusion:
The vulnerability is confirmed as CWE-787 (Out-of-bounds Write) on line 20. The memcpy operation could potentially write beyond the bounds of `nexthop_mac[]` if it's not properly sized to hold ETH_ALEN bytes. To fix this, the code should either ensure that `nexthop_mac[]` is always allocated with sufficient size or add a runtime check to verify the buffer size before performing the memcpy operation.