<p></p>
<p><b>@benpicco</b> commented on this pull request.</p>

<hr>

<p>In <a href="https://github.com/RIOT-OS/RIOT/pull/16536#discussion_r699413960">sys/net/gnrc/routing/ipv6_auto_subnets/gnrc_ipv6_auto_subnets.c</a>:</p>
<pre style='color:#555'>> +
+        /* first remove old prefix if the prefix changed */
+        _remove_old_prefix(downstream, &new_prefix, new_prefix_len, &ext_opts);
+
+        /* configure subnet on downstream interface */
+        gnrc_netif_ipv6_add_prefix(downstream, &new_prefix, new_prefix_len,
+                                   valid_ltime, pref_ltime);
+
+        /* start advertising subnet */
+        gnrc_ipv6_nib_change_rtr_adv_iface(downstream, true);
+
+        /* add route information option with new subnet */
+        tmp = gnrc_ndp_opt_ri_build(&new_prefix, new_prefix_len, valid_ltime,
+                                    NDP_OPT_RI_FLAGS_PRF_NONE, ext_opts);
+        if (tmp == NULL) {
+            DEBUG("auto_subnets: No space left in packet buffer. Not adding RIO\n");
</pre>
<p>I thought it would be still worthwhile to configure the downstream networks even if the packet buffer is currently exhausted to inform upstream.</p>
<p>We might also bail out here to make the breakage more visible.</p>

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