I've had this problem for over a year and didn't find a solution until now, when I did a dive into the code and put debugging around routing commands.
I'm sharing my findings in case it helps you and others who Google this error.
Symptoms:
Reset a WAN interface and the routing dies.
In the logs:
Dec 6 06:46:35 fw kernel: arpresolve: can't allocate llinfo for 192.168.21.1 on lagg0_vlan21
In the routing table:
Destination Gateway Flags Netif Expire
192.168.21.0/24 link#14 U lagg0_vl
192.168.21.1 192.168.21.1 UGHS lagg0_vl
192.168.21.10 link#14 UHS lo0
Root cause:
The DNS server and gateway are the same.
Fix:
Either change DNS servers, or patch the code (pfSense 2.2 and 2.3.5):
/etc/inc/system.inc, near line 257:
Change
mwexec("/sbin/route {$cmd} -host {$inet6}{$dnsserver} {$gatewayip}");
```To
if( $dnsserver != $gatewayip )
mwexec("/sbin/route {$cmd} -host {$inet6}{$dnsserver} {$gatewayip}");