Bizarre du coup. Que donnes un 'show ip route' sur l'ERL ? Est-ce que tu as des routes spécifiques sur eth1.838 ?
Chez moi:
admin@gateway:~$ show ip route
Codes: K - kernel, C - connected, S - static, R - RIP, B - BGP
O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
> - selected route, * - FIB route, p - stale info
IP Route Table for VRF "default"
S *> 0.0.0.0/0 [210/0] via 90.116.160.1, eth1.832
C *> 10.138.104.0/21 is directly connected, eth1.838
K *> 80.10.117.120/31 [0/0] via 10.138.111.254, eth1.838
K *> 80.10.204.0/22 [0/0] via 10.138.111.254, eth1.838
K *> 81.253.206.0/24 [0/0] via 10.138.111.254, eth1.838
K *> 81.253.210.0/23 [0/0] via 10.138.111.254, eth1.838
K *> 81.253.214.0/23 [0/0] via 10.138.111.254, eth1.838
C *> 90.116.160.0/21 is directly connected, eth1.832
C *> 127.0.0.0/8 is directly connected, lo
K *> 172.19.20.0/23 [0/0] via 10.138.111.254, eth1.838
K *> 172.20.224.167/32 [0/0] via 10.138.111.254, eth1.838
K *> 172.23.12.0/22 [0/0] via 10.138.111.254, eth1.838
C *> 192.168.66.0/24 is directly connected, eth0.66
C *> 192.168.67.0/24 is directly connected, eth0.67
C *> 192.168.68.0/24 is directly connected, eth0.68
C *> 192.168.69.0/24 is directly connected, eth0.69
C *> 192.168.70.0/24 is directly connected, eth0.70
C *> 192.168.255.254/32 is directly connected, eth1.840
K *> 193.253.67.88/29 [0/0] via 10.138.111.254, eth1.838
K *> 193.253.153.227/32 [0/0] via 10.138.111.254, eth1.838
K *> 193.253.153.228/32 [0/0] via 10.138.111.254, eth1.838
Sinon, dans /etc/dhcp/dhclient-exit-hooks.d tu as quoi ? (Il y a plusieurs dossiers possibles pour les scripts dhclient) en 1.8.5 je n'ai que ddclient.