secondary IPs

Two external networks on the ASR, where 1.109.1/24 and 1.109.2/24 are the external network subnets and 1.1.1.0/24 and 2.2.2.0/24 are extra subnets used for SNAT:

interface GigabitEthernet0/1/6.1066
 description OPENSTACK_NEUTRON_EXTERNAL_INTF
 encapsulation dot1Q 1066
 ip address 2.2.2.254 255.255.255.0 secondary
 ip address 1.109.1.4 255.255.255.0
 ip nat outside
 standby delay minimum 30 reload 60
 standby version 2
 standby 1064 ip 1.109.1.3
 standby 1064 timers 1 3
 standby 1064 name neutron-hsrp-1064-1066
!
interface GigabitEthernet0/1/6.1067
 description OPENSTACK_NEUTRON_EXTERNAL_INTF
 encapsulation dot1Q 1067
 ip address 1.1.1.254 255.255.255.0 secondary
 ip address 1.109.2.4 255.255.255.0
 ip nat outside
 standby delay minimum 30 reload 60
 standby version 2
 standby 1064 ip 1.109.2.3
 standby 1064 timers 1 3
 standby 1064 name neutron-hsrp-1064-1067

We've also added other subnets that we use for additional floating IP address allocation (something we support with GBP).

These external VLAN sub-interfaces on the ASR connect to two VLAN interfaces on a VM:

eth1      Link encap:Ethernet  HWaddr 52:54:00:b1:ad:82
          inet addr:1.109.1.1  Bcast:1.109.1.255  Mask:255.255.255.0
          inet6 addr: fe80::5054:ff:feb1:ad82/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:954994 errors:0 dropped:728525 overruns:0 frame:0
          TX packets:527 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:73998042 (73.9 MB)  TX bytes:43222 (43.2 KB)

eth2      Link encap:Ethernet  HWaddr 52:54:00:aa:15:6d
          inet addr:1.109.2.1  Bcast:1.109.2.255  Mask:255.255.255.0
          inet6 addr: fe80::5054:ff:feaa:156d/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:1278265 errors:0 dropped:728467 overruns:0 frame:0
          TX packets:116460 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:104850036 (104.8 MB)  TX bytes:11265520 (11.2 MB)

with their associated routes:
noiro@fab9-ext-rtr:~$ netstat -nr
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
...
1.1.1.0         1.109.2.4       255.255.255.0   UG        0 0          0 eth2
1.109.1.0       0.0.0.0         255.255.255.0   U         0 0          0 eth1
1.109.2.0       0.0.0.0         255.255.255.0   U         0 0          0 eth2
2.2.2.0         1.109.1.4       255.255.255.0   UG        0 0          0 eth1
....