Loopback

Loopback interfaces are virtual logical interfaces that provide a stable, always-up reference point for network services including routing protocol router IDs, management access, and service endpoints that remain accessible regardless of physical interface states. They support full Layer 3 functionality including IPv4 and IPv6 addressing, routing protocol participation (OSPF, BGP), MPLS services, and advanced features such as BFD for fast failure detection and network monitoring applications. Loopback interfaces are essential for network design best practices, providing consistent reachability for critical services, enabling effective network troubleshooting, and serving as anchor points for overlay networks, VPN services, and network management systems.

Diagram

Classes

interfaces (iosxe.devices.configuration)

Name	Type	Constraint	Mandatory	Default Value
loopbacks	List	[loopbacks]	No

loopbacks (iosxe.devices.configuration.interfaces)

Name	Type	Constraint	Mandatory	Default Value
id	Integer		Yes
interface_groups	List	String	No
interface_group_policy	Choice	merge, replace	No
description	String		No
shutdown	Boolean	true, false	No
ip_mtu	Integer	min: 68, max: 18000	No
vrf_forwarding	String		No
ipv4	Class	[ipv4]	No
ipv6	Class	[ipv6]	No
arp_timeout	Integer	min: 0, max: 2147483	No
load_interval	Integer	min: 30, max: 600	No
mpls	Class	[mpls]	No
ospf	Class	[ospf]	No
ospfv3	Class	[ospfv3]	No
isis	Class	[isis]	No
pim	Class	[pim]	No
igmp	Class	[igmp]	No
zone_member_security	String		No

ipv4 (iosxe.devices.configuration.interfaces.loopbacks)

Name	Type	Constraint	Mandatory	Default Value
address	IP		No
address_mask	IP		No
proxy_arp	Boolean	true, false	No
access_group_in	String		No
access_group_out	String		No
redirects	Boolean	true, false	No
unreachables	Boolean	true, false	No
nat_inside	Boolean	true, false	No
nat_outside	Boolean	true, false	No
unnumbered_interface_type	Choice	Loopback, Vlan, GigabitEthernet, TwoGigabitEthernet, FiveGigabitEthernet, TenGigabitEthernet, TwentyFiveGigabitEthernet, FortyGigabitEthernet, FiftyGigabitEthernet, HundredGigabitEthernet, PortChannel	No
unnumbered_interface_id	Any	String or Integer[min: 0]	No
address_dhcp	Boolean	true, false	No

ipv6 (iosxe.devices.configuration.interfaces.loopbacks)

Name	Type	Constraint	Mandatory	Default Value
enable	Boolean	true, false	No
addresses	List	[addresses]	No
link_local_addresses	List	IP	No
address_autoconfig_default	Boolean	true, false	No
address_dhcp	Boolean	true, false	No
mtu	Integer	min: 1280, max: 9976	No
pim	Class	[pim]	No

mpls (iosxe.devices.configuration.interfaces.loopbacks)

Name	Type	Constraint	Mandatory	Default Value
ip	Boolean	true, false	No
mtu	Integer		No

ospf (iosxe.devices.configuration.interfaces.loopbacks)

Name	Type	Constraint	Mandatory	Default Value
authentication_key_chain	String		No
authentication_message_digest	Boolean	true, false	No
authentication_null	Boolean	true, false	No
cost	Integer	min: 1, max: 65535	No
dead_interval	Integer	min: 1, max: 65535	No
hello_interval	Integer	min: 1, max: 65535	No
message_digest_keys	List	[message_digest_keys]	No
mtu_ignore	Boolean	true, false	No
multi_area_ids	List	Any[String or Integer[min: 0]]	No
network_type	Choice	broadcast, non-broadcast, point-to-multipoint, point-to-point	No
priority	Integer	min: 0, max: 255	No
process_ids	List	[process_ids]	No
ttl_security_hops	Integer	min: 1, max: 254	No

ospfv3 (iosxe.devices.configuration.interfaces.loopbacks)

Name	Type	Constraint	Mandatory	Default Value
bfd	Boolean	true, false	No
cost	Integer	min: 1, max: 65535	No
dead_interval	Integer	min: 1, max: 65535	No
hello_interval	Integer	min: 1, max: 65535	No
mtu_ignore	Boolean	true, false	No
network_type	Choice	broadcast, non-broadcast, point-to-multipoint, point-to-point	No
priority	Integer	min: 0, max: 255	No

isis (iosxe.devices.configuration.interfaces.loopbacks)

Name	Type	Constraint	Mandatory	Default Value
area_tag	String		No
ipv4_metric_levels	List	[ipv4_metric_levels]	No
network_point_to_point	Boolean	true, false	No

pim (iosxe.devices.configuration.interfaces.loopbacks)

Name	Type	Constraint	Mandatory	Default Value
passive	Boolean	true, false	No
dense_mode	Boolean	true, false	No
sparse_mode	Boolean	true, false	No
sparse_dense_mode	Boolean	true, false	No
bfd	Boolean	true, false	No
border	Boolean	true, false	No
bsr_border	Boolean	true, false	No
dr_priority	Integer	min: 0, max: 4294967294	No

igmp (iosxe.devices.configuration.interfaces.loopbacks)

Name	Type	Constraint	Mandatory	Default Value
version	Integer	min: 1, max: 3	No

addresses (iosxe.devices.configuration.interfaces.loopbacks.ipv6)

Name	Type	Constraint	Mandatory	Default Value
prefix	IP		Yes
eui_64	Boolean	true, false	No

pim (iosxe.devices.configuration.interfaces.loopbacks.ipv6)

Name	Type	Constraint	Mandatory	Default Value
pim	Boolean	true, false	No
bfd	Boolean	true, false	No
bsr_border	Boolean	true, false	No
dr_priority	Integer	min: 0, max: 4294967295	No

message_digest_keys (iosxe.devices.configuration.interfaces.loopbacks.ospf)

Name	Type	Constraint	Mandatory	Default Value
id	Integer	min: 1, max: 255	Yes
md5_auth_key	String		No
md5_auth_type	Choice	0, 7	No

process_ids (iosxe.devices.configuration.interfaces.loopbacks.ospf)

Name	Type	Constraint	Mandatory	Default Value
id	Integer	min: 1, max: 65535	No
areas	List	Any[String or Integer[min: 0]]	No

ipv4_metric_levels (iosxe.devices.configuration.interfaces.loopbacks.isis)

Name	Type	Constraint	Mandatory	Default Value
level	Choice	level-1, level-2	Yes
value	Integer	min: 1, max: 16777214	Yes

Guidelines and Limitations

IP Address Reassignment Between Interfaces

IOS-XE enforces IP address uniqueness within the same VRF — no two interfaces can hold the same IP address simultaneously. When swapping IP addresses between two interfaces (for example, moving 10.1.1.1 from Loopback101 to Loopback102 and vice versa), terraform apply will fail because Terraform updates both interfaces in parallel without awareness of the cross-resource conflict. The device rejects the new IP assignment with an “inconsistent value: Device refused one or more commands” error because the target IP still exists on the other interface.

To perform an IP swap, apply the change in two steps:

1. Remove the IP addresses from both interfaces (delete the ipv4 block or assign temporary addresses) and run terraform apply.
2. Set the new desired IP addresses and run terraform apply a second time.

Examples

interface Loopback0
 description Loopback0
 ip address 5.5.5.5 255.255.255.0
 ipv6 address dhcp

iosxe:
  devices:
    - name: Device1
      configuration:
        interfaces:
          loopbacks:
            - id: 0
              description: Loopback0
              ipv4:
                address: 5.5.5.5
                address_mask: 255.255.255.0
              ipv6:
                enable: true
                address_dhcp: true

Full example:

iosxe:
  devices:
    - name: Device1
      configuration:
        interfaces:
          loopbacks:
            - id: 1
              description: Production Loopback 2
              interface_groups:
                - management
              shutdown: false
              ip_mtu: 1400
              vrf_forwarding: prod
              arp_timeout: 30
              load_interval: 30
              ipv4:
                address: 192.168.2.1
                address_mask: 255.255.255.0
                address_dhcp: true
                proxy_arp: true
                access_group_in: ACL_IN
                access_group_out: ACL_OUT
                redirects: false
                unreachables: false
                verify_unicast_source_reachable_via: any
                verify_unicast_source_allow_default: true
              ipv6:
                enable: true
                addresses:
                  - prefix: 2001::1
                    prefix_length: 64
                link_local_addresses:
                  - fe80::1
                mtu: 1450
                pim:
                  dr_priority: 100
              bfd:
                enable: true
                local_address: 172.16.1.1
                interval: 120
                interval_multiplier: 3
                interval_min_rx: 100
                echo: false
              mpls:
                mtu: 1450
                ip: true
              ospf:
                cost: 50
                dead_interval: 3
                hello_interval: 1
                mtu_ignore: true
                network_type: point-to-multipoint
                priority: 1
                ttl_security_hops: 2
                process_ids:
                  - id: 1
                    areas:
                      - "0.0.0.0"
              ospfv3:
                network_type: point-to-point
                cost: 50
                bfd: true
                dead_interval: 40
                hello_interval: 10
                mtu_ignore: true
                priority: 100
              pim:
                dense_mode: true
                bfd: true
                border: true
                bsr_border: true
                dr_priority: 100
              igmp:
                version: 2
              isis:
                area_tag: TEST  # Must match an isis_processes.area_tag defined under routing
                ipv4_metric_levels:
                  - level: level-1
                    value: 50

NAT Inside Loopback

Configure a loopback interface as a NAT inside interface. This is commonly used when the loopback serves as a source for traffic that should be translated by NAT.

Note: nat_inside and nat_outside are mutually exclusive on a single interface (only one can be configured at a time).

Cisco IOS-XE CLI Equivalent:

interface Loopback100
 description NAT Inside Loopback
 ip address 10.0.100.1 255.255.255.255
 ip nat inside

iosxe:
  devices:
    - name: Device1
      configuration:
        interfaces:
          loopbacks:
            - id: 100
              description: NAT Inside Loopback
              ipv4:
                address: 10.0.100.1
                address_mask: 255.255.255.255
                nat_inside: true

NAT Outside Loopback

Configure a loopback interface as a NAT outside interface. This is used when the loopback represents a public-facing address that receives translated traffic.

Cisco IOS-XE CLI Equivalent:

interface Loopback200
 description NAT Outside Loopback
 ip address 203.0.113.1 255.255.255.255
 ip nat outside

iosxe:
  devices:
    - name: Device1
      configuration:
        interfaces:
          loopbacks:
            - id: 200
              description: NAT Outside Loopback
              ipv4:
                address: 203.0.113.1
                address_mask: 255.255.255.255
                nat_outside: true

Unicast Reverse Path Forwarding (uRPF)

Configure uRPF source verification on a loopback interface. The reachable-via mode can be rx (strict mode — source must be reachable via the receiving interface) or any (loose mode — source must be reachable via any interface).

Cisco IOS-XE CLI Equivalent:

interface Loopback200
 description uRPF Loopback
 ip address 200.0.0.1 255.255.255.255
 ip verify unicast source reachable-via any allow-default

iosxe:
  devices:
    - name: Device1
      configuration:
        interfaces:
          loopbacks:
            - id: 200
              description: uRPF Loopback
              ipv4:
                address: 200.0.0.1
                address_mask: 255.255.255.255
                verify_unicast_source_reachable_via: any
                verify_unicast_source_allow_default: true

Loopback interfaces are virtual logical interfaces that provide a stable, always-up reference point for network services including routing protocol router IDs, management access, and service endpoints that remain accessible regardless of physical interface states. They support full Layer 3 functionality including IPv4 and IPv6 addressing, routing protocol participation (OSPF, BGP), MPLS services, and advanced features such as BFD for fast failure detection and network monitoring applications. Loopback interfaces are essential for network design best practices, providing consistent reachability for critical services, enabling effective network troubleshooting, and serving as anchor points for overlay networks, VPN services, and network management systems.

Diagram

Classes

interfaces (iosxe.devices.configuration)

Name	Type	Constraint	Mandatory	Default Value
loopbacks	List	[loopbacks]	No

loopbacks (iosxe.devices.configuration.interfaces)

Name	Type	Constraint	Mandatory	Default Value
id	Integer		Yes
interface_groups	List	String	No
description	String		No
shutdown	Boolean	true, false	No
vrf_forwarding	String		No
ipv4	Class	[ipv4]	No
ipv6	Class	[ipv6]	No
arp_timeout	Integer	min: 0, max: 2147483	No
load_interval	Integer	min: 30, max: 600	No
mpls	Class	[mpls]	No
ospf	Class	[ospf]	No
ospfv3	Class	[ospfv3]	No
pim	Class	[pim]	No
igmp	Class	[igmp]	No

ipv4 (iosxe.devices.configuration.interfaces.loopbacks)

Name	Type	Constraint	Mandatory	Default Value
address	IP		No
address_mask	IP		No
proxy_arp	Boolean	true, false	No
access_group_in	String		No
access_group_out	String		No
redirects	Boolean	true, false	No
unreachables	Boolean	true, false	No
unnumbered_interface_type	Choice	Loopback, Vlan, GigabitEthernet, FiveGigabitEthernet, TenGigabitEthernet, FortyGigabitEthernet, HundredGigabitEthernet, PortChannel	No
unnumbered_interface_id	Any	String or Integer[min: 0]	No

ipv6 (iosxe.devices.configuration.interfaces.loopbacks)

Name	Type	Constraint	Mandatory	Default Value
enable	Boolean	true, false	No
addresses	List	[addresses]	No
link_local_addresses	List	IP	No
address_autoconfig_default	Boolean	true, false	No
address_dhcp	Boolean	true, false	No
mtu	Integer	min: 1280, max: 9976	No
pim	Class	[pim]	No

mpls (iosxe.devices.configuration.interfaces.loopbacks)

Name	Type	Constraint	Mandatory	Default Value
ip	Boolean	true, false	No
mtu	Integer		No

ospf (iosxe.devices.configuration.interfaces.loopbacks)

Name	Type	Constraint	Mandatory	Default Value
cost	Integer	min: 1, max: 65535	No
dead_interval	Integer	min: 1, max: 65535	No
hello_interval	Integer	min: 1, max: 65535	No
mtu_ignore	Boolean	true, false	No
network_type	Choice	broadcast, non-broadcast, point-to-multipoint, point-to-point	No
priority	Integer	min: 0, max: 255	No
ttl_security_hops	Integer	min: 1, max: 254	No
process_ids	List	[process_ids]	No
message_digest_keys	List	[message_digest_keys]	No

ospfv3 (iosxe.devices.configuration.interfaces.loopbacks)

Name	Type	Constraint	Mandatory	Default Value
network_type	Choice	broadcast, non-broadcast, point-to-multipoint, point-to-point	No
cost	Integer	min: 1, max: 65535	No

pim (iosxe.devices.configuration.interfaces.loopbacks)

Name	Type	Constraint	Mandatory	Default Value
passive	Boolean	true, false	No
dense_mode	Boolean	true, false	No
sparse_mode	Boolean	true, false	No
sparse_dense_mode	Boolean	true, false	No
bfd	Boolean	true, false	No
border	Boolean	true, false	No
bsr_border	Boolean	true, false	No
dr_priority	Integer	min: 0, max: 4294967294	No

igmp (iosxe.devices.configuration.interfaces.loopbacks)

Name	Type	Constraint	Mandatory	Default Value
version	Integer	min: 1, max: 3	No

addresses (iosxe.devices.configuration.interfaces.loopbacks.ipv6)

Name	Type	Constraint	Mandatory	Default Value
prefix	IP		Yes
eui_64	Boolean	true, false	No

pim (iosxe.devices.configuration.interfaces.loopbacks.ipv6)

Name	Type	Constraint	Mandatory	Default Value
pim	Boolean	true, false	No
bfd	Boolean	true, false	No
bsr_border	Boolean	true, false	No
dr_priority	Integer	min: 0, max: 4294967295	No

process_ids (iosxe.devices.configuration.interfaces.loopbacks.ospf)

Name	Type	Constraint	Mandatory	Default Value
id	Integer	min: 1, max: 65535	No
areas	List	Any[String or Integer[min: 0]]	No

message_digest_keys (iosxe.devices.configuration.interfaces.loopbacks.ospf)

Name	Type	Constraint	Mandatory	Default Value
id	Integer	min: 1, max: 255	Yes
md5_auth_key	String		No
md5_auth_type	Choice	0, 7	No

Examples

iosxe:
  devices:
    - name: Device1
      configuration:
        interfaces:
          loopbacks:
            - id: 0
              description: Loopback0
              ip_address: 5.5.5.5
              ip_address_mask: 255.255.255.0

Full example:

iosxe:
  devices:
    - name: Device1
      configuration:
        interfaces:
          loopbacks:
            - id: 1
              description: Production Loopback 2
              shutdown: false
              vrf_forwarding: prod
              arp_timeout: 30
              load_interval: 30
              ipv4:
                address: 192.168.2.1
                address_mask: 255.255.255.0
                proxy_arp: true
                access_group_in: ACL_IN
                access_group_out: ACL_OUT
                redirects: false
                unreachables: false
              ipv6:
                enable: true
                addresses:
                  - prefix: 2001::1
                    prefix_length: 64
                link_local_addresses:
                  - fe80::1
                mtu: 1450
              bfd:
                enable: true
                local_address: 172.16.1.1
                interval: 120
                interval_multiplier: 3
                interval_min_rx: 100
                echo: false
              mpls:
                mtu: 1450
                ip: true
              ospf:
                cost: 50
                dead_interval: 3
                hello_interval: 1
                mtu_ignore: true
                network_type: point-to-multipoint
                priority: 1
                ttl_security_hops: 2
                process_ids:
                  - id: 1
                    areas:
                      - "0.0.0.0"
              ospfv3:
                network_type: point-to-point
                cost: 50
              pim:
                dense_mode: true
                bfd: true
                border: true
                bsr_border: true
                dr_priority: 100
              igmp:
                version: 2