Network > Interfaces > Loopback

Use the following fields to configure a loopback interface:

Loopback Interface Settings	Configure In	Description
Interface Name	Loopback Interface	The read-only Interface Name is set to loopback . In the adjacent field, enter a numeric suffix (1-9999) to identify the interface.
Comment		Enter an optional description for the interface.
Netflow Profile		If you want to export unidirectional IP traffic that traverses an ingress interface to a NetFlow server, select the server profile or click Netflow Profile to define a new profile (see Device > Server Profiles > NetFlow). Select None to remove the current NetFlow server assignment from the interface.
Virtual Router	Loopback Interface Config	Assign a virtual router to the interface, or click Virtual Router to define a new one (see Network > Virtual Routers). Select None to remove the current virtual router assignment from the interface.
Virtual System		If the firewall supports multiple virtual systems and that capability is enabled, select a virtual system (vsys) for the interface or click Virtual System to define a new vsys.
Security Zone		Select a security zone for the interface, or click Zone to define a new zone. Select None to remove the current zone assignment from the interface.
Management Profile	Tunnel Interface Advanced Other Info	Management Profile —Select a profile that defines the protocols (for example, SSH, Telnet, and HTTP) you can use to manage the firewall over this interface. Select None to remove the current profile assignment from the interface.
MTU		Enter the maximum transmission unit (MTU) in bytes for packets sent on this interface (576-9,192; default is 1,500). If machines on either side of the firewall perform Path MTU Discovery (PMTUD) and the interface receives a packet exceeding the MTU, the firewall returns an ICMP fragmentation needed message to the source indicating the packet is too large.
Adjust TCP MSS		Select to adjust the maximum segment size (MSS) to accommodate bytes for any headers within the interface MTU byte size. The MTU byte size minus the MSS Adjustment Size equals the MSS byte size, which varies by IP protocol: IPv4 MSS Adjustment Size —Range is 40-300; default is 40. IPv6 MSS Adjustment Size —Range is 60-300; default is 60. Use these settings to address the case where a tunnel through the network requires a smaller MSS. If a packet has more bytes than the MSS without fragmentation, this setting enables the adjustment. Encapsulation adds length to headers, so it helps to configure the MSS adjustment size to allow bytes for such things as an MPLS header or tunneled traffic that has a VLAN tag.
For an IPv4 address
IP	Loopback Interface IPv4	Click Add , then perform one of the following steps to specify a static IP address and network mask for the interface. Type the entry in Classless Inter-Domain Routing (CIDR) notation: ip_address / mask (for example, 192.168.2.0/24). Select an existing address object of type IP netmask . Click Address to create an address object of type IP netmask . You can enter multiple IP addresses for the interface. The forwarding information base (FIB) your system uses determines the maximum number of IP addresses. To delete an IP address, select the address and click Delete .
For an IPv6 address
Enable IPv6 on the interface	Loopback Interface IPv6	Select to enable IPv6 addressing on this interface.
Interface ID		Enter the 64-bit extended unique identifier (EUI-64) in hexadecimal format (for example, 00:26:08:FF:FE:DE:4E:29). If you leave this field blank, the firewall uses the EUI-64 generated from the MAC address of the physical interface. If you enable the Use interface ID as host portion option when adding an address, the firewall uses the interface ID as the host portion of that address.
Address		Click Add and configure the following parameters for each IPv6 address: Address —Enter an IPv6 address and prefix length (e.g. 2001:400:f00::1/64). You can also select an existing IPv6 address object or click Address to create an address object. Enable address on interface —Select to enable the IPv6 address on the interface. Use interface ID as host portion —Select to use the Interface ID as the host portion of the IPv6 address. Anycast —Select to include routing through the nearest node.