Managing loopback interfaces
Adding a loopback interface
![[NOTE: ]](images/note.gif) | NOTE: This task is not support by Switch 2530 (J9772A–J9783A) products. |
Prerequisites
Before you use the CLI to add a loopback interface, you must be at the global configuration level.
Adding a loopback interface
To add a loopback interface on the switch:
Use the
interface loopback <number>command to create the loopback interface. The number argument is a value from 1 to 7.Use the
ip address <ip address>command to configure an IP address for the loopback interface.You can configure up to 32 IP addresses on a loopback interface. You do not need to specify a network mask. A loopback interface uses the default subnet mask 255.255.255.255.
Examples
A loopback interface configured with one IP address
switch(config)# interface loopback 1 switch(config)# ip address 10.1.1.1
Loopback interfaces configured with multiple IP addresses
This example shows the addition of two loopback interfaces, each with two IP addresses configured.
switch(config)# interface loopback 0 switch(lo0)# ip address 172.16.101.8 switch(lo0)# ip address 172.16.101.9 switch(lo0)# exit switch(config)# interface loopback 1 switch(lol)# ip address 172.16.102.1 switch(lol)# ip address 172.16.102.2
Removing a loopback interface
| NOTE: This task is not support by Switch 2530 (J9772A–J9783A) products. |
Prerequisites
Before you use the CLI to remove a loopback interface, you must be at the global configuration level.
Removing a loopback interface
Use the no interface loopback <number> command
to remove a loopback interface. The number argument
is a value from 1 to 7, specifying the loopback interface to be removed.
You cannot remove the default loopback interface (number 0) with IP address 127.0.0.1.
Displaying loopback interface configurations
Do one of the following:
Use the
show ipcommand to display a list of the loopback interfaces, which are sorted according to the loopback number.The list of loopback interfaces is displayed below other IP configuration parameters, such as packet TTL and ARP age-out values, and VLAN IP configurations. The default loopback interface (lo0) with IP address 127.0.0.1 is not displayed in the list because it is permanently configured on the switch.
Use the
show ip routecommand to display the loopback interfaces in a list of IP routing entries. The list is sorted according to the destination IP address.
Examples
The show ip command output
This example displays the IP addresses configured for two user-defined loopback interfaces (lo1 and lo2).
switch# show ip
IP Routing : Enabled
Default Gateway : 15.255.128.1
Default TTL : 64
Arp Age : 20
Domain Suffix :
DNS server :
| Proxy ARP
VLAN | IP Config IP Address Subnet Mask Std Local
---------------- + ---------- --------------- -------------- ----------
DEFAULT_VLAN | Manual 10.0.8.121 255.255.0.0 No No
VLAN2 Manual 192.168.12.1 255.255.255.0 No No
VLAN3 Disabled
Loopback Addresses
Loopback IP Config IP Address Subnet Mask
-------- ---------- ------------------ ---------------
lol Manual 172.16.110.2 255.255.255.255
lo2 Manual 172.16.112.2 255.255.255.255
lo2 Manual 172.16.114.1 255.255.255.255
The show ip route command output
This example displays the configuration of the default loopback interface (lo0) and one user-defined loopback interface (lo2).
switch# show ip route
IP Route Entries
Destination Gateway VLAN Type Sub-Type Metric Dist.
------------------ --------------- ---- --------- ---------- --------- ----
10.0.0.0/16 DEFAULT_VLAN 1 connected 1 0
127.0.0.0/8 reject static 0 0
127.0.0.1/32 lo0 connected 1 0
172.16.10.121/32 lo2 static 1 0
172.16.100.0/24 10.0.8.11 1 ospf IntraArea 1 1
172.16.102.0/24 VLAN2 2 connected 1 0
Summary of loopback interface configuration
Summary of loopback interface configuration:
You can configure a loopback interface only from the CLI; you cannot configure a loopback interface from the WebAgent or Menu interface.
Loopback interfaces share the same IP address space with VLAN configurations. The maximum number of IP addresses supported on a switch is 2048, which includes all IP addresses configured for both VLANs and loopback interfaces (except for the default loopback IP address 127.0.0.1).
Each IP address that you configure on a loopback interface must be unique in the switch. This means that the address cannot be used by a VLAN interface or another loopback interface.
For example, if you configure a VLAN with IP address 172.16.100.8/24, you cannot configure a loopback interface with IP address 172.16.100.8. In the same way, if you configure a loopback interface (
lo1) with IP address 172.16.101.8, you cannot configure another loopback interface (lo2) with IP address 172.16.101.8.You can configure multiple IP addresses on a loopback interface (
lo0tolo7). Up to 32 IP addresses are supported on a loopback interface.
Overview of loopback interfaces
By default, each switch has an internal loopback
interface (lo0) with the IP address 127.0.0.1.
This IP address is used only for internal traffic transmitted within
the switch and is not used in packet headers in egress traffic sent
to network devices.
You can configure up to seven other loopback
interfaces (lo1, lo2, lo3,
and so on) on the switch to use to transmit network across the network.
Each loopback interface can have multiple IP addresses. Routing protocols,
such as RIP and OSPF, advertise the configured loopback addresses
throughout a network or autonomous system.
User-defined loopback addresses provide the following benefits:
A loopback interface is a virtual interface that is always up and reachable as long as at least one of the IP interfaces on the switch is operational. As a result, a loopback interface is useful for debugging tasks since its IP address can always be pinged if any other switch interface is up.
You can use a loopback interface to establish a Telnet session, ping the switch, and access the switch through SNMP, SSH, and HTTP (WebAgent).
A loopback IP address can be used by routing protocols. For example, you can configure the loopback IP address as the router ID used to identify the switch in an OSPF area. Because the loopback interface is always up, you ensure that the switch's router ID remains constant and that the OSPF network is protected from changes caused by downed interfaces.
OSPF does not require that you use an IP address as the router ID. OSPF only requires the router ID to be a unique value within the autonomous system (AS). However, if you configure the loopback IP address as the router ID, OSPF can reach the switch if any switch interface is up. (Normally, OSPF automatically configures the router ID with the IP address of a switch interface. The disadvantage is that if the interface goes down, OSPF can no longer ping the switch using the router ID even if other interfaces are operational.)
For more information about how to configure a loopback IP address to participate in an OSPF broadcast area, see the titled "(Optional) Assigning Loopback Addresses to an Area" in the Multicast and Routing Guide.