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xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
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Loopback IP Interface
Loopback IP interfaces are network interfaces that are not associated with a physical interface. Physical interfaces have some form
of physical element, for example VLAN, physical ports or even interface MAC address. Unlike a physical interface, the loopback
interface exists only in software; there are no physical elements. In DES3800, you identify an individual loopback interface using
the interface name and IP address which is an independent subnet.
The benefits of loopback interfaces provide:
1. A stable interface on which you can assign a layer-3 address such as an IP address. The address should be a host address
instead of network address.
2. A stable source interface to ensure that the IP address assigned to the interface is always reachable as long as the IP routing
protocol on the physical interfaces continue to advertise this subnet or host IP assigned to the loopback interface.
3. The loopback interface can be considered stable because once you enable it, it will remain enabled until you shut it down
(disable it).
4. It can also be used to establish a Telnet session or Web session.
The loopback interface doesn’t participate in routing or application relay. The path to the loopback interface is via a routing
mechanism. Otherwise, there is no path to the loopback interface. The following defines more clearly the behavior of the loopback
interface:
1. There is no routing protocol running on the loopback interface.
2. It doesn’t have both MAC addresses and ARP because of loopback IP interface. When it needs a source MAC address, it
takes the outbound physical interface MAC address.
3. It is never link down even if none of the ports are link up.
4. The route to the loopback interface needs to be advertised by routing protocols running on other interfaces or it is only
reachable by the static route.
5. For the loopback interface, there is no physical connection through the loopback interface. And, because the loopback
interface is software-only, it has no corresponding VLAN.
6. Users can use the loopback interface as the termination address for open shortest path first (OSPF) sessions. In applications
where other routers or access servers attempt to reach this loopback interface, users should configure a routing protocol to
distribute the subnet assigned to the loopback address.
7. OSPF uses the highest IP address configured on the interfaces as its router ID. If this interface is down or removed, the
OSPF process must recalculate a new router ID and resend all its routing information out over its interfaces. If a loopback
interface is configured with an IP address, OSPF uses this IP address as its router ID, even if other interfaces have higher IP
addresses. Because loopback interfaces never fail, this provides
greater stability. OSPF automatically prefers a loopback
interface over other interfaces, and it chooses the highest IP address among all loopback interfaces.
In the following example, ip1 and ip3 are up, and ip2 is down. A user (10.1.1.2) wants to access the device through a telnet
session. “telnet 20.1.1.1” will fail because ip2 is down, however, “telnet 40.1.1.1” will succeed, because this loopback interface
is always up. But if another user (20.1.1.2) wants to access the loopback ipif (40.1.1.1), it will fail because ip2 is down. This rule
is applied to other access methods, for example, ping.