![]() ![]() ![]() A loopback interface can provide a stable interface on which you can assign a Layer 3 address such as an IP or IPX address.The ID is unique per virtual interface type to make the entire name string unique for example both a loopback 0 interface and a null 0 interface can exist, but two loopback 0 interfaces cannot exist in a single networking device.Ĭisco IOS software supports four types of virtual interfaces For example: loopback 0, tunnel 1, and fastethernet 0/0.1. You identify an individual virtual interface using a numerical ID after the virtual interface name. Virtual interfaces exist only in software there are no physical elements. Physical interfaces have some form of physical element-for example, an RJ-45 male connector on an Ethernet cable. Virtual interfaces are network interfaces that are not associated with a physical interface. ![]() Loopback Interfaces Versus Loopback Mode.To configure virtual interfaces, you should understand the following concepts: Information About Configuring Virtual Interfaces Feature Information for Configuring Virtual Interfaces Prerequisites for Configuring Virtual Interfacesīefore virtual interfaces can be used in your network, you must have some physical (hardware) interfaces configured and be able to communicate between the networking devices on which you wish to use virtual interfaces.Configuration Examples for Virtual Interfaces.Information About Configuring Virtual Interfaces.Prerequisites for Configuring Virtual Interfaces.To find information about feature support and configuration, use the "Feature Information for Configuring Virtual Interfaces" section. Your Cisco IOS software release may not support all features. This module was first published on May 2, 2005, and last updated on May 2, 2005.įinding Feature Information in This Module This module describes the four common types of virtual, or logical, interfaces that can be configured using Cisco IOS software: Virtual interfaces do not have a hardware component such as the RJ-45 female port on a 100BASE-T Fast Ethernet network interface card. Virtual interfaces are software-based interfaces that you create in the memory of the networking device using Cisco IOS commands. ICMP Unreachable Messages from Null InterfacesĬonfiguration Examples for Virtual InterfacesĬonfiguring a Loopback Interface: Exampleįeature Information for Configuring Virtual Interfaces Prerequisites for Configuring Virtual Interfaces And this is the case where loopback address for router ID in OSPF is beneficial.Configuring Virtual Interfaces Table Of Contents But (!!) if any other router ( A, B, C or D) will have one (or more) of its interface(s) down, then if the router ID was not "set deterministically" - new advertisement will have to be sent onto the network, which will affected the overall bandwidth of it. On the other hand, if the router ID was set "deterministically" by configuring it to be the loopback address (or if there is any loopback address in the OSPF process), that will never go down (unless of course, the entire router/OSPF process will go down), then if any one of the interfaces of the router goes down, the router ID won't be affected, therfor no multicast OSPF "new router ID" messages will be sent onto the network.Ĭonsidering the above topology, in case router E (or more precisely its only interface) goes down, then anyway, when it will go up again it will still advertise its router ID "all over again". So, on any link failure for a specific router - if the logic of the router ID selection is still set on the "highest IP address" and there is no loopback address configured as well in the OSPF process (or there is no loopback address in the router at all) - then this link failure will trigger a new router ID selection procedure "within" the router and, perhaps more importantly, will obligate this router to advertise its "newly elected" router ID, meaning send OSPF messages again onto the network. This ID is chosen among ALL available interfaces on a given router UNLESS explicitly configured otherwise. As others mentioned, every OSPF router chooses a router ID. Adding to Maupin great answer I would further say that the (wise) choice of the router ID to be the loop back interface will be more "powerful" upon link failure scenarios. ![]()
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