Junos OS

  • 1.  Understanding loopback interface

    Posted 12-15-2014 13:27

    I am a bit confused at loopback interfaces and internal links interfaces..

    As Junos JNCIA study guide mentions

     

    "Management interfaces: Used to connect the device running the Junos OS to a management network. The actual
    designation for this interface is platform-specific; examples include fxp0 and me0."

     

    "Internal interfaces: Used to connect the control and forwarding planes. The actual designation for this interface is
    platform-specific; examples include fxp1 and em0."

     

    "Use the lo0 interface in conjunction with
    routing protocols to facilitate routing in a redundant environment that is independent of the individual physical
    links within that environment."

     

    Very well, so fxp1 and em0 are the ports for internal link between RE and PFE, Why is it then in the same study guide when we want to filter traffic destined to the router itself like telnet/ssh/http we will use lo0? Shouldn't we use either fxp1 or em0 since its the link between RE and em0?

     

    And is lo0 a routed port?

    Why can't we just use the ingress interface as the destination ip address for telnet traffic or ssh traffic instead of using lo0 or fxp0?

     

    I'm sorry for the confusion but I just want clarification on real-life applications for when to use each, fxp0 em0 lo0 and which is routed and which isnt?

     

    Thanks in advance



  • 2.  RE: Understanding loopback interface
    Best Answer

    Posted 12-15-2014 21:15

     Here is a link that will help you out. I copied some relevant info that should get you started.
    http://chimera.labs.oreilly.com/books/1234000001633/ch04.html#management_interface
    Very well, so fxp1 and em0 are the ports for internal link between RE and PFE, Why is it then in the same study guide when we want to filter traffic destined to the router itself like telnet/ssh/http we will use lo0? Shouldn't we use either fxp1 or em0 since its the link between RE and em0?

     " All data center SRX devices come with a built-in physically dedicated fxp0 port. On the branch devices, the fxp0 interface is only created on entering cluster mode (discussed in Chapter 7). The original design of the fxp0 interface was to provide a dedicated physical interface that is directly connected to the route engine, the idea being that even if the data plane would be completely utilized, you would still have direct access to the route engine for management.

    In a typical service provider network, there is a dedicated segment to just device management. This way, all of the devices could be reachable no matter the state of the transit network, and no management protocols would need to be turned on where customer’s traffic passes"

    And is lo0 a routed port?
    The lo0 interface is a software interface that allows network connectivity if the direct interface is down. Some protocols such as ibgp and OSPF require full mesh connectivity. Imagine doing that hundreds of routers. So the loopback interface which always up will allow communication with the other interfaces to facilitate this full mesh when there are no direct links to the devices.
    Why can't we just use the ingress interface as the destination ip address for telnet traffic or ssh traffic instead of using lo0 or fxp0?
    You can use any interface you want to as the Management Interface. In the some cases we use ge-0/0/0.0 for management. Not all devices that have an fxpo e.g. the branch SRX.
     

    I'm sorry for the confusion but I just want clarification on real-life applications for when to use each, fxp0 em0 lo0 and which is routed and which isnt?
    None of the managent interfaces are used for routing traffic. the internal interface fxp1 connecting the RE to the PFE is not configurable.

    http://kb.juniper.net/InfoCenter/index?page=content&id=KB13389&smlogin=true  (em0 info)