loopback interface, routed port and vlans questions

[SJ K]

Hi all,

I have created a few loopback interfaces on my routers for simulating routes and has to started to google around for reasons on the usage of loopback interfaces. Some of the main reasons are that it can be use as an indication of the router status, as physical interface might be down (but does not means that the router is down).  I have also seen another reason that it is use for management.

q1) for the above, how can 1 manage to use/reach the loopback interface IP, if the physical interface is down ?. These mean loopback interface can be reached via other interfaces/network/routes as well right ?

q2) how can i prevent my loopback interface/network from reaching certain networks/routes in my router and vice versa ?

q3) On my L3 switch, I have created a routed port. But on executing show VLAN, i still see that the interface is still inside the default VLAN.
Does routed port need to be in any VLAN for any purposes ?

q4) I read from cisco website that (*notice the bold*)

Access Ports

An access port belongs to and carries the traffic of only one VLAN (unless it is configured as a voice VLAN port). Traffic is received and sent in native formats with no VLAN tagging. Traffic arriving on an access port is assumed to belong to the VLAN assigned to the port. If an access port receives a tagged packet (Inter-Switch Link [ISL] or 802.1Q tagged) for the VLAN assigned to the port, the packet is forwarded. If the port receives a tagged packet for another VLAN, the packet is dropped, the source address is not learned, and the frame is counted in the No destination statistic.

Does it means that VLAN tag only "exists" in a frame while transgressing between switches, once it is out a switch access port, the vlan tag will be "removed" unless it is a trunk port ?

Thanks!

Regards,
Noob

[Richard Burts · Accepted solution]

Noob

1) If one of the physical interfaces is down but other physical interfaces are still up then it may be possible to reach the loopback interface via one of the other physical interfaces. If all of the physical interfaces are down then the loopback interface will not be reachable.

2) If you want to control access for the loopback interface to or from other interfaces then you need to configure and apply access lists to the interfaces.

3) If you have configured a switch interface as a routed port by configuring no switchpport then I am surprised that the switch port still shows up in the default vlan. But if that is the case then I assume that it shows up in the default vlan because it has not been assigned to a different vlan and if it is not assigned any where else then it would logically be grouped in the default vlan.

4) Yes it does imply that the vlan tag only exists while passing between switches on a trunk port and that if it gets to an access port that the tag is removed.

HTH

Rick

[SJ K]

Hi Rick,

If you have configured a switch interface as a routed port by configuring no switchpport then I am surprised that the switch port still shows up in the default vlan. But if that is the case then I assume that it shows up in the default vlan because it has not been assigned to a different vlan and if it is not assigned any where else then it would logically be grouped in the default vlan.

I configured the switchport to access VLAN2 before configuring it as a no switchport and setting an IP on it.  In show VLAN, it is now assigned to the VLAN2.

But should a routed port belongs to any VLAN ? What should be the correct behaviour

Regards,
Noob

[Richard Burts · Accepted solution]

Noob

My understanding is that when you configure a port with "no switchport" that the port should no longer be a member of a vlan.

HTH

Rick

[SJ K]

Hi Rick,

Thanks for coming back.

Meaning that once you configure a routed port -> the interface will not be display in show VLAN right ?  (same as a trunk port).

Regards,
Noob

[Richard Burts · Accepted solution]

Noob

Yes that would be my expectation.

HTH

Rick

[SJ K]

Thanks Richard. Appreciate the reply.