How to backup sub-interfaces

Unanswered Question
Aug 3rd, 2007


I have one router 3725 connected with two switches 3560 ,switches connected between itch other,net works under OSPF. On router I have few sub-interfaces F0/0.1 (server 1), F0/0.2 (...) ,F0/1.1 (server 2). When one link between router and switch crashes,switch with crashed link cat rich router throe another switch.Its all work fine?,but I cant understand what the problem with sub-interfaces on the router,when link crashed the status of interface/sub-interfaces is "Ethernet is up,line protocol is down" and all sub-int on this F0/. link become unrichible. How I can backup this?


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Edison Ortiz Fri, 08/03/2007 - 05:43

The sub-interfaces are logical interfaces and rely on the physical main interface. If you have a physical connection to a switch and the switch dies, the main interfaces and its sub-interfaces will be down.

If you want redundancy, you need another physical main interface (another port in the router) connected to a separate switch.

seducer666 Sun, 08/05/2007 - 21:28


But if I have sub-interface


ip address

and f0/0 goes down,sub-int f0/0.1 goes down to.I cant make sub-int with same ip address.


ip address

How I can do redundancy?

spremkumar Sun, 08/05/2007 - 22:45


You wont be able to assign ip address belongs to a subnet which is already configured in one of the interfaces of the router.

Also if the main physical interface goes down the subsequent subinterfaces configured will also go down.


Edison Ortiz Mon, 08/06/2007 - 05:57

The redundancy would take place by having another physical interface and a different subnet on these point-to-point connections.

For instance,


ip add


ip add

The other end of each of these connections would have and respectively.

alindh Wed, 08/22/2007 - 12:51

I have seen two solutions (stated on this forum) to support one router with two ethernet interfaces on one network:

1) Backup interface. Cisco allows you to have two interfaces (or sub interfaces for vlans) on the same router with the same IP config, as long as you use the "backup interface" command. This uses ONE interface at a time and does not lose performance and should not cause any problems. This is mostly a hardware backup of the interface (uses linktest keepalive). If your good equipment is on the backup network and the router did not switch then you are disconnected.

2) Bridge interface. You can configure BVI as your main IP interface and then setup the physical interface (or sub interfaces) into a bridge group. This uses both interfaces and ethernet STP to find the best path. Performance can be very bad with IRB and may also cause other strange problems due to unexpected bridging. This is mostly a software backup of the interface (uses STP for best path). There is no primary/backup interface so both should work and may pass traffic between the two networks if there is no other path (and kill the CPU on our router).

Note: Cisco access points use bridging between radios and ethernets, so IRB is in use every day by IOS....

Remember your subinterfaces (VLANS) are using the main physical interface, so it's all or nothing on that interface.

There is no good simple choice....use two routers and HSRP/VRRP and dynamic routing....

Cisco needs to make "backup interface" more of a bridge interface without the performance loss.

Is there somthing better?

chris.rosan Wed, 08/22/2007 - 23:18

I have 2 routers that had multiple sub interfaces on each router. EG

router 1





Router 2





You can specify HSRP on each of the sub interfaces, and have a HSRP address for each sub interface. You do this the same as you would HSRP for a normal interface.

This obviously means that you need 2 routers. Otherwise you can do the same with a second interface on the same router, and have sub interfaces of that but if the router dies then you'll lose everything. HSRP is the better way to go i'd suggest.


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