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Ethernet interface keeplives to detect loops

Hi,

I am not sure to understand well how keepalives work on Ethernet interfaces.

I indeed would like to use these packets and the funtionnality which put an interface into "err-disable" state when the interface see a keepalive packet it sent to come back (loopback error).

I made a test with a C3560, creating a physical loop betwwen 2 interfaces, keepalives enabled, loopack detect enabled, but nothing happens.

Another example is a real case where we had a loop, provocated on a hub itself connected on a C2960 with a single physical connection. With the loop on the hub (with no STP), the switch C2960 should see the keepalive packet to come back on itself . What happens actually is that the "loopback error" sometimes occurs immediatly, sometimes never.

Has someone deeper knowledge about keepalives which coul help me to understand that?

Thanks in advance

P.

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Re: Ethernet interface keeplives to detect loops

Hello Paris,

an ethernet keepalive is a frame with source MAC address = destination MAC address and a specific encapsulation

The original objective of an ethernet keepalive is just to check if the local interface can send frame out on wire.

ethernet frames with  LLC/SNAP encapsulation with protocol 0x9000 and with MAC SA = MAC DA =  ethernet interface bia.

see

http://www.groupstudy.com/archives/cisco/200112/msg01021.html

The best way to detect loops in the real world should be the use of BPDU guard to react when the port receives back its own STP BPDU.

I would not rely on ethernet keepalives as a way to detect loops.

Hope to help

Giuseppe

New Member

Ethernet interface keeplives to detect loops

Hi Giuseppe,

I indeed would like to use something more "official" like STP BPDU Guard to detect loop. However, it is not so simple. In my topology, there is a switch A, administrated by myself), and switch B connected to A, but B is administrated by another entity, so I have no control on it.  So I can't activate BPDU Guard on the uplink interface. Ideally, the solution todetect a loop that occur on switch B would be to activate BPDU Guardonly to detect my own BPDUs, but it's not possible.

For the moment, I can only use features like stormcontrol to mitigate the effect of the loop on the dowstrem routers which suddenly has to treat plenty of ARP or multicast packets.

Regards,

P.

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