What packet will switch send when config with keepalive ?

[henrybb]

When I config keepalive on switch interface, what will happen ? Is it multicast packet or broadcast packet ?

There is syslog information on my switch which I make loop to test BPDUguard.

"%ETHCNTR-3-LOOP_BACK_DETECTED: Keepalive packet loop-back detected on FastEthernet0/7"

Where can I find detail document about it ?

thanks!

[leonvd79]

The specifics of the keepalive packet are not disclosed on CCO. However, just as with CDP, VTP, and STP packets they are not forwarderd beyond the remote (switch)port.

To learn more about the Keepalive function, see the linked command reference:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cr/hif_r/int_i1h.htm#wp1168538

HTH

--Leon

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[henrybb]

thanks for your reply.

I sniffe it and get following:

ETHER: ----- Ether Header -----

ETHER:

ETHER: Packet 106 arrived at 18:49:46.07

ETHER: Packet size = 60 bytes

ETHER: Destination = 0:3:e3:9b:ad:c0,

ETHER: Source = 0:3:e3:9b:ad:c0,

ETHER: Ethertype = 9000 (Loopback)

ETHER:

Source MAC and DEST mac is same and belong to switch static mac.

I can't understand why my test will result in "ETHCNTR-3-LOOP_BACK_DETECTED: Keepalive packet loop-back detected on FastEthernet XX".

My lab is like this:

switch-----hub(portA,portB),one cable is connected portA and portB of hub. So there is loop on hub.

If I disconnect cable between portA and portB, switch will not report ETHCNTR-3-LOOP_BACK_DETECTED.

It looks like that loop on hub result in the error. But I don't know why ?

Who can tell me detail information ?

thanks!

[leonvd79]

You created a physical loop by cabling two ports of the switch to one hub.

The hub receives the frames on port-A and forwards it to all other ports (including port-B) the switch receives the frame from port-B and inspects the source and destination address. Since it is a loopback frame which originated locally the switchs displays an error.

A hub is considered an OSI layer-1 device. It copies and amplifies the recieved signal to all connected ports on the hub.

Whereas the switch (aka. multiport bridge) is an OSI layer-2 device and forwards frames based on the destination address to the recepient or the nearest path to the recipient.

Break the loop by removing cable from port-B.

HTH

--Leon

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