keepalive format??

Answered Question
Jan 21st, 2009
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I wonder keepalive format.

also how the device(switch or router) checks the keepalive about attated host.


I know "TCP keepalive" and "keepalive"(used in switch/router)

are different.

(I know TCP keepalive well)


could anyone explain me about keepalive check method and format??



Correct Answer by Giuseppe Larosa about 8 years 5 months ago

Hello Sanghee,

I've found a better explanation:


an ethernet keepalive is an ethernet frame with


MAC SA = MAC DA = router interface MAC address

and with ethertype 0x9000


this special ethertype calls for loopback: the device that receives the frame sends it back.


So by receiving its own frames with SA= DA = router interface MAC a router can put its interface as up/up.


Actually a trick I used some years ago to have ethernet ports to stay up/up even if unplugged was to disable keepalive (it was for lab tests).


see


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


Hope to help

Giuseppe


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Giuseppe Larosa Wed, 01/21/2009 - 23:54
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Hello Sanghee,


L2 keepalives are different from TCP keepalive.


First of all they are not technology independent and they don't carry an IP packet in their payload.


keepalives on an ethernet interface are different then those on a serial interface.


On a serial interface the format of the keepalive depends from the encapsulation used:


hdlc keepalive is an HLDC frame

ppp keepalive is a PPP frame


if you mismatch the encapsulation the two sides complain of receiving wrong keepalive and interface state is up/down


HDLC keepalive and PPP keepalives contain parameters like mysequence number your sequence number myseen yourseen you can check this on the output of

debug serial interface



These sequence numbers are used to determine the health level of the link and provide a way to calculate the reliability of link that you can see on

sh int serx/y expressed as x/255


255/255 means that the link is healthy


if some keepalives are missed the two routers see some hops in the sequence fields and can calculate an error rate.


PPP has additional features like link quality monitor that move to a more detailed check but the idea is the same.


PPP keepalives also contain a magic number that allows for loopback detection you can see it with

debug ppp negotiation



frame-relay keepalive is an LMI frame, LMI enquiry sent to the switch port

you can check them with

sh frame-relay lmi


An ethernet keepalive should be an ethernet frame:


the interesting aspect is that the MAC SA = MAC DA = mac address of interface


the frame is sent out the device on the other side of the link sends it back to the sender.


Hope to help

Giuseppe


Sanghee Han Thu, 01/29/2009 - 20:56
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Thanks for your reply.

it's very helpful to me.


but I can't understand about ethernet keepalive a little.


which type does the sender(ex.router) use?

I means routers use a broadcast mac as destination mac? or use a unicast mac for every hosts?


As you said,

if the routers use the keepalive with SA_MAC==inteface_MAC and DA_MAC==interface_MAC, how can the hosts reply for the keepalive frame??


sorry.

I wonder everything. ^^


and thanks a lot.






Correct Answer
Giuseppe Larosa Fri, 01/30/2009 - 11:16
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Hello Sanghee,

I've found a better explanation:


an ethernet keepalive is an ethernet frame with


MAC SA = MAC DA = router interface MAC address

and with ethertype 0x9000


this special ethertype calls for loopback: the device that receives the frame sends it back.


So by receiving its own frames with SA= DA = router interface MAC a router can put its interface as up/up.


Actually a trick I used some years ago to have ethernet ports to stay up/up even if unplugged was to disable keepalive (it was for lab tests).


see


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


Hope to help

Giuseppe


Sanghee Han Thu, 02/05/2009 - 15:57
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Thanks for your reply.

it's very helpful to me.


I understand the function of the keepalive by now.


Thanks, giuslar.


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