uplink and dummy frames

Unanswered Question
Sep 4th, 2008

hi every body!

Let say we have an access switch sw1 which is connected to distribution switches sw2 and sw3 via port f0/2 and f0/3 respectively.

swi port f0/2 is root port while f0/3 is blocked port. Sw1 is using uplink feature

sw1 has following mac table

mac1 f0/2

mac2 f0/2

mac3 f0/4 ( host connected to sw1 f0/4)

when the f0/2 link goes down, sw1 will send dummay frames ,using mac addresses as sourse addreses. My point is even for those hosts such as mac1 and mac2 which are reachable via f0/2, which are not directly connected to sw1, will sw1 send dummy frames? or does it(sw1) send dummy frames for the hosts which are directly connected to sw1?

thanks a lot!

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andrew.prince@m... Fri, 09/05/2008 - 00:40

Your topology makes no sense - if sw1 has connections to sw2(sw1-fa0/2) and sw3(sw1-fa0/3)

Sw1-fa0/2 will be forwarding - sw1-fa0/3 will be forwarding.

If there is NO connection between sw2 & sw3 - there is no loop, and STP has no requirement on any switch to place any ports in blocking.

sarahr202 Fri, 09/05/2008 - 08:23

sorry for this omission, please assume sw2 and sw3 are connected as well.All i want to know about dummy frames that a switch configured with uplink feature, send at multicast address 0100.0ccd.cdcd, using mac address from the mac table as source address.Will switch send dummy frames for all mac addresses present in mac table? or

will it send only dummy frames for only those mac addresses belonging to hosts which are directly connected to switch?

thanks a lot!

sarahr202 Fri, 09/05/2008 - 10:40

thanks for the link Andrew!

But my question remain unresolved.For example in the link you forwarded,after uplink feature brings the blocked port up, swA sends dummy packet for host C. Will swA also send dummy frame for host s which is connected to swD2?

If yes, then it will cause swD2 to learn errornously host S is reachable via port connected to swA, while in fact host S is directly connected to swD2.

thanks a lot!

sarahr202 Fri, 09/05/2008 - 18:49

thanks for your reply

i am talking about uplink feature that use dummy frames. Please check the link you forwarded to me.

andrew.prince@m... Sat, 09/06/2008 - 02:04

OK - now I understand what you are refering to, it's is the update of CAM tables. I mis-understood the "dummy" wording, when actually they are carefully crafted BPDU's send by the switch to force the rest of the switches to update the CAM tables - when there have been a change in the topology - which can take some time, as the TCN bit is set by the switch that has the link failure - and NO other switch in the layer 2 topology will "timeout there MAC address tables" until they recevie a BPDU from the root switch with the TCN ACK bit set - which then triggers the switches that recevie it to re-learn the mac address table.

The page quotes:-

"The backup link is brought up so quickly, however, that the CAM tables are no longer accurate. If S sends a packet to C, it is forwarded to D1, where it is dropped. Communication between S and C is interrupted as long as the CAM table is incorrect. Even with the topology change mechanism, it can take up to 15 seconds before the problem is solved.

In order to solve this problem, switch A begins to flood dummy packets with the different MAC addresses that it has in its CAM table as a source. In this case, a packet with C as a source address is generated by A. Its destination is a Cisco proprietary multicast MAC address that ensures that the packet is flooded on the whole network and updates the necessary CAM tables on the other switches. "

A clearer undertstanding of the TCN and CAm table update is available in basic STP explanations.


sarahr202 Sun, 09/07/2008 - 12:02

thanks for your reply Andrew! I applogise for the confusion. I just used the word dummy because that is what cisco book bscmsn guide refer to them.Anyway in the last paragrah you said" In order to solve this problem, swA begins to flood dummy packets with different Mac addresses that it has in its Cam table as a source. In this case , a packet with C as source address is generated by A"

My question is since swA cam table has entry for host S which is directly connected to swD2, would switchA send dummy packet using S mac address as a soouce mac address?

If yes, then swD2, which previouldy knew that s is directly connected to its port,will relearn that host S is now reachable by port connected to swA.

thanks a lot!


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