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Beginner

L2(Ethernet) unknown unicast frame

I have 2 switches connected each other (switch A and switch B).

Switch A and B is connected to Ixia also.

Assume that, I send l2 Ethernet traffic from Ixia to Switch A, and my destination mac is for Switch B(some port). Traffic is only only unidirectional (switch a-> switch b). So here the dest mac will not be there in Switch A cam table, so it will flood. After flooding what will happen? How the packet from ixia reaches the l2 interface on Switch B? Or switch A will always flood the packets ?

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Hall of Fame Cisco Employee

Re: L2(Ethernet) unknown unicast frame

Hello,

You are correct - if there is no frame going to be sent back from the destination of the flow, the only way to prevent the flooding is to set up a static MAC address table entry. The syntax should be fairly simple:

mac address-table static <MAC-address> vlan <vlan-id> interface <iface>

On SwA, this should point to the port toward SwB. On SwB, this should point to the port where the destination is connected.

Best regards,
Peter

View solution in original post

6 REPLIES 6
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Hall of Fame Guru

Re: L2(Ethernet) unknown unicast frame

 

Not sure what lxia is but for that device to send packets it would have sent an arp request for the device on switch B and as arp is a broadcast switch A would have flooded the packet. 

 

When the device on switch B responded to the arp request switch A would have recorded the mac address so when lxia sends packets switch A does not need to flood. 

 

Jon

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Hall of Fame Cisco Employee

Re: L2(Ethernet) unknown unicast frame

Hi Jon,

Ixia is a traffic generator. You can use it to construct the entire frame synthetically, with all fields pre-populated by values you have entered during the configuration of the stream.

Assuming that the flow generated by Ixia truly does not elicit back any response from the device on the switch B, the unicast flooding will never stop - in fact, both switches A and B will be flooding this stream because they will never learn the location of the destination MAC address. The stream will reach its destination host, but it will also reach other hosts (who will ignore it, though, since the destination MAC address differs from theirs).

Best regards,

Peter

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Hall of Fame Guru

Re: L2(Ethernet) unknown unicast frame

 

Hi Peter 

 

Thanks for the clarification. 

 

Jon

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Beginner

Re: L2(Ethernet) unknown unicast frame

So is there any solution for this (I mean to avoid the flooding) ?

Can we statically add an entry in the mac table for both Switch A and switch B to avoid the flooding of packets?

Highlighted
Hall of Fame Cisco Employee

Re: L2(Ethernet) unknown unicast frame

Hello,

You are correct - if there is no frame going to be sent back from the destination of the flow, the only way to prevent the flooding is to set up a static MAC address table entry. The syntax should be fairly simple:

mac address-table static <MAC-address> vlan <vlan-id> interface <iface>

On SwA, this should point to the port toward SwB. On SwB, this should point to the port where the destination is connected.

Best regards,
Peter

View solution in original post

Highlighted

Re: L2(Ethernet) unknown unicast frame

Because Switch B MAC Address is not on the MAC Table of Switch A, Switch A will duplicate the frame and send it out to all ports(except the port where IXIA packet came) from). Switch B will receive the frame and look at the Destination MAC address field to deliver the packet to the correct port.

 

Hi Peter,

From what I understood, The traffic from IXIA will reach the device on Switch B however since the reply/response from that device is not received, IXIA will continue generating traffic resulting to continuous flooding on both SWA and SWB? Thank you!.

 

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