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Line card blocking ports and shared port

Amr Hafez
Level 1
Level 1


If we have a catalyst switch with slot capacity x Gbps, then if we insert a 12 port line card with total BW of 4x Gbps, then we will have only 3 non-blocking ports in the line card and other 9 ports will be blocked (depending on line card port group)

as an example if we have 4507R-E and we insert line card WS-X4712-SFP+E

and the chassis slot capacity is 48 Gbps so we will only have 4 non-blocking ports in this line card

from data sheet Line Card Datasheet I found the following:

Bandwidth is allocated across four 3-port groups, providing 12 Gbps per port group (2.5:1)

does this means that we will not have blocking port in port group in case of the traffic does not exceed 4 Gbps in each port?

and what will happen in case of port speed exceed 4 Gbps ? does another 2 ports block or block instantaneously? 

3 Replies 3

Joseph W. Doherty
Hall of Fame
Hall of Fame


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I believe that should be understood as there's a 12 Gbps "channel" to the fabric, that's shared by three 10g ports.  So the issue is any combination of the three 10g ports, that tries to push more than 12 Gbps, will queue, and a port's packets could be "blocked" even though its port utilization is well under 10 Gbps.

For example, if port 1 and 2 were both accepting 6 Gbps, and port 3, 1 Gbps, all ports utilization is under their 10, but the combination of 13 Gbps, will create congestion on the channel to/from the fabric.  Packets on port 3, again even though it's only using 1 Gbps, could be "blocked" waiting behind packets from ports 1 and 2.

so the blocking time (in case of more that 12 Gbps in the group) could be any random time even long time or short time (msec or minutes) 

I tried before to connect four contiguous ports of WS-X4712-SFP+E without traffic on the ports but only one port work and other ports in the same group has been blocked

Right, "random" blocking time, but they should be pretty brief on a 10g port.  Nothing near minutes, as it's unlikely there's sufficient buffer capacity for that.  Much more likely, over subscription will cause there to be drops.

Don't know why you didn't see the other ports work.  The reason for the 2.5:1 over subscription, that's 30Gbps(3x10):12Gbps(channel) or 2.5:1.

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