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New Member

relation between switch fabric and forwarding rate

what is the relation between switch fabric and forwarding rate.

Hall of Fame Super Silver

Re: relation between switch fabric and forwarding rate

Hello Rajan,

a non blocking switching fabric allows for using ports at line rate.

A port can work at line rate it can support minimum frame size frames

minimum frame size = 64 bytes

interframe-gap including 8 bytes preamble= 20.1 bytes

8 = bits/byte

packet rate = rate / ((framesize+IFG)*8)

1 Gbps = 1488000 packets per second per direction.

10 Gbps = 14880000 pps

Be aware that usually numbers are given in this way:

1Gbps TX and 1 Gbps RX is considered as 2 Gbps.

Hope to help


New Member

Re: relation between switch fabric and forwarding rate

If you have MLS enable on the box, then check out this command:

sh mls cef adjacency mac-address << sup mac-add>> det or slot

It will give you the traffic traversing each module.




Super Bronze

Re: relation between switch fabric and forwarding rate

The switch fabric provides the bandwidth between ports. To guarantee all ports can pass full line rate, you would need 2x ports bandwidths (assuming duplex ports), so for example, for 8 gig ports you would need 16 Gbps bandwidth.

Many switches do not provide enough bandwidth for all their ports to run concurrently at full capacity. If the switch doesn't, the switch will delay or drop packets when the fabric bandwidth is exceeded.

Besides raw fabric bandwidth, architecture of the fabric can also limit performance. Sometimes multiple ports share bandwidth to the main fabric and/or queuing is done on ingress to the fabric rather than egress. This latter situation can lead to head-of-line blocking (i.e. one busy port blocks other ports). ([edit] head-of-line block example - real traffic - vehicle ahead making left [and stopped due to on-coming traffic], but you can't pass it although road ahead is clear)

The impact of what's described above, to forwarding performance, can be very similar to the impact of a switch uplink. If you have an 8 port switch, all same port speed, and use one port for an uplink, all the other ports can bottleneck on the uplink. If the 8 port switch had a 10x faster dedicated uplink, those ports would not bottleneck on the uplink. If instead of 8 ports, you had 24 ports, but again a 10x uplink, you can still bottleneck on the uplink, but perhaps not as often as when there were 8 same type of ports because the bandwidth ratios between edge ports and uplink is better (7:1 vs. 8:10 vs. 24:10). It's similar for fabric bandwidths.

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