Buffers on Gig ethernet switches ??? 2

[Max5Pier]

We installed Cisco 2950 switches and I'm doing some testing. I'm asking for some info regarding the RAM buffer on the switches. I have servers connected at 1000Mbps and clients connected at 100Mbps.

Will activating the "Flow Control" option on the servers to make sure I don't overload my buffers or should I try to do some additionnal tunning on the switches - the setting by default is store-an-forward. I know that I don't want to change the size of the packets (1.5K).

Thanks for the input ;-)

 

[gunthnp]

store and foward is the slowest type of swcithing cut through is much faster

gunthnp

 

[rwullems]

Yep, Cut trough is faster. but it will let pass damaged frames also trough your switch.
I assume you are using Full Duplex Gb. (Half duplex does exist, but is rarely used).
FD GB supports a kind of back pressure when the buffers of your switch will runn out of space. This is done by a MAC Control frame (You can find out, it is using Ethertype 8808, use google to find some info if you need)

Figure out if your switch supports this (i think it will), but also if the adapter in your server supports the MAC Control frame. If the switch is sending it out and your server won't pick it up, the system is useless and then the possibility is there that the switch is gonna drop some frames.

Regards,
Robert

Robert A.H. Wullems
Sniffer University Instructor
SCM / CNX / MCP
Citee Education
the Netherlands

 

[gunthnp]

unless you have porblem with bad frame cut through is the beter way to go and if you do have a porlbem
you need to fix the line anyway

gunthnp

 

[bcastner]

max5,

The debate is endless on this issue. For 100 mbs connections use store and forward for its ability to reject damaged frames.

My vote.

By the way, no matter if you choose cut through or store and forward, the difference is minimal as a practical matter except in high latency or lossy connection situations.

 

[Max5Pier]

This is all very,very relevant information, thank you.

But, there is still one thing on wich I'm not too sure. I'm afraid that sending data from my server at 1000Mbps to a client wich receives it at 100mbps will fill the switch's buffer very quickly frames will be droped. Will the 802.3ab protocol take care of the flow control efficiently, will it be enough ?

 

[bcastner]

Max,

Consider the timing involved in the entire transaction. The Cisco 2950 will not even break a sweat passing packets from the 1 gig server to the 100mbs clients.

 

[Max5Pier]

bcastner,

Would you say the same thing for the Dell PowerConnect 2124 (non-manageable switch)?

I know I'm getting into picky details, but the performance issue is a big one in my environnment - Finance/banking.

Thanks