Will flow control help on over subscribed card?

Unanswered Question
Feb 21st, 2007

Hello. We have a 4507R as our core switch, with a 48 port gig line card installed in it (WS-X4548-GB-RJ45). This card has many servers plugged into it, most running gig speeds. We are seeing Rx-No-Pkt-Buff on a lot of the interfaces, especially after the servers have completed being backed up.

I opened a case with TAC, and it seems as though these errors are being caused due to the fact that the card is over subscribed, and we are pushing too much data through the 8 port groupings at one time. If I enabled flow control on the servers that are plugged into this card, will it improve performance and prevent the errors from showing up?

I have this problem too.
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thomas.chen Tue, 02/27/2007 - 07:08

The WS-X4448-GB-RJ45, WS-X4548-GB-RJ45, and WS-X4548-GB-RJ45V switching modules have 48 oversubscribed ports in six groups of eight ports each:

?Ports 1, 2, 3, 4, 5,6, 7, 8

?Ports 9, 10, 11, 12, 13, 14, 15, 16

?Ports 17, 18, 19, 20, 21, 22, 23, 24

?Ports 25, 26, 27, 28, 29, 30, 31, 32

?Ports 33, 34, 35, 36, 37, 38, 39, 40

?Ports 41, 42, 43, 44, 45, 46, 47, 48

The eight ports within each group use common circuitry that effectively multiplexes the group into a single, nonblocking, full-duplex Gigabit Ethernet connection to the internal switch fabric. For each group of eight ports, frames received are buffered and sent to the common Gigabit Ethernet link to the internal switch fabric. If the amount of data received for a port begins to exceed buffer capacity, flow control sends pause frames to the remote port to temporarily stop traffic and prevent frame loss.

Note: You can disable flow control on a port if it is the only port within a group that is connected.

When transmitting, each port in the group shares the bandwidth of a Gigabit Ethernet link. However, each port operates independently so that flow control, or configuration of one port, does not block or degrade the performance of another port in that group.

When the Gigabit Ethernet switching module is not fully utilized, you can connect balancing port connections across port groupings to maximize available bandwidth. For example, with the WS-X4448-GB-RJ45 10/100/1000 switching module, you can connect ports from different groups such as ports 4, 12, 20, or 30 (in any order) before you connect ports from the same group, such as ports 1, 2, 3, 4, 5, 6, 7 and 8.

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