OpenBSD Journal

[Ask OBSDJ] Another filtering bridge question

Contributed by Dengue on from the brconfig-bridge0-add-rl0-add-xl0-up dept.

Tom Carpenter writes :
"I'm trying to spec-out an x86 machine to run OpenBSD to act as a filtering bridge. The 10/100 Mb NICs currently supported under OpenBSD that I've looked at have anywhere from 4KB to 256KB of on-board buffer. Is there a recommended minimum amount of buffer to avoid dropped packets for a 100Mb network under full load (100Mb transmitting, 100Mb receiving)?


(Comments are closed)

  1. By Anonymous Coward () on

    It is probably not a huge concern, since your average intel box can easily keep up with a few 100Mb/s cards. On receiving a packet, all that needs to happen is a DMA transfer to main memory.

    The only potential concern is 33MHz/32bit PCI which only has about 1Gb of bus bandwidth, which could become a limiting factor for large numbers of interfaces.

    If you are hitting a bottleneck with CPU or PCI, and all the interfaces are running fully loaded as you describe (not that likely?) then any amount of buffering won't help, as packets will have to be dropped anyway, it's just a matter of after how long they start getting dropped.

    In short, don't worry too much about the buffer size, but do make sure you buy good quality, well supported cards if performance is a priority.

  2. By Anonymous Coward () on

    just curious...which NIC did you find with 256KB onboard? Usually I only see 4 to 6 KB..

    the alacritech ones looks quite nifty though (


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