Thus spake Tony Hain:
Hence my original question. Packets across the GE will be 1500 unless you are packing them.
Or unless you actually have >1500 MTU to the hosts, which is quite possible. A traffic study from MCI's backbone (obviously years ago) showed nearly 40% (byte-wise) of their traffic was in packets >1500 bytes. With the death of FDDI, this has probably come down, but GE-attached servers in colos should push it back up.
Assuming you are just passing the packets as received from the aggregation switch, this would only happen if your router hardware was better at managing jumbo buffer allocations than 1500B ones. Clearly it will waste large chunks of memory, so do you have measurements to show the actual performance increase?
Routers usually have separate buffer pools for common packet sizes (or use buffer vectors), so the MTU of the interface does not noticeably affect memory usage. Router performance is, however, directly related to packet size, since forwarding overhead is per-packet and not per-byte. It is much easier to fill big pipes with 9000 byte packets than 1500 byte packets.
Tony
S -- Stephen Sprunk "So long as they don't get violent, I want to CCIE #3723 let everyone say what they wish, for I myself have K5SSS always said exactly what pleased me." --Albert Einstein
Thanks Stephen, I understand all those points; I was reacting to the comment that the only jumbo frames were between the routers, so the only ones coming in would be 1500. Look at John's notes from 4/25 8:10pm & 4/26 9:40am. There is no way I know of for a larger frame by itself to increase the speed of a packet. What was missing was the subsequent comment about tunneling those packets (thus increasing the size beyond 1500) across the GE links to the distribution router. Avoiding fragmentation clearly has performance gains, but fragmentation is unnecessary if the packet size remains the same from end to end. The original scenario was simply standard-size feeders into a router, with jumbo frame GE between the core routers, then standard-size distribution on the other end. The claim was that jumbo frames in the middle made it go faster. If this is true I want to know how. Tony -----Original Message----- From: owner-nanog@merit.edu [mailto:owner-nanog@merit.edu]On Behalf Of Stephen Sprunk Sent: Friday, April 27, 2001 12:11 PM To: Tony Hain Cc: North American Noise and Off-topic Gripes Subject: Re: jumbo frames Thus spake Tony Hain:
Hence my original question. Packets across the GE will be 1500 unless you are packing them.
Or unless you actually have >1500 MTU to the hosts, which is quite possible. A traffic study from MCI's backbone (obviously years ago) showed nearly 40% (byte-wise) of their traffic was in packets >1500 bytes. With the death of FDDI, this has probably come down, but GE-attached servers in colos should push it back up.
Assuming you are just passing the packets as received from the aggregation switch, this would only happen if your router hardware was better at managing jumbo buffer allocations than 1500B ones. Clearly it will waste large chunks of memory, so do you have measurements to show the actual performance increase?
Routers usually have separate buffer pools for common packet sizes (or use buffer vectors), so the MTU of the interface does not noticeably affect memory usage. Router performance is, however, directly related to packet size, since forwarding overhead is per-packet and not per-byte. It is much easier to fill big pipes with 9000 byte packets than 1500 byte packets.
Tony
S -- Stephen Sprunk "So long as they don't get violent, I want to CCIE #3723 let everyone say what they wish, for I myself have K5SSS always said exactly what pleased me." --Albert Einstein
participants (2)
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Stephen Sprunk -
Tony Hain