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Mixed Networks - more

The logical approach depicted in Figure 6, uses a Layer 2 (or 3 if you have it) switch that supports gigabit Ethernet, jumbo frames, VLANs and 802.1q tagging or port based VLANs. An example is the Linksys SRW2008, reviewed here. In addition, you'll need a router that supports 802.1q tagging or port based VLANs. An example is the D-Link DFL-CPG310, reviewed here.

VLAN jumbo frames

Figure 6: Setting up jumbo frames using a VLAN model

In both scenarios above, we've isolated the jumbo frame devices to a VLAN that supports gigabit Ethernet and jumbo frame transmission, allowing inter-device jumbo frame transmission. Once again, even if the link to the router is Fast Ethernet, the devices on the jumbo frame LAN can still access the Internet. The traffic from the jumbo frame devices to the Internet (recall that HTTP uses TCP) will just throttle down to a lower frame size utilizing Packet MTU Discovery, or the router will automatically fragment them. As noted in the earlier ping test discussion, if you have a PPPoE Internet connection, your router is already fragmenting packets.

If you experience lower Internet throughput with jumbo frames enabled, try running a few ping tests to determine if the problem is fragmentation or ICMP blocking. If a large packet ping with the -f flag from your jumbo frame PC to a website is not coming back successfully, that could indicate there is a problem with ICMP blocking. A lower frame size may be necessary in this case.

So mixed networks with jumbo frame devices and non-jumbo frame devices can work nicely. For example, I have a gigabit Ethernet switch with jumbo frames enabled. A PC on this switch with jumbo frames enabled can print to my network printer, which has a 100 Mbps Ethernet interface connected to this same switch. HP network printers use TCP port 9100, so print requests from the jumbo frame PC use TCP MSS discovery to send standard Ethernet frames to the printer. On the other hand, UDP flows from the jumbo frame PC to this printer, if there were any, would likely fail because UDP can't perform MSS discovery.


To wrap things up, here are the key points for using jumbo frames on small networks:

  1. Jumbo frames require gigabit Ethernet.
  2. The highest frame size for a connection is the lowest end-to-end maximum frame size.
  3. Gigabit Ethernet Layer 2 switches forward or drop jumbo frames; they don't fragment.
  4. Fragmentation is a Layer 3 (routing) function.
  5. TCP can adjust frame size between different devices. UDP can't.
  6. Using jumbo frames for low latency applications (gaming, VoIP) can be counterproductive
  7. Bigger isn't necessarily better. Jumbo frame sizes need to be matched to device computing power.

The bottom line is that jumbo frames sizes aren't standardized, and it may take some investigation and testing on your part to get the most benefit for your particular LAN and its clients. I hope that this article has provided some insight into jumbo frames and the tools to intelligently implement them on your network.

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