Re: exchange point media (was: Re: MAE-EAST Moving? ...)
I was heard to say a few days ago:
Please be aware that any traffic passing through FDDI XP's using DEC gigaswitches is biased. These switches break path mtu discovery.
This was in the context of measuring packet size in the Internet. After receiving a couple notes asking for clarification, I figured it was worthwhile to pass the same info back on to NANOG. Should one install a (fast|gig)ethernet card into one's Gigaswitch and said switch have FDDI interfaces, the switch will go from having a 4500+ byte MTU to a 1500 byte MTU. This is even between FDDI card to FDDI card on the same switch. The switch will happily fragment the packet if you leave DF off. However, if you turn it on, the switch will drop the packet silently. The switch is unable to generate ICMP and route it back to the sender. Given all of this, PMTU-D breaks in these circumstances. This is important because at least two XP's that I run route server service experienced problems due to this - Mae-West and PAIX. PAIX solved the problem by heavily encouraging customers to get off the Gigaswitch - I don't know if its still in service. Mae-West (SJ) still has this issue to the best of my knowledge. Mae-West (NASA) has been migrating to a SSR fast ether network to get off the Gigaswitches. Collectively, Mae-West still has this issue when transiting across the bridge. So, to make a long story short, try some PMTU probes on your switched fabric before trusting statistics about packet sizes. IM(probably undereducated)O, larger MTUs are not going to do you a lot of good across public exchange point fabric for a long time to come. Large MTU's across private interconnects, or specially targeted interconnects between "Service Providers" will probably be of greater benefit. -- Jeffrey Haas - Merit RSng project - jeffhaas@merit.edu
jeffhaas@merit.edu said:
his is important because at least two XP's
LINX also runs mixed media (sadly) - FDDI and more sane things (GigE and Fast Ethernet). The switches here fragment when switching toward Fast Ethernet. They would certainly move >1500 byte pings between FDDI ports, but I have no FDDI ports now to test whether this works with DF set (i.e. it doesn't fragment FDDI to FDDI), nor whether ICMP is *ever* generated. However, I believe best practice at LINX is, and perhaps should be elsewhere, to set MTU on your FDDI interfaces to that supported by ethernet. Given most exchanges started with ethernet and worked upwards, this should have been best practice everywhere from teh start. -- Alex Bligh VP Core Network, Concentric Network Corporation (formerly GX Networks, Xara Networks)
This is a problem for many switches that support non-ethernet interfaces. RFC1191 does not apply to switches (only routers), but it probably should. I've run into this problem with switches from Fore and Cabletron. I would bet that this is a problem for most switches. JMH Jeff Haas wrote: ...
Should one install a (fast|gig)ethernet card into one's Gigaswitch and said switch have FDDI interfaces, the switch will go from having a 4500+ byte MTU to a 1500 byte MTU. This is even between FDDI card to FDDI card on the same switch.
The switch will happily fragment the packet if you leave DF off. However, if you turn it on, the switch will drop the packet silently. The switch is unable to generate ICMP and route it back to the sender.
Given all of this, PMTU-D breaks in these circumstances. ... -- Jeffrey Haas - Merit RSng project - jeffhaas@merit.edu
-- John Hall <j.hall@f5.com> F5 Networks, Inc. Senior Test Engineer 206-505-0800 It is up to us to produce better-quality movies. -- Lloyd Kaufman, producer of "Stuff Stephanie in the Incinerator"
participants (3)
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Alex Bligh -
Jeff Haas -
John Hall