<html><body><div style="color:#000; background-color:#fff; font-family:Helvetica Neue-Light, Helvetica Neue Light, Helvetica Neue, Helvetica, Arial, Lucida Grande, Sans-Serif;font-size:16px"><div dir="ltr" id="yui_3_16_0_1_1429408051078_14272"><span id="yui_3_16_0_1_1429408051078_14524">Another one that comes to mind,</span></div><div dir="ltr" id="yui_3_16_0_1_1429408051078_14163"><span><br></span></div><div dir="ltr" id="yui_3_16_0_1_1429408051078_14161"><span id="yui_3_16_0_1_1429408051078_14162">- Don't assuming that a inter-router or PE-CE protocol validates or tests that MTUs match or are sufficient. EIGRP doesn't check MTUs when it comes up, and if I recall correctly it isn't required of IS-IS either.</span></div><div dir="ltr" id="yui_3_16_0_1_1429408051078_14161"><span><br></span></div><div dir="ltr" id="yui_3_16_0_1_1429408051078_14161"><span id="yui_3_16_0_1_1429408051078_14376">I don't know if the default has changed, however a while back TCP connections from a Cisco router used the minimum maximum TCP segment size of 576 octets and didn't use the TCP MSS option. That included for BGP TCP connections. This is a safe choice, however it means that BGP TCP connections didn't take advantage of much larger interface MTUs to reduce BGP establishment times, and also wouldn't uncover mismatched MTUs on both ends of the link. So BGP coming up wasn't as much of an absolute indication of that the link was fully and properly configured. If I recall correctly, the command to enable full utilisation of link MTU for BGP and other TCP connections is the global 'ip tcp path-mtu-discovery' command.</span></div><br> <div style="font-family: Helvetica Neue-Light, Helvetica Neue Light, Helvetica Neue, Helvetica, Arial, Lucida Grande, Sans-Serif; font-size: 16px;" id="yui_3_16_0_1_1429408051078_14139"> <div style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, Sans-Serif; font-size: 16px;" id="yui_3_16_0_1_1429408051078_14138"> <div dir="ltr" id="yui_3_16_0_1_1429408051078_14137"> <hr size="1" id="yui_3_16_0_1_1429408051078_14275"> <font size="2" face="Arial" id="yui_3_16_0_1_1429408051078_14140"> <b id="yui_3_16_0_1_1429408051078_14448"><span style="font-weight:bold;" id="yui_3_16_0_1_1429408051078_14447">From:</span></b> Mark ZZZ Smith <markzzzsmith@yahoo.com.au><br> <b><span style="font-weight: bold;">To:</span></b> James Mcintosh <james.mcintosh@rocketmail.com>; "ausnog@lists.ausnog.net" <ausnog@lists.ausnog.net> <br> <b><span style="font-weight: bold;">Sent:</span></b> Sunday, 19 April 2015, 12:41<br> <b id="yui_3_16_0_1_1429408051078_14160"><span style="font-weight: bold;" id="yui_3_16_0_1_1429408051078_14159">Subject:</span></b> Re: [AusNOG] Sensible MTU For Core Routers<br> </font> </div> <div class="y_msg_container" id="yui_3_16_0_1_1429408051078_14141"><br><div id="yiv4253926429"><div id="yui_3_16_0_1_1429408051078_14469"><div style="color:#000;background-color:#fff;font-family:Helvetica Neue-Light, Helvetica Neue Light, Helvetica Neue, Helvetica, Arial, Lucida Grande, Sans-Serif;font-size:16px;" id="yui_3_16_0_1_1429408051078_14468"><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026">Hmm, you've opened the MTU can of worms.</div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026"><br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026">It's wise to set your internal interface MTUs to be both consistent and large enough for the outset to support any sort of tunnelling you might possibly perform across your network internally at any time in the future e.g., GRE, L2TP, IPsec, MPLS. Having a too small MTU is can be a much worse problem to have than a too big MTUs, so in the past I've just set it to the maximum supported by the device. Changing an MTU at a later date can either be very disruptive to the traffic flowing across the interface (because it may have to redo memory buffer allocations, or renegotiate it with the other end e.g., PPP) or may even require a reboot of a device (so service outage, out of hours work etc. and I like to say it takes 10 times the effort to work out of hours, so you avoid it everywhere you can.)</div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026"><br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026">A few other rules about MTUs:</div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026"><br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026">- know what the MTU value actually is describing. Is it a payload MTU (e.g. 1500 octets for Ethernet), a "whole of frame" MTU, which includes overhead fields of the frame or packet (e.g., for Ethernet, 12 octets Dst Addr. & Src. Addr. + 2 octet type/length + 1500 octet payload + 4 octet FCS (checksum) = 1518 octets. (Not that you'll likely to be exposed to it when setting an MTU, however there is also 8 octets of per-frame preamble in Ethernet, as well as an interframe gap that is also measured in bits, if you happen to want to calculate the maximum frames per second a layer 1 link speed of e.g., 10Mbps supports. ))</div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026"><br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026">- VLAN tags or other tunnelling headers/fields may or may not be included in the MTU that you set. For example, if you're setting the "whole of frame MTU" on an interface, the vendor might be limiting it to allow 1 or more VLAN tags to be added. IOW, you're not actually setting the "whole of frame MTU".<br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026"><br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026">- Layer 2 payload MTUs are layer 3 whole of packet MTUs. This becomes important to know because if you use the term "IPv4 MTU" are you describing the IPv4 whole of packet MTU, or the IPv4 payload MTU? The IPv4 payload can vary because of IPv4 options (they're rare, however the one you're likely to see is the Router Alert option), as can the IPv6 payload because of Extension Headers.<br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026"><br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026">- We're now in a multiprotocol world, so it may be possible to set different payload and/or whole of frame MTUs for different protocols. Typically you'll be setting the "Layer 2 MTU", either the payload or whole of frame MTU. However, on routers it is likely to be possible to set different MTUs for different layer 3 protocols, and they'll be the "whole of frame MTU" for the layer 3 protocol. This can be useful to know in a few scenarios, but also explains why there might be multiple per-interface MTU parameters you can set on a router.</div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026"><br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026">- Some links can support different "MTUs" in different directions e.g., PPP. PPP supports setting Max Receive Unit and Maximum Transmit Unit values, and they're announced to each other (or negotiatied, I'm getting a bit rusty)</div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026"><br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026">- in general make try to make sure all devices attached to the same link have the same MTU, otherwise one of the devices can generate a frame that is too big for other devices to receive, and it will be silently dropped. You can be a bit loose with this rule as long as you ensure that all frames that will ever appear on the link will be no larger than the smallest MTU that any of the attached devices (routers, switches, hosts.) support. This is a bit in conflict with my practice of setting it to as large as the device will support. I've figured that a bit of effort to ensure too large frames are originated or forwarded is a lesser effort than the possible effort (out of hours reboot etc.) involved in changing an MTU. I would strictly follow this rule when hosts are attached to the link that I don't administer.</div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026"><br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026">- avoid "dumbell MTU" scenarios i.e., large/small/large MTU values. They'll likely trigger packet fragmentation or path MTU discovery. Fragmentation can't or is costly to do in hardware, and PMTUD can be unreliable either because the ICMP Packet Too Big messages are dropped by overly zealous security people or are rate limited. In the later case, the rate limits can be increased, however they're still generated by the router control plane, which taxes the general purpose control plane CPU. Examples of dumbell MTU scenarios are residential PPPoE access without large PPPoE MTU support (i.e., customer LAN MTU:1500, PPPoE MTU:1492, ISP MTU:1500) or VPN scenarios (e.g., branch LAN MTU:1500, IPsec/L2TP tunnel MTU (much < 1500), central LAN MTU:1500). Lowering the outer MTUs to suit the middle of the dumbell MTU is better, and in IPv4 can be achieved with the DHCP MTU option (assuming the hosts support it) or the IPv6 RA MTU option. (The other option to 'fix' this is the TCP MSS hack, however that only works for TCP, not UDP, IPsec, GRE etc., etc., etc., and is ugly when you look at what it is actually having to do to pull it off (because the TCP MSS option doesn't have to be in a fixed location in the TCP segment.))</div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026"><br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026">- IPv4 minimum whole of packet MTU is 68 octets, IPv6 minimum whole of packet MTU is 1280 octets.</div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026"><br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026"><br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026">With all those worms, it's probably time to go fishing.</div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026"><br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026"><br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026"><br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6026"><br clear="none"></div><br clear="none"> <div id="yiv4253926429yui_3_16_0_1_1429408051078_6030" style="font-family:Helvetica Neue-Light, Helvetica Neue Light, Helvetica Neue, Helvetica, Arial, Lucida Grande, Sans-Serif;font-size:16px;"> <div id="yiv4253926429yui_3_16_0_1_1429408051078_6029" style="font-family:HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, Sans-Serif;font-size:16px;"> <div class="qtdSeparateBR"><br><br></div><div class="yiv4253926429yqt3301906262" id="yiv4253926429yqt10291"><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429408051078_6028"> <hr size="1"> <font id="yiv4253926429yui_3_16_0_1_1429408051078_6027" size="2" face="Arial"> <b id="yiv4253926429yui_3_16_0_1_1429408051078_7225"><span id="yiv4253926429yui_3_16_0_1_1429408051078_7224" style="font-weight:bold;">From:</span></b> James Mcintosh <james.mcintosh@rocketmail.com><br clear="none"> <b><span style="font-weight:bold;">To:</span></b> "ausnog@lists.ausnog.net" <ausnog@lists.ausnog.net> <br clear="none"> <b id="yiv4253926429yui_3_16_0_1_1429408051078_7227"><span id="yiv4253926429yui_3_16_0_1_1429408051078_7226" style="font-weight:bold;">Sent:</span></b> Sunday, 19 April 2015, 10:53<br clear="none"> <b><span style="font-weight:bold;">Subject:</span></b> [AusNOG] Sensible MTU For Core Routers<br clear="none"> </font> </div> <div class="yiv4253926429y_msg_container" id="yiv4253926429yui_3_16_0_1_1429408051078_6031"><br clear="none"><div id="yiv4253926429"><div id="yiv4253926429yui_3_16_0_1_1429408051078_6857"><div id="yiv4253926429yui_3_16_0_1_1429408051078_6856" style="color:#000;background-color:#fff;font-family:HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, Sans-Serif;font-size:16px;"><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429404584935_3026">Hey Noggers,</div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429404584935_3026"><br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429404584935_3026">With router MTU's being configurable up to a whopping 9216 what are people setting for their interface MTU's on internet connected core and PE routers?</div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429404584935_3026"><br clear="none"></div><div dir="ltr" id="yiv4253926429yui_3_16_0_1_1429404584935_3026">Is it is still just the default of 1500?</div><div class="yiv4253926429" dir="ltr" id="yiv4253926429yui_3_16_0_1_1429404584935_3026" style=""> </div><div class="yiv4253926429" dir="ltr" id="yiv4253926429yui_3_16_0_1_1429404584935_3026" style="">What about core switches? What are you setting there to get get interoperability with various ethernet providers?</div><div class="yiv4253926429" dir="ltr" id="yiv4253926429yui_3_16_0_1_1429404584935_3026" style=""><br clear="none"></div><div class="yiv4253926429" dir="ltr" id="yiv4253926429yui_3_16_0_1_1429404584935_3026" style=""><br clear="none"></div><div class="yiv4253926429" dir="ltr" id="yiv4253926429yui_3_16_0_1_1429404584935_3026" style="">-James</div></div></div></div><br clear="none">_______________________________________________<br clear="none">AusNOG mailing list<br clear="none"><a rel="nofollow" shape="rect" ymailto="mailto:AusNOG@lists.ausnog.net" target="_blank" href="mailto:AusNOG@lists.ausnog.net">AusNOG@lists.ausnog.net</a><br clear="none"><a rel="nofollow" shape="rect" target="_blank" href="http://lists.ausnog.net/mailman/listinfo/ausnog">http://lists.ausnog.net/mailman/listinfo/ausnog</a><br clear="none"><br clear="none"><br clear="none"></div></div> </div> </div> </div></div></div><br><div class="yqt3301906262" id="yqt28978">_______________________________________________<br clear="none">AusNOG mailing list<br clear="none"><a shape="rect" ymailto="mailto:AusNOG@lists.ausnog.net" href="mailto:AusNOG@lists.ausnog.net">AusNOG@lists.ausnog.net</a><br clear="none"><a shape="rect" href="http://lists.ausnog.net/mailman/listinfo/ausnog" target="_blank">http://lists.ausnog.net/mailman/listinfo/ausnog</a><br clear="none"></div><br><br></div> </div> </div> </div></body></html>