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So basically do the modern routers in the internet still use 1500 as the MTU of Ethernet? or the 1500 Byte was for the old days?

What I'm asking here is what is the MTU of Ethernet in modern Networks?

and if it is 1500 bytes, what will happen if we for example use 15000 bytes instead and what will happen to the Internet if we force all the routers to use 15000 bytes instead?

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    Oh and i've never heard of an MTU size of 30k. – user36472 Jul 16 '18 at 12:07
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    @Cown. ehm... "and reassembly of the packets". That would never be a router's task. That's up to the end host, tunneling involved or not. – Marc 'netztier' Luethi Jul 16 '18 at 12:14
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    @Marc'netztier'Luethi you should read up about this command: ip virtual reassembly (router will reassemble fragments) – Mike Pennington Jul 16 '18 at 13:45
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    Router reassembly is very common in pppoe and firewall configurations. supportforums.cisco.com/t5/wan-routing-and-switching/… – Mike Pennington Jul 16 '18 at 14:28
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    I stand corrected. Reassembly may happen on routers. – Marc 'netztier' Luethi Jul 16 '18 at 14:56
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Standard maximum payload size for Ethernet is still 1500 bytes.

While the maximum frame size has grown slightly, from 1518 to 1522 byte with 802.1Q and further with 802.1AD, the payload, "MAC client data" size, or Maximum Service Data Unit (MSDU) hasn't been changed as per IEEE standards for compatibility reasons - maintaining the payload size enables transparent switching within the whole range from 10 Mbit/s up to 400 Gbit/s.

In Ethernet, there is no concept for negotiating a frame size nor for fragmenting a frame oversized for forwarding nor for providing an error message to the sender, so the frame can only be dropped. Each node in a segment needs to use the same MTU.

However, "jumbo" frames exceeding this maximum have been popular for a while in closed, controlled networks. The network administrator has to make sure that all nodes on a network can handle the non-standard size before actually using that frame size. Very common is a payload size of 9000 bytes, six times the official size.

There are also "baby giant" implementations where a tunnel's outside MTU is increased somewhat to enable encapsulation of full-sized packets without eating into the inside MTU (or for similar reasons).

On the Internet the requirement is just a minimum link MTU of 68 bytes for IPv4 and 1280 bytes for IPv6. Note that while large parts of the Internet use Ethernet nowadays, not all do.

If you pass an IPv4 packet larger than 1500 bytes to your Internet router it should fragment it according to its uplink MTU. Without fragmentation, the next hop router will likely just drop the packet. For IPv6, there's no router fragmentation and path MTU discovery is mandatory, so your client should never send a packet exceeding the destination path's MTU.

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    Jumbo frames are especially useful when dumping large amounts of data from device to device in a UDP manner. The decrease in overhead by having more data per packet is significant enough to be worth it at times and I've seen it in use for such in data-heavy environments. – Mast Jul 16 '18 at 18:51
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    Jumbo frames are more significant for reducing the processing overhead - or rather were, the overhead has been significantly reduced by offloading features since. By bandwith, UDP throughput increases by just 3.7% (1.5k vs 9k). – Zac67 Jul 16 '18 at 19:32
  • Wouldn't that be "grown slightly, from 1514 to 1518 bytes with 802.1Q VLAN and further with 802.1AD"? (MTU of 1500 + 14 byte Ethernet header (excluding preamble)) – Jonathon Reinhart Sep 10 at 18:01
  • @JonathonReinhart The FCS is part of the frame as well, so that's 18 bytes overhead for L2 (22 for 802.1Q, ...). – Zac67 Sep 10 at 18:09
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The ethernet (IEEE 802.3) standard is still 1500 octets for the MTU, but some vendors support jumbo frames. Unfortunately, there is no standard for jumbo frames, and different vendors support different sizes for a jumbo MTU, even across the product line, or sometimes even different sizes for a jumbo MTU on different interfaces in the same switch.

If you try to send a jumbo frame, and it encounters an interface anywhere along its switched path with a smaller MTU, it will be dropped as a giant frame, and it will simply be lost. Switches do not fragment frames. Routers may fragment packets to fit an MTU on a different interface, but switches do not fragment frames because ethernet has no facility for fragmentation. Even with routers fragmenting packets, most businesses are now dropping fragmented packets to prevent fragment DoS attacks. Fragmentation is expensive to router resources, and IPv6 has eliminated fragmentation in the path, requiring hosts to use PMTUD to discover the minimum MTU in a path, and to pre-fragment packets before sending.

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