MTU refers to the largest size packet that can be transmitted out of a network interface. The path MTU is the lowest MTU of any interface along the transmission path. If a packet is larger than the path MTU, it will be fragmented or dropped. PMTUD allows endpoints to determine the optimal packet size to avoid fragmentation by starting with the local MTU and reducing size if packets are dropped.

1. MTU AND MRU
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2. • MTU or Maximum transmission unit is a topic that pops up every once in a
while in different discussions.
• MTU typically becomes an issue of concern during network changes, like
adding new vendors equipment or upgrading to a new software.
• One reason for that is the difference in implementations used by different
vendors or even between different OS versions or equipment from the same
vendor.

3. What is a data packet?
• A packet is the single unit of data that is routed between a source and a
destination on the network. Each packet contains information to help devices
or switch the packet to its destination and actual data known as payload.
What is MTU?

4. • A maximum transmission unit (MTU) is the largest length of a packet that can be
transmitted out of an interface toward a destination. When the word MTU is used
plainly, we are typically referring to the interface MTU, but when talking about a
protocol MTU (e.g IP MTU, MPLS MTU) we are typically referring to the maximum
payload of the protocol itself.
• Whether the headers are included or not is an implementation that can vary from box
to another and from OS to another, so it should always be tested out specially when
operating on a multivendor environment.
• We can’t really understand how MTU is playing a part in network operations, without
understanding the concept of path MTU.
• Path MTU: is to the lowest MTU of an interface on the path between the source and
destination. Path MTU is a very important aspect because it has a huge impact on the
overall performance of the network and end user experience.

5. • The topology below presents an analogy to clarify the path MTU concept. As we can see in the
topology the yellow rectangles height represents the exit interface MTU of the router (don’t confuse
this with bandwidth). The packet in the topology can easily fit the MTU of the interfaces in the first
segment connecting routers A and B; in the second segment, the MTU is smaller and such big
packet doesn’t fit the yellow rectangle (interface MTU) in one chunk. Therefore the path MTU is
actually the MTU of the second exist interface (The smallest), because the whole path will only be
able to pass packets that fits this MTU.

6. DEFAULT INTERFACE MTU VALUES: SOURCE WIKIPEDIA
Media Maximum Transmission Unit (bytes) Notes
Internet IPv4 Path MTU At least 68, max of 64KB
Practical path MTUs are generally
higher. Systems may use Path MTU
Discovery to find the actual path
MTU.
Internet IPv6 Path MTU
At least 1280,max of 64KB, but up to
4GB with optional jumbogram
Practical path MTUs are generally
higher. Systems must use Path
MTU Discovery to find the actual
path MTU.

7. Ethernet v2 1500
Nearly all IP over Ethernet
implementations use the
Ethernet V2 frame format.
Ethernet Jumbo Frames 1501 – 9198
The limit varies by vendor. For
correct interoperation, the
Ethernet network must have the
same MTU.Jumbo frames are
usually only seen in special-
purpose networks.
PPPoE over Ethernet v2 1492
= Ethernet v2 MTU (1500) –
PPPoE Header (8)
PPPoE over Ethernet Jumbo
Frames
1493 – 9190
= Ethernet Jumbo Frame MTU
(1501 – 9198) – PPPoE Header
(8)

8. WHAT HAPPENS IF A PACKET SIZE IS BIGGER THAN PATH MTU?
• If one host is sending packets that are having a bigger length than the path MTU and IPv4 is in
play, these packets will be fragmented if Don’t Fragment (DF) bit is not set. If they can’t be
fragmented they will be dropped by the device processing them and an ICMP message with code
“fragmentation needed” will be sent to the source to warn it about the problem. Fragmentation is
generally a bad thing; it increases network overhead, consumes routers resources and results in
many unwanted side effects.
• When the source receives “fragmentation needed” ICMP packets, it needs to lower it’s packet size
to match to avoid packets being dropped by middle routers.
• If IPv6 is in play, we know that it doesn’t support fragmentation, such large packets will be
dropped and ICMPv6 message “Packet too big” will be sent to the source to inform it that it needs
to lower the packet size to avoid the drops.
• There are some mechanism that are used solely to avoid these problems in the first place. Two of
them are PMTUD and TCP MSS adjust.

9. WHAT IS PATH MTU DISCOVERY (PMTUD) ?
• Path MTU discovery is a standardized mechanism that is used by end hosts to avoid
fragmentation or packet drops. The basic idea is that the source host will assume that the path
MTU is equal to it’s exit interface MTU and will send all packets on the path with (DF bit) set. If
any of the packets is bigger than the path MTU, it will be dropped by the middle routers and
an ICMP message will be sent to the source to inform it that it needs to lower the packet size.
• The process will continue by the host until it determines the suitable packet size and to detect
any changes in the paths or it will remove the DF bit and allow the packets to be fragmented.
• The process is pretty similar when using IPv6, with the difference that fragmentation is not
allowed in IPv6 and there is No DF bit to set.

10. WHAT IS TCP
MSS?
• TCP MSS is an option in the TCP header that is used by
the two ends of the connection independently to
determine the maximum segment size that can be
accepted by each host on this connection. The
maximum segment size is simply the maximum data
payload that a TCP packet can accommodate on the
connection.
• This option can be manipulated by network operators
using a feature known as TCP MSS adjust. The feature
allows middle routers to intercept and alter this value if
configured to do so as a technique to avoid MTU
problems mentioned above.

11. MRU
• On the other hand maximum receive unit (MRU) is the largest packet size that an interface
can receive, so it’s an ingress interface parameter. In most of the cases MRU equals MTU but
it’s not a requirement. You can configure different values for both MTU and MRU to achieve
some benefits.
What if packets received are bigger than interface MRU?
• If a device is receiving packets that are bigger in length than interface MRU due to some
reason, the packets will be considered “Too big” or oversized. Usually there will be a counter
incrementing on the interface and those packets will likely be dropped by the router’s
forwarding plane.

12. MTU AND PING SIZE CONFUSION
• Cisco IOS when you specify the size with ping you are actually specifying the
datagram size (IP header + Transport header + Application Data), this means that
Cisco includes the IP header (20 bytes) and the ICMP header (8 bytes) and thus you’ll
have a total packet size of what you have specified in the size option of a ping.
• Cisco’s method more appealing, since what you choose is what you get, with zero
confusion, but still it seems that the general common behaviour is not Cisco’s, and
thus you need to take care.

13. Description of MTU

14. OUTPUTS FROM DEVICE
Cisco IOS
• Router#ping 10.10.1.1 size 1500 df-bit
• Type escape sequence to abort.
Sending 5, 1500-byte ICMP Echos to 10.10.1.1, timeout is 2 seconds:
Packet sent with the DF bit set
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms
• Router#ping 10.10.1.1 size 1501 df-bit
• Type escape sequence to abort.
Sending 5, 1501-byte ICMP Echos to 10.10.1.1, timeout is 2 seconds:
Packet sent with the DF bit set
…..
Success rate is 0 percent (0/5)

15. Cisco IOS XR
• Router#ping 20.20.20.1 size 1500 donnotfrag
Type escape sequence to abort.
Sending 5, 1500-byte ICMP Echos to 20.20.20.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms
• Router#ping 20.20.20.1 size 1501 donnotfrag
Type escape sequence to abort.
Sending 5, 1501-byte ICMP Echos to 20.20.20.1, timeout is 2 seconds:
M.M.M
Success rate is 0 percent (0/5)
• Cisco IOS excludes the Layer 2 header from the interface MTU, while IOS XR includes the
Layer 2 header in the interface MTU. For example the default MTU for Ethernet interface is:
IOS:1500byte / IOS XR:1514bytes

16. • We must take care when changing the default MTU setting, the whole point is to always
remember whether the header is included or not – The second thing, Cisco accommodates
the extra 4 bytes of dot1q (Cisco IOS doesn’t include the header anyway, while IOS XR
accommodates the extra 3 bytes for the dot1q sub interfaces)
Cisco IOS
• Router#sh interfaces g0/1 | i MTU
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
router#sh ip interface g0/1.1 | i MTU
MTU is 1500 bytes
After Changing the MTU to 1600:
• Router#sh interfaces g0/1 | i MTU
MTU 1600 bytes, BW 1000000 Kbit, DLY 10 usec,
router#sh ip interface g0/1 | i MTU
MTU is 1600 bytes

17. Cisco IOS XR
• Router#sh int Gi0/1/0/9 | i MTU
MTU 1514 bytes, BW 1000000 Kbit
Router#sh ip int g0/1/0/9 | i MTU
MTU is 1514 (1500 is available to IP)
After Changing the MTU to 1614:
• Router#sh int g0/1/0/1 | i MTU
MTU 1614 bytes, BW 1000000 Kbit
Router#sh ip int g0/1/0/1 | i MTU
MTU is 1614 (1600 is available to IP)

18. After Changing the MTU to 1614 with dot1q:
• Router#sh int g0/1/0/9 | i MTU
MTU 1614 bytes, BW 1000000 Kbit
Router#sh ip int g0/1/0/9 | i MTU
MTU is 1614 (1600 is available to IP)
Router#sh ip int g0/1/0/9.1 | i MTU
MTU is 1618 (1600 is available to IP)