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New Member


I keep running into issues with MTU and TCP MTU, yet I'm not following why it causes so many problems

ie.  For mpls, you need to decrease the MTU to account for the MPLS labels.

ie.. For tunnel encapsulation (ipsec, GRE, etc), you need to decrease the mtu to account fro the additional overhead..

ie...BGP..if the mtu somewhere along the path is lower than what is negotiated, the peering will fail

The logic makes sense on why to lower the mtu to prevent fragmentation.

However, it would seem if these settings weren't set, the router/L3 device would just fragment.  Other than the extra processing power to reassemble, it would still seem like fragmentation wouldn't cause so many connectivity issues.

If most hosts sent packets with the DF bit set, then I could easily see why fragmentation would cause issues.   Most packets dont have the df bit set though so fragmentation shouldnt cause an issue.

Cisco Employee



Here is a best document to read for fragmentation.

Issues with IP Fragmentation

There are several issues that make IP fragmentation undesirable. There is a small increase in CPU and memory overhead to fragment an IP datagram. This holds true for the sender as well as for a router in the path between a sender and a receiver. Creating fragments simply involves creating fragment headers and copying the original datagram into the fragments. This can be done fairly efficiently because all the information needed to create the fragments is immediately available.

Fragmentation causes more overhead for the receiver when reassembling the fragments because the receiver must allocate memory for the arriving fragments and coalesce them back into one datagram after all of the fragments are received. Reassembly on a host is not considered a problem because the host has the time and memory resources to devote to this task.

But, reassembly is very inefficient on a router whose primary job is to forward packets as quickly as possible. A router is not designed to hold on to packets for any length of time. Also a router doing reassembly chooses the largest buffer available (18K) with which to work because it has no way of knowing the size of the original IP packet until the last fragment is received.

Another fragmentation issue involves handling dropped fragments. If one fragment of an IP datagram is dropped, then the entire original IP datagram must be resent, and it will also be fragmented. You see an example of this with Network File System (NFS). NFS, by default, has a read and write block size of 8192, so a NFS IP/UDP datagram will be approximately 8500 bytes (including NFS, UDP, and IP headers). A sending station connected to an Ethernet (MTU 1500) will have to fragment the 8500 byte datagram into six pieces; five 1500 byte fragments and one 1100 byte fragment. If any of the six fragments is dropped because of a congested link, the complete original datagram will have to be retransmitted, which means that six more fragments will have to be created. If this link drops one in six packets, then the odds are low that any NFS data can be transferred over this link, since at least one IP fragment would be dropped from each NFS 8500 byte original IP datagram.

Firewalls that filter or manipulate packets based on Layer 4 (L4) through Layer 7 (L7) information in the packet may have trouble processing IP fragments correctly. If the IP fragments are out of order, a firewall may block the non-initial fragments because they do not carry the information that would match the packet filter. This would mean that the original IP datagram could not be reassembled by the receiving host. If the firewall is configured to allow non-initial fragments with insufficient information to properly match the filter, then a non-initial fragment attack through the firewall could occur. Also, some network devices (such as Content Switch Engines) direct packets based on L4 through L7 information, and if a packet spans multiple fragments, then the device may have trouble enforcing its policie