1.1 Network theory _ IP Fragmentation _ IP MTU

If sweep range and DF bit are set on ping command, and we see !!!!!!!!!!!!!!!!!!MMMMM What kind of problem is detected?

The maximum MTU size that can be used is 1492

IPv4 and IPv6 minimum MTU?

IPv4 Min MTU = 576 IPv4 Min MTU = 1280

IPv4 and IPv6 header length?

IPv4 header length = 20 octets IPv6 header length = 40 octets

IPv6 and IPv4 routers fragment packets?

IPv6 routrers *not support* packet fragmentation. IPv4 routrers support packet fragmentation if DF bit = 0

Which two methods change the IP MTU value for an interface?

- Configure the *interface* MTU - Configure the *interface* IP MTU

An IPv6 network has different MTUs on different segments, if the network is experiencing reliability issues, which option is the most likely reason? A. ICMPv6 is filtered. B. The Do Not Fragment bit is marked. C. HSRPv6 is configured incorrectly. D. The MTU size is greater than 1470 bytes.

A. ICMPv6 is filtered; so router that dropped the packet cannot let source (host) knows that its interface MTU is not big enough to can handle packet and it dropped. There will be no mechanism to alert sender to fragment/reduce packet size.

Which statements about PMTUD is true? A. It is supported by TCP and UDP. B. It increases the connection's send MSS value to prevent fragmentation. C. GRE tunnels use PMTUD to fragment data packets by default. D. It is used to prevent fragmentation of packets at the endpoint of a TCP connection. E. It is used to prevent fragmentation of packets traveling across a smaller MTU link between

A. What Is PMTUD? TCP MSS as described earlier takes care of fragmentation at the two endpoints of a TCP connection, but it does not handle the case where there is a smaller MTU link in the middle between these two endpoints. PMTUD was developed in order to *avoid fragmentation in the path between the endpoints*. It is used *to dynamically determine the lowest MTU along the path* from a packet's source to its destination. Note: PMTUD is only supported by TCP and UDP. Other protocols do not support it. If PMTUD is enabled on a host, and it almost always is, all TCP/IP or UDP packets from the host will have the DF bit set.