IP Fragmentation

NIL – The Never-Ending Story of IP Fragmentation.

Listing 1
Permitting PMTUD-related ICMP packets in an extended ip access-list
permit icmp any any packet-too-big
deny icmp any any fragments

Clear the don’t fragment bit for UDP traffic

ip access-list extended BrokenUDP
remark The UDP filter should be more specific
permit udp any any
route-map ClearDF permit 10
match ip address BrokenUDP
set ip df 0
interface FastEthernet0/0
ip address
ip policy route-map ClearDF

IP Fragmentation and Tunnels
Tunnel Path MTU Discovery display

Rtr#show interface tunnel 0 | include protocol|Path
Tunnel0 is up, line protocol is up
Tunnel protocol/transport GRE/IP
Path MTU Discovery, ager 10 mins, min MTU 92, MTU 776, expires 00:01:57
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