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Community Member

PMTUD Question

Hi All,

I want to ask a question which is that I have a Cisco 2821 router running GRE over IPSec which point to another Cisco 2821 which has same configuration. I see so many fragmentation shown in "show ip traffic". Then I have read the Cisco document "". But there are some points which I cannot understand. As follows:

1. If I don't set the tunnel interface a command "tunnel path-mtu-discovery", the tunnel0 interface will not perform PMTUD as it will clear the DF bit after encap the packets and then send to IPSec for encryption. This is the current configuration of my router. But why I also can see the icmp unreachable(type 3, code 4) sent by this router to my hosts in LAN? Does the tunnel0 interface still sends icmp unreachable to hosts which send packets with DF bit 1 even though the "tunnel path-mth-discovery" not be enabled?

2. After I read the Cisco document mentioned above, I decide to set "ip tcp adjusted-mss 1360" to ethernet interface which point to my LAN in order to tell the hosts to send MSS which meet the IPSec packet size and set "tunnel path-mth-discovery" to let the tunnel0 interface to add DF bit 1 to new encapsulated GRE packets' IP header so that IPSec will not fragment those GRE packets. Is this decision right?

The IPSec crypto map is applied to one of the Gigabit Ethernet interface of the 2821. Another Gi0/1 is for LAN. One tunnel0 interface. Loopback0's IP is the tunnel source.

Additional configuration:

MTU 1476 in "show ip int tunnel0"

MTU 1514 in "show int tunnel0" and "show int loopback0"

MTU 1500 in "show int gi0/1" and "show int gi0/0"

Path MTU 1500 in "show crypto ip sa"(Gi0/1 has crypto map point to another Cisco 2821)

Anybody could answer my question?




Re: PMTUD Question


1) If you do not set the PMTUD - it does not "clear" the DF bit. It just does not copy the DF bit in the GRE header.

2) If you have already set the tcp mss adjust command - there should be no requirement for you to also enable the PMTUD - as any packet that traverses the tunnel AFTER TCP SYN/TCP SYN ACK will NEVER exceed the tunnel MTU even with the DF bit set.

Personally I have found that when dealing with "Windows" PC's and Servers PMTU and PMTUD does not work. Within networking I have found, the below:-

1) Calculate the best MTU/MSS for GRE/IPSEC - giving some "fudge"

2) When using tcp-adjust-mss - place on the tunnel interfaces.



Community Member

Re: PMTUD Question

Hi Andrew,

1) Is that the router still tell the host to reduce the packet size when the DF bit is 1 even though the PMTUD not enabled on the router?

2) Shouldn't I set the "ip tcp adjusted-mss" on tunnel0 interface? Should it work just only enable on the interface which facing the LAN?



Re: PMTUD Question

1) If a router tries to forward an IP datagram, with the DF bit set, onto the tunnel that has a lower MTU than the size of the packet, the router will drop the packet and return an ICMP "Destination Unreachable" message is sent back to the source of the IP datagram, with the code indicating "fragmentation needed and DF set". When the source station receives the ICMP message, it will lower the send MSS, and when TCP retransmits the segment, it will use the smaller segment size. BUT I have found that windows IGNORES the icmp "fragmentation needed" - even when PMTU and PMTUD are enabled on the servers/workstations.

2) I personally have implemented it on the tunnel interfaces in networks where I have found this issue. Personally I configure as close to the source as the problem as possible - in this instance the tunnel, but it's not really close to the issue. What you could do is just lower the MTU of the NIC's for all your devices at the site - however this does not scale well if you have to manually "touch" hundreds or thousands of machines.


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