Setting pmtu value

[Patrick Chim]

Hi,

I perform the following action and it shown errors. I see the man page and it doesn't show any limitation. I try e.g. '-p 525' and it's OK. Then what's wrong with '-p 1500' ?

> route -p 1500 add net 167.247.133.64 netmask 255.255.255.224 172.19.1.50 1
add net 167.247.133.64: gateway 172.19.1.50: Invalid argument

Regards,
Patrick

[Patrick Wallek]

I think part of your problem is your gateway is not going to work with your specified netmask and IP address.

You need a gateway that is in the same address range as your IP.

[Patrick Chim]

Hi,

My default gateway is 172.19.130.5 .
Is it impossible to set the pmtu ?

Regards,
Patrick

[Bill Hassell]

Based on the subnet mask you are using, and that your computer is on the 167.xx subnet but your gateway has a 172.xx network address. The gateway and your computer are not on the same network based on these numbers.

[Patrick Wallek]

With those IP numbers, it is not only impossible to set the pmtu, it is impossible to set anything. Your default gateway and machine's IP MUST be on the same subnet.

With your 172.19.1.50 default gateway, your machine should be on a 172.19.1.??? network.

At least that is as far as I know.

[Patrick Chim]

Hi,

With more information, the IP address 172.19.1.50 is the firewall !

167.247.152.32 172.19.1.50 UGH 0 lan3 0
167.247.152.23 172.19.1.50 UGH 0 lan3 0
167.247.133.64 172.19.1.50 UG 0 lan3 0
default 172.19.130.5 UG 0 lan1 0

Regards,
Patrick

[Patrick Chim]

Hi,

Furthermore, the route can be added if I did not specific the '-p' option and the connection can be established.

Regards,
Patrick

[Patrick Wallek]

Does your NIC allow a pmtu of 1500? According to the route man page:

-p pmtu Specifies a path maximum transmission unit (MTU) value for a static route. The minimum value allowed is 68 bytes; the maximum is the MTU of the outgoing interface for this route. This option can be applied to both host and network routes.

[Patrick Chim]

Hi,

Does you mean the setting of the lan card ? If so, then all the lan cards have been set to a MTU value 1500.

Why I raise this question is that our Network administrator discover that some packet sending between the specified hosts has split into few parts and each part about 500 in size and so decrease the performance.

Regards,
Patrick

[sven verhaegen]

hi

what you are seeing i s probably fragmenting , this has nothing tto do with the actual PMTU set at cardlevel , in a HP machine each card has apredefined mtu size , for ethernet cards that is standard 1500 , but in case for instance that you are passing trough dfifferent media which has a smaller bandwith e.g. x.25 , leased lines a.o. they might have but a mtu of 600 then if you machine has fragmenting set to on the communication and negotiation could end up in the paket size being deciuded as way lower the normal capable size of the interface , you would get instead of 1500 bytes , 2 * 600 bytes and 1 * 300 bytes packets , most probably this is what is happening , there are ways of deactivating fragmenting , by adpating pmtu_strategy in the ndd tunable parameters but I would not advise for this unless you are sure the large packet size is handled correctly by your routing devices

[rick jones]

it would be good if you could provide the complete output of netstat -rn from your system.

also, just out of curiousity, what is the value of ip_pmtu_strategy on your system?

[Patrick Chim]

Hi,

I have set ip_pmtu_strategy to 1 by using the ndd command but the packet is still break down into 3 parts.

Below is the output of the 'netstat -nr' command.

Routing tables
Destination Gateway Flags Refs Interface Pmtu
127.0.0.1 127.0.0.1 UH 0 lo0 4136
10.0.0.12 10.0.0.12 UH 0 lan0 4136
172.19.1.111 172.19.1.111 UH 0 lan3 4136
172.19.128.11 172.19.128.11 UH 0 lan1 4136
172.29.13.18 172.19.1.50 UGH 0 lan3 0
10.0.16.21 172.19.1.50 UGH 0 lan3 0
10.0.16.1 172.19.1.50 UGH 0 lan3 0
10.0.16.2 172.19.1.50 UGH 0 lan3 0
172.29.28.28 172.19.1.50 UGH 0 lan3 0
172.16.0.15 172.19.1.50 UGH 0 lan3 0
167.247.152.70 172.19.1.50 UGH 0 lan3 0
167.247.152.69 172.19.1.50 UGH 0 lan3 0
167.247.152.68 172.19.1.50 UGH 0 lan3 0
167.247.152.67 172.19.1.50 UGH 0 lan3 0
167.247.152.66 172.19.1.50 UGH 0 lan3 0
167.247.152.32 172.19.1.50 UGH 0 lan3 0
167.247.152.23 172.19.1.50 UGH 0 lan3 0
10.0.16.111 172.19.1.50 UGH 0 lan3 0
192.168.1.10 172.19.1.50 UGH 0 lan3 0
172.29.240.86 172.19.1.50 UGH 0 lan3 0
192.168.185.241 172.19.1.50 UGH 0 lan3 0
167.247.152.68 172.19.1.50 UGH 0 lan3 0
167.247.152.67 172.19.1.50 UGH 0 lan3 0
167.247.152.66 172.19.1.50 UGH 0 lan3 0
167.247.152.32 172.19.1.50 UGH 0 lan3 0
167.247.152.23 172.19.1.50 UGH 0 lan3 0
10.0.16.111 172.19.1.50 UGH 0 lan3 0
192.168.1.10 172.19.1.50 UGH 0 lan3 0
172.29.240.86 172.19.1.50 UGH 0 lan3 0
192.168.185.241 172.19.1.50 UGH 0 lan3 0
192.168.185.240 172.19.1.50 UGH 0 lan3 0
192.168.185.243 172.19.1.50 UGH 0 lan3 0
192.168.185.245 172.19.1.50 UGH 0 lan3 0
192.168.185.246 172.19.1.50 UGH 0 lan3 0
172.19.128.0 172.19.128.11 U 2 lan1 1500
172.19.0.0 172.19.1.111 U 2 lan3 1500
10.0.0.0 10.0.0.12 U 2 lan0 1500
128.7.55.0 172.19.1.50 UG 0 lan3 0
192.1.10.0 172.19.1.50 UG 0 lan3 0
167.247.133.64 172.19.1.50 UG 0 lan3 0
128.123.19.128 172.19.1.50 UG 0 lan3 0
127.0.0.0 127.0.0.1 U 0 lo0 0
192.168.0.0 172.19.1.50 UG 0 lan3 0
default 172.19.130.5 UG 0 lan1 0

Regards,
Patrick

[U.SivaKumar_2]

Hi,
This link will explain you everything.

http://www.cisco.com/warp/public/105/38.shtml

regards,
U.SivaKumar

[rick jones · Accepted solution]

ip_pmtu_strategy = 1 may not appear to "take" right away for a given destination if there is already something called an IRE (internet routing entry, iirc) for that destination in the routing tables. It may be that the change in ip_ptmu_strategy will not take until that entry expires and is refreshed.

it has been a long time, but seeing all those host routes (flag "H" showing) with a 0 pmtu looks strange to me. I thought that when a dynamic PMTU host route expired the PMTU was reset to that of the local interface. Of course, that was back on 10.20 - it might be different on 11.X

[Patrick Chim]

Hi,

Rick, you are right. After about 1 hour that I set the ip_pmtu_strategy=1, the packet sent will not break down into small pieces.

Many thanks,
Patrick :)