ā10-03-2011 08:54 AM - edited ā03-04-2019 01:48 PM
Hi all,
I have many point to point GRE over IPsec tunnels for my OSPF routing.
I've been working on correcting MSS and MTU values to prevent issues. But something puzzles me :
agde06rtr03#show int tunnel 1
Tunnel1 is up, line protocol is up
Hardware is Tunnel
Description: *** blablabla ***
Internet address is 10.5.4.46/30
MTU 17916 bytes, BW 50000 Kbit/sec, DLY 50000 usec,
reliability 255/255, txload 1/255, rxload 1/255
...
Tunnel transport MTU 1476 bytes
Tunnel transmit bandwidth 8000 (kbps)
Tunnel receive bandwidth 8000 (kbps)
Why is that MTU value so high ? What does it represent ?
If anyone can help, thanks much in advance.
Tom
Solved! Go to Solution.
ā10-04-2011 02:42 AM
Hello Thomas,
The MTU value in the show interface command was changed in the later IOS versions The value is calculated based on the platform buffer sizes. So what you are seeing is the maximum value the router can support for a tunnel interface.
A new field, tunnel transport MTU, was added to check the actual MTU value of the interface.
Warm Regards,
Rose
ā10-03-2011 09:54 AM
Tom
Would you post the configuration of tunel1? My guess is that something in the configuration is setting the MTU that high. Your post of show interface is a good start. But without seeing exactly what is configured it is hard to give a good answer.
HTH
Rick
ā10-04-2011 12:58 AM
Hi Rick,
Here is the complete output :
interface Tunnel1
description *** blablabla ***
dampening
bandwidth 50000
ip address 10.5.4.46 255.255.255.252
no ip redirects
no ip unreachables
no ip proxy-arp
ip mtu 1400
ip pim dense-mode
ip tcp adjust-mss 1360
ip ospf hello-interval 1
ip ospf dead-interval 5
load-interval 30
qos pre-classify
cdp enable
tunnel source GigabitEthernet0/1.606
tunnel destination 10.5.5.77
ā10-04-2011 02:42 AM
Hello Thomas,
The MTU value in the show interface command was changed in the later IOS versions The value is calculated based on the platform buffer sizes. So what you are seeing is the maximum value the router can support for a tunnel interface.
A new field, tunnel transport MTU, was added to check the actual MTU value of the interface.
Warm Regards,
Rose
ā10-04-2011 04:43 AM
Rose
Thank you for this very helpful information.
HTH
Rick
ā10-04-2011 04:54 AM
Thanks you both for the time you took to answer my question !
Tom
ā10-04-2011 05:07 PM
Hi Rose,
Thanks for that info. May I quickly ask what are the advantages of calculating the MTU based on platform buffers.
Thanks in advance
Kishore
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