confused about mpls mtu command

[flyjunli]

hi,

i confuse about mpls mtu command

test platforms are 76 pfc3b,mpls,gigabit sip400 spa interface

if i didn't config mpls mtu command ,using default,ping command is successful,if more than 1496 packets, i can see fragment from show ip traffic.

if i config mpls mtu override 1504,ping command is sucessful too. there is fragment too when i use 1501 byte packet.

if i config mpls mtu override 1524 byte.

ping command failed if i use packet more than 1500, , all packet are droped,even 1501 byte.only 1500 byte packet can success.

all config above interface mtu is 1500.

this confused me.

why i use default 1500 interace mtu, mpls mtu override 1504 ,ping packet can fragment,ping success, but i use mpls mtu override 1524, i can see fragment in show ip traffic,but ping command failed. i can't see packet in destination router,how this work.

thank you!

jun

[mohammedmahmoud]

Hi,

Can you please elaborate on the topology, since if you are testing with two directly connected routers, and you are pinging directly connected IP addresses from the neighbor router, then according to the PHP behavior the traffic will be ordinary IP traffic and not MPLS traffic and thus the mpls mtu command has no effect over it.

BR,

Mohammed Mahmoud.

[flyjunli]

topology is simple

7609-1--sip GE spa----7609-2--pos---7609-3--flexwan E1-----7604-1--ge--ce

i config mpls mtu 1524 between 7609-1 and 7609-2 . and keep interface mtu 1500 default.

ping packet from 7609-1 to 7604-1 loopback 0.

ping 1500 byte packet is ok, but ping 1501 byte packet is totally lost.then i config mtu 1524 between 7604-1 and 7609-3, it is useless,notwork, i can't see packet coming from 7609-1 on 7604-1.

but i add config mtu 1524 between 7609-1 and 7609-2. config mtu 1500 between 7604-1 and 7609-3,ping 1501 bytes from 7609-1 to 7604-1 loopback0 is ok. but i can see fragment from show ip traffic command in 7609-3.

i have a question, why we need mpls mtu command. if we don't change interface mtu,just only config mpls mtu 1524, it doesn't work, if we just change mpls mtu,how it work in the ios. if we config interface mtu 1524,interface mtu size is big enough, it seems mpls mtu command is useless, we don't need mpls mtu command, just interface mtu 1524 is ok.

why we need mpls mtu command. we just only change interface mtu is enough.

thank you!

jun

[a.alekseev]

http://www.cisco.com/en/US/docs/ios/12_2/switch/command/reference/xrfscmd3.html#wp1059294