MTU RELATED ISSUES ON MPLS NETWORK

[2D-Technology Services]

Hello Community Members.,

Recently I started noticing MTU related issues on my network .

Some internet users can no-longer open some key websites, in a bid to resolve this  I would sometime adjust the MSS : '''ip tcp adjust-mss   1360'' on either the sub or Physical Interface as the case may be.

The PE is on MTU ,

#sh int BDI208 | incl MTU
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,

Fa0/0

ip address 10.3.0.89 255.255.255.248
no ip redirects
ip tcp adjust-mss 1360

Interconnecting Switch MTU:

sh sys mtu
Global Ethernet MTU is 9198 bytes.

Ping response to  the user terminal:

ping 10.3.0.94 repeat 100 size 1500
Type escape sequence to abort.
Sending 100, 1500-byte ICMP Echos to 10.13.0.94, timeout is 2 seconds:
!!!!!!!!!!!!.!!!!!!!!!!!!.!!!!!!!!!!!!.!!!!!!!!!!!!.!!!!!!!!!!!!.!!!!!
!!!!!!!.!!!!!!!!!!!!.!!!!!!!!!
Success rate is 93 percent (93/100), round-trip min/avg/max = 1/7/48 ms

I  am wondering if anyone has come across this kind of issue and how he resolved it without impacting the overall network activity.

[filopeter]

Try to find out the maximum mtu you can transfer without fragmenting and then start solving this issue for the device/link that causes the problem. Use a df-bit (don't fragment bit) option in the ping command

ping 10.3.0.94 repeat 100 size 1500 df-bit

[2D-Technology Services]

#ping  10.3.0.94 repeat 100 size 1500 df-bit
Type escape sequence to abort.
Sending 100, 1500-byte ICMP Echos to 10.13.0.94, timeout is 2 seconds:
Packet sent with the DF bit set
!!!!!!!!!!!!.!!!!!!!!!!!!.!!!!!!!!!!!!.!!!!!!!!!!!!.!!!!!!!!!!!!.!!!!!
!!!!!!!.!!!!!!!!!!!!.!!!!!!!!!
Success rate is 93 percent (93/100), round-trip min/avg/max = 1/6/48 ms

[filopeter]

This ping output does not point to a MTU or fragmentation related issue. 

Using the "ip tcp adjust-mss" command is a good workaround for networks which have issues with the Path MTU discovery (PMTUD). DMVPN environment is one example of such networks, where you can run into an MTU related issues. 

[MHM Cisco World]

Do 

Show mpls forward-table

You will see mpls mtu use by neighbor 

Then you can adjust mpls mtu by 

#MPLS MTU <VALUE>

MHM

[2D-Technology Services]

Thanks MHM..,

Below is the output of the vrf forwarding:

sh mpls forwarding-table 10.3.0.88
Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or Tunnel Id Switched interface
None 60 0.0.0.0/0 9334 Gi0/0/4 10.1.255.2

[MHM Cisco World]

sh mpls forwarding-table 10.3.0.88 detail 

also why there is no local Label ?

MHM

[2D-Technology Services]

Hello MHM..,

the MTU adjustment is to me done on the PE side Right ? Other users connected to the same PE are not having this Issue only this particular user.

On the CE side the client is using '' ip tcp adjust-mss 1360 '' on his dmvpn tunnel interface , runing on eigrp Protocol.