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sanka
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Mtu and Mpls

Mon Dec 18, 2017 12:57 pm

Hi,

I my network I see a very strange behaivour.
I have a chain of 10 device, six are Mikrotik and 4 Cisco.
First is Cisco, in the middle are Mikrotik and at the end Cisco..
Some Vpls beetween Cisco and Cisco work, but Mtu that have on encapsulated traffic isn't 1500 but 1498.
The Vpls are correctly configured and mtu conf are coehrent.
The only strange things is that 1 mirkotik in the middle of network have problem with MTU.
This CCR1072 have 2 interface: 1 to Cisco, 1 to another Mikrotik, and also bridge interface for loopback purpose.

the mtu that I have configured is 1546 but if I try to ping the ip address configured on himself interface receive the error on MTU.
The real mtu are 1500, but on the inerface I have set 1546!

Any idea of this very strange issue?

Thanks
 
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Anumrak
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Re: Mtu and Mpls

Mon Dec 18, 2017 2:29 pm

Try to ping with specific source IP.
 
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mrz
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Re: Mtu and Mpls

Mon Dec 18, 2017 3:19 pm

Note that there are 3 different MTU values that you can configure
L3 MTU on physical interface - this one will not affect MTU of VPLS traffic
MPLS MTU - by default it is 1508 and will not be able to pass unfragmented VPLS
L2MTU - this one determines how big Layer2 frame can be sent over the interface.
 
sanka
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Re: Mtu and Mpls

Tue Dec 19, 2017 1:03 pm

L3 mtu, all interface have 1546
L2 mtu, all interface have 1580
Mpls mtu have as mtu 1546.

We create a vpls tunnel from remote end that must encapsulate double tagged traffic.
I set up tunnel with advertised mtu at 1508 but when try to ping from remote end into vpls tunnel
I reach only 1496.

So, I think that I see this problem at L3 layer, mpls layer can be have also some problem of fragmentation packet
that generate drops on final tunnels and I cant' have full mtu also with double tagged traffic.

ping with source have same results.

Thanks
 
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Re: Mtu and Mpls

Tue Dec 19, 2017 4:39 pm

Is the L3 transport over a connection that you control or is it a private circuit?
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sanka
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Re: Mtu and Mpls

Wed Dec 20, 2017 10:05 am

We use L3 and mpls connection on our dark fiber network to create interconnection (transport network).
Over this network we create vpls tunnel for overlay services.

I see that mismatch at L3 side on transport network, but also on services deployed inside a tunnel we see issue with mtu,
as the problem with L3 and mpls mtu reflects on vpls tunnel.

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