[j-nsp] MTU signalling on l2circuit

[Harry Reynolds]

Hmm. So much for the first guess. I just looked at the spec and indeed
the circuit will not come up. You can configure the MTU explicitly:


http://www.juniper.net/techpubs/software/junos/junos72/swconfig72-vpns/h
tml/l2circuits-config7.html#1051093


"By default, the MTU used to advertise a Layer 2 circuit is determined
by taking the interface MTU for the associated physical interface and
subtracting the encapsulation overhead for sending IP packets based on
the encapsulation. 

However, encapsulations that support multiple logical interfaces (and
multiple Layer 2 circuits) rely on the same interface MTU (since they
are all associated with the same physical interface). This can prove to
be a limitation for VLAN Layer 2 circuits using the same Ethernet
interface or for Layer 2 circuit DLCIs using the same Frame Relay
interface.

This can also affect multivendor environments. For example, if you had
three PE devices supplied by different vendors and one of the devices
could only support an MTU of 1500, even if the other devices could
support larger MTUs you would have to configure the MTU as 1500 (the
smallest MTU of the three PE devices).

You can explicitly configure what MTU is advertised for a Layer 2
circuit, even if the Layer 2 circuit is sharing a physical interfaces
with other Layer 2 circuits. When you explicitly configure an MTU for a
Layer 2 circuit, be aware of the following:

An explicitly configured MTU is signalled to the remote PE device. The
configured MTU is also compared to the MTU received from the remote PE
device. If there is a conflict, the Layer 2 circuit is taken down. 
If you configure an MTU for an ATM cell relay interface on an ATM II
PIC, the configured MTU is used to compute the cell bundle size
advertised for that Layer 2 circuit rather than using the default
interface MTU. 
A configured MTU is used only in the control plane. It is not enforced
in the data plane. You need to ensure that the CE device for a given
Layer 2 circuit uses the correct MTU for data transmission. 
To configure the MTU for a Layer 2 circuit, include the mtu statement:

mtu mtu-number;


" 

> -----Original Message-----
> From: Alexander Koch [mailto:efraim at clues.de] 
> Sent: Thursday, June 02, 2005 9:40 AM
> To: Harry Reynolds
> Cc: juniper-nsp at puck.nether.net
> Subject: Re: [j-nsp] MTU signalling on l2circuit
> 
> On Thu, 2 June 2005 09:26:23 -0700, Harry Reynolds wrote:
> > If I follow, the circuit should come up, and operate fine, 
> as long as 
> > the lesser of the two MTUs is not exceeded. L2 VPNs cannot 
> fragment. 
> > If the CPEs are set for different MTUs you will likely see 
> issues with 
> > OSPF adjacency formation (stuck in exchange mode).
> 
> Hmh. I get 'MM' for a l2circuit. I cannot set MTU per VLAN 
> sub-int, obviously, and I need high MTU for some normal VLANs 
> on that trunk.
> 
> Currently the l2circuit is not up.
> 
> Alexander
>