ELTEX ESR-3100 - Assignment of MTU When Operating with MPLS

ESR series service routers.ESR-Series. User manual
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2. Vlan (Tagged) mode:
If AC is a subinterface, the vlan tag is saved before putting it in the bridge. The vlan tag can be saved or
overwritten depending on the configuration when you exit the bridge.
If AC is an interface, traffic modification does not occur in either direction.
12.10 Assignment of MTU when operating with MPLS
It is very important to correctly configure the MTU parameter on the interfaces through which a packet is
transmitted. This is true for the installation of the pseudowire and for the transmission of service traffic. There
are two important moments:
Sizes of the Ethernet header (18 bytes), inner tag (4bytes), outer tag (4 bytes) are not taken into account
on the AC interfaces;
On the interfaces participating in the forwarding of MPLS traffic, it is necessary to increase the MTU by
the number of labels (each label is equal to 4 bytes).
MTU value is involved in signaling when constructing a pseudowire in both LDP-signaling and BGP-signaling.
Examples for both cases are given below.
For signaling (LDP, BGP) the default MTU value is 1500.
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The MTU values involved in signaling do not affect the actual packet size passing through the
pseudowire.
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