Label Range and TTL

From the /mpls settings menu it is possible to assign specific dynamic label range and TTL propagation. If for some reason static label mapping is used then the dynamic range can be adjusted to exclude statically assigned label numbers from being dynamically assigned by any of the label distribution protocols.

MPLS MTU

Configuration of MPLS MTU (path MTU + MPLS tag size) is useful in cases when there is a large variety of possible MTUs along the path. Configuring MPLS MTU to a minimum value that can pass all the hops will ensure that the MPLS packet will not be silently dropped on the devices that do not support big enough MTU.

MPLS MTUs are configured from the /mpls interface menu.

[admin@rack1_b35_CCR1036] /mpls/interface> print 
Flags: X - disabled; * - builtin 
 0    ;;; router-test
      interface=ether1 mpls-mtu=1580 input=yes 

 1    ;;; router-test
      interface=ether2 mpls-mtu=1580 input=yes 

 2    interface=all mpls-mtu=1500  

Properties

The order of the entries is important due to the possibility that different interface lists can contain the same interface and in addition, that interface can be referenced directly.

Selection of the MPLS MTU happens in the following manner:

• If the interface matched the entry from this table, then try to use configured MPLS MTU value
• If the interface does not match any entry then consider MPLS MTU equal to L2MTU
• If the interface does not support L2MTU, then consider MPLS MTU equal to L3 MTU

On the MPLS ingress path, MTU is chosen by min(MPLS MTU - tagsize, l3mtu). This means that on interfaces that do not support L2MTU and default L3 MTU is set to 1500, max path MTU will be 1500 - tag size (the interface will not be able to pass full IP frame without fragmentation). In such scenarios, L3MTU must be increased by max observed tag size.

Read more on MTUs in the MTU in RouterOS article.

Forwarding Table

Entries in the /mpls forwarding-table menu show label bindings for specific routes that will be used in MPLS label switching. Properties in this menu are read-only.

[admin@rack1_b35_CCR1036] /mpls/forwarding-table> print 
Flags: L, V - VPLS
Columns: LABEL, VRF, PREFIX, NEXTHOPS
#   LABEL  VRF   PREFIX         NEXTHOPS                                            
0 L    16  main  10.0.0.0/8     { nh=10.155.130.1; interface=ether12 }              
1 L    18  main  111.111.111.3  { label=impl-null; nh=111.12.0.1; interface=ether2 }
2 L    17  main  111.111.111.2  { label=impl-null; nh=111.11.0.1; interface=ether1 }