91 lines
3.5 KiB
ReStructuredText
91 lines
3.5 KiB
ReStructuredText
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.. _mplsfib:
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MPLS FIB
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----------
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There is a tight coupling between IP and MPLS forwarding. MPLS forwarding
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equivalence classes (FECs) are often an IP prefix Рthat is to say that traffic
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matching a given IP prefix is routed into a MPLS label switch path (LSP). It is
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thus necessary to be able to associated a given prefix/route with an [out-going]
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MPLS label that will be imposed when the packet is forwarded. This is configured
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as:
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.. code-block:: console
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$ ip route add 1.1.1.1/32 via 10.10.10.10 GigabitEthernet0/8/0 out-label 33
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packets matching 1.1.1.1/32 will be forwarded out GigabitEthernet0/8/0 and have MPLS label
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33 imposed. More than one out-going label can be specified. Out-going MPLS labels
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can be applied to recursive and non-recursive routes, e.g;
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.. code-block:: console
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$ ip route add 2.2.2.0/24 via 1.1.1.1 out-label 34
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packets matching 2.2.2.0/24 will thus have two MPLS labels imposed; 34 and 33.
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This is the realisation of, e,g, an MPLS BGP VPNv4.
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To associate/allocate a local-label for a prefix, and thus have packets to that
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local-label forwarded equivalently to the prefix do;
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.. code-block:: console
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$ mpls local-label 99 2.2.2.0/24
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In the API this action is called a *bind*.
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The router receiving the MPLS encapsulated packets needs to be programmed with
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actions associated which each label value Рthis is the role of the MPLS FIB.
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The MPLS FIB Is a table, whose key is the MPLS label value and end-of-stack (EOS)
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bit, which stores the action to perform on packets with matching encapsulation.
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Currently supported actions are:
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#. Pop the label and perform an IPv[46] lookup in a specified table
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#. Pop the label and forward via a specified next-hop (this is penultimate-hop-pop, PHP)
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#. Swap the label and forward via a specified next-hop.
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These can be programmed respectively by:
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#. mpls local-label 33 ip4-lookup-in-table X
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#. mpls local-label 33 via 10.10.10.10 GigabitEthernet0/8/0
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#. mpls local-label 33 via 10.10.10.10 GigabitEthernet0/8/0 out-label 66
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the latter is an example of an MPLS cross connect. Any description of a next-hop,
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recursive, non-recursive, labelled, non-labelled, etc, that is valid for an IP
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prefix, is also valid for an MPLS local-label.
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Implementation
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^^^^^^^^^^^^^^^
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The MPLS FIB is implemented using exactly the same data structures as the IP FIB.
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The only difference is the implementation of the table. Whereas for IPv4 this is
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an mtrie and for IPv6 a hash table, for MPLS it is a flat array indexed by a 21
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bit key (label & EOS bit). This implementation is chosen to favour packet
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forwarding speed.
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MPLS Tunnels
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^^^^^^^^^^^^^
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VPP no longer supports MPLS tunnels that are coupled to a particular transport,
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i.e. MPLSoGRE or MPLSoEth. Such tight coupling is not beneficial. Instead VPP supports;
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#. MPLS LSPs associated with IP prefixes and MPLS local-labels (as described above) which are transport independent (i.e. the IP route could be reachable over a GRE tunnel, or any other interface type).
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#. A generic uni-directional MPLS tunnel interface that is transport independent.
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An MPLS tunnel is effectively an LSP with an associated interface. The LSP can be
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described by any next-hop type (recursive, non-recursive etc), e.g.:
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mpls tunnel add via 10.10.10.10 GigabitEthernet0/8/0 out-label 66
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IP routes and/or MPLS x-connects can be routed via the interface, e.g.
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.. code-block:: console
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$ ip route add 2.2.2.0/24 via mpls-tunnel0
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packets matching the route for 2.2.2.0/24 would thus have label 66 imposed since
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it is transmitted via the tunnel.
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These MPLS tunnels can be used to realise MPLS RSVP-TE tunnels.
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