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Docs: update MPLS FIB section with text from the wiki

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Change-Id: I9d903db89facc916fb5dd23d564417230e1c76e0
Signed-off-by: Neale Ranns <nranns@cisco.com>
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Neale Ranns
2018-08-31 00:45:19 -07:00
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213
docs/gettingstarted/developers/fib20/mplsfib.rst
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@ -1,45 +1,87 @@
.. _mplsfib:
MPLS FIB
----------
--------
There is a tight coupling between IP and MPLS forwarding. MPLS forwarding
equivalence classes (FECs) are often an IP prefix Рthat is to say that traffic
matching a given IP prefix is routed into a MPLS label switch path (LSP). It is
thus necessary to be able to associated a given prefix/route with an [out-going]
MPLS label that will be imposed when the packet is forwarded. This is configured
as:
Implementation
^^^^^^^^^^^^^^^
The MPLS FIB is implemented using exactly the same data structures as
the IP FIB. The only difference is the implementation of the
table. Whereas for IPv4 this is an mtrie and for IPv6 a hash table,
for MPLS it is a flat array indexed by a 21 bit key (label & EOS
bit). This implementation is chosen to favour packet forwarding speed.
Basics
^^^^^^
MPLS is not enabled by default. There are two steps to get
started. First, create the default MPLS FIB:
.. code-block:: console
$ ip route add 1.1.1.1/32 via 10.10.10.10 GigabitEthernet0/8/0 out-label 33
$ mpls table add 0
packets matching 1.1.1.1/32 will be forwarded out GigabitEthernet0/8/0 and have MPLS label
33 imposed. More than one out-going label can be specified. Out-going MPLS labels
can be applied to recursive and non-recursive routes, e.g;
With '0' being the magic number for the 'default' table (just like it
is for IPv[46]). One can create other MPLS tables, but, unlike IP
tables, one cannot 'bind' non-default MPLS tables to interfaces, in
other words all MPLS packets received on an interface will always
result in a lookup in the default table. One has to be more inventive
to use the non-default tables...
Secondly, for *each* interface on which you wish to *receive* MPLS
packets, that interface must be MPLS 'enabled'
.. code-block:: console
$ ip route add 2.2.2.0/24 via 1.1.1.1 out-label 34
$ set interface mpls GigEthernet0/0/0 enable
packets matching 2.2.2.0/24 will thus have two MPLS labels imposed; 34 and 33.
This is the realisation of, e,g, an MPLS BGP VPNv4.
there is no equivalent enable for transmit, all that is required is to
use an interface as an egress path.
To associate/allocate a local-label for a prefix, and thus have packets to that
local-label forwarded equivalently to the prefix do;
Entries in the MPLS FIB can be displayed with:
.. code-block:: console
$ sh mpls fib [table X] [label]
There is a tight coupling between IP and MPLS forwarding. MPLS
forwarding equivalence classes (FECs) are often an IP prefix – that is
to say that traffic matching a given IP prefix is routed into a MPLS
label switch path (LSP). It is thus necessary to be able to associated
a given prefix/route with an [out-going] MPLS label that will be
imposed when the packet is forwarded. This is configured as:
.. code-block:: console
$ ip route add 1.1.1.1/32 via 10.10.10.10 GigEthernet0/0/0 out-labels 33
packets matching 1.1.1.1/32 will be forwarded out GigEthernet0/0/0 and have
MPLS label 33 imposed. More than one out-going label can be
specified. Out-going MPLS labels can be applied to recursive and
non-recursive routes, e.g;
.. code-block:: console
$ ip route add 2.2.2.0/24 via 1.1.1.1 out-labels 34
packets matching 2.2.2.0/24 will thus have two MPLS labels imposed; 34
and 33. This is the realisation of, e,g, an MPLS BGP VPNv4.
To associate/allocate a local-label for a prefix, and thus have
packets to that local-label forwarded equivalently to the prefix do;
.. code-block:: console
$ mpls local-label 99 2.2.2.0/24
In the API this action is called a *bind*.
The router receiving the MPLS encapsulated packets needs to be programmed with
actions associated which each label value Рthis is the role of the MPLS FIB.
The MPLS FIB Is a table, whose key is the MPLS label value and end-of-stack (EOS)
bit, which stores the action to perform on packets with matching encapsulation.
Currently supported actions are:
In the API this action is called a ‘bind’.
The router receiving the MPLS encapsulated packets needs to be
programmed with actions associated which each label value – this is
the role of the MPLS FIB. The MPLS FIB Is a table, whose key is the
MPLS label value and end-of-stack (EOS) bit, which stores the action
to perform on packets with matching encapsulation. Currently supported
actions are:
#. Pop the label and perform an IPv[46] lookup in a specified table
#. Pop the label and forward via a specified next-hop (this is penultimate-hop-pop, PHP)
@ -47,44 +89,105 @@ Currently supported actions are:
These can be programmed respectively by:
#. mpls local-label 33 ip4-lookup-in-table X
#. mpls local-label 33 via 10.10.10.10 GigabitEthernet0/8/0
#. mpls local-label 33 via 10.10.10.10 GigabitEthernet0/8/0 out-label 66
#. mpls local-label 33 eos ip4-lookup-in-table X
#. mpls local-label 33 [eos] via 10.10.10.10 GigEthernet0/0/0
#. mpls local-label 33 [eos] via 10.10.10.10 GigEthernet0/0/0 out-labels 66
the latter is an example of an MPLS cross connect. Any description of a next-hop,
recursive, non-recursive, labelled, non-labelled, etc, that is valid for an IP
prefix, is also valid for an MPLS local-label.
the latter is an example of an MPLS cross connect. Any description of
a next-hop, recursive, non-recursive, labelled, non-labelled, etc,
that is valid for an IP prefix, is also valid for an MPLS
local-label. Note the use of the 'eos' keyword which indicates the
programming is for the case when the label is end-of-stack. The last
two operations can apply to both eos and non-eos packets, but the pop
and IP lookup only to an eos packet.
Implementation
^^^^^^^^^^^^^^^
The MPLS FIB is implemented using exactly the same data structures as the IP FIB.
The only difference is the implementation of the table. Whereas for IPv4 this is
an mtrie and for IPv6 a hash table, for MPLS it is a flat array indexed by a 21
bit key (label & EOS bit). This implementation is chosen to favour packet
forwarding speed.
MPLS VPN
^^^^^^^^
MPLS Tunnels
^^^^^^^^^^^^^
To configure an MPLS VPN for a PE the follow example can be used.
VPP no longer supports MPLS tunnels that are coupled to a particular transport,
i.e. MPLSoGRE or MPLSoEth. Such tight coupling is not beneficial. Instead VPP supports;
#. 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).
#. A generic uni-directional MPLS tunnel interface that is transport independent.
An MPLS tunnel is effectively an LSP with an associated interface. The LSP can be
described by any next-hop type (recursive, non-recursive etc), e.g.:
mpls tunnel add via 10.10.10.10 GigabitEthernet0/8/0 out-label 66
IP routes and/or MPLS x-connects can be routed via the interface, e.g.
Step 1; Configure routes to the iBGP peers - note these route MUST
have out-going labels;
.. code-block:: console
$ ip route add 2.2.2.0/24 via mpls-tunnel0
$ ip route add 10.0.0.1/32 via 192.168.1.2 Eth0 out-labels 33
$ ip route add 10.0.0.2/32 via 192.168.2.2 Eth0 out-labels 34
packets matching the route for 2.2.2.0/24 would thus have label 66 imposed since
it is transmitted via the tunnel.
Step 2; Configure the customer 'VRF'
These MPLS tunnels can be used to realise MPLS RSVP-TE tunnels.
.. code-block:: console
$ ip table add 2
Step 3; add a route via the iBGP peer[s] with the MPLS label
advertised by that peer
.. code-block:: console
$ ip route add table 2 10.10.10.0/24 via 10.0.0.2 next-hop-table 0 out-label 122
$ ip route add table 2 10.10.10.0/24 via 10.0.0.1 next-hop-table 0 out-label 121
Step 4; add a route via the eBGP peer
.. code-block:: console
$ ip route add table 2 10.10.20.0/24 via 172.16.0.1 next-hop-table 2
Step 5; depending on the label allocation scheme used, add routes to
the MPLS FIB to accept incoming labelled packets:
#. per-prefix label scheme - this command 'binds' the label to the same
forwarding as the IP route
.. code-block:: console
$ mpls local-label 99 10.10.20.0/24
#. per-CE label scheme - this pops the incoming label and forwards via
the next-hop provided. Append config for 'out-labels' if so desired.
.. code-block:: console
$ mpls local-label 99 via 172.16.0.1 next-hop-table 2
#. per-VRF label scheme
.. code-block:: console
$ mpls local-label 99 via ip4-lookup-in-table 2
MPLS Tunnels
^^^^^^^^^^^^
MPLS tunnels are unidirectional and can impose a stack of labels. They
are 'normal' interfaces and thus can be used, for example, as the
target for IP routes and L2 cross-connects. To construct a tunnel:
.. code-block:: console
$ mpls tunnel add via 10.10.10.10 GigEthernet0/0/0 out-labels 33 44 55
and to then have that created tunnel to perform ECMP:
.. code-block:: console
$ mpls tunnel add mpls-tunnel0 via 10.10.10.11 GigEthernet0/0/0 out-labels 66 77 88
use
.. code-block:: console
$ sh mpls tunnel [X]
to see the monster you have created.
An MPLS tunnel interface is an interface like any other and now ready
for use with the usual set of interface commands, e.g.:
.. code-block:: console
$ set interface state mpls-tunnel0 up
$ set interface ip address mpls-tunnel0 192.168.1.1/30
$ ip route 1.1.1.1/32 via mpls-tunnel0