+++ /dev/null
-.. _mplsfib:
-
-MPLS FIB
---------
-
-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.
-
-It can be the case in MPLS forwarding that packets received with the
-EOS bit set in the MPLS label need to be forwarded differently from
-those without. The most common example of this is if the path set
-contains a path that does not have an output label. In this case the
-non-EOS packets cannot take this path, because to do so would expose
-the neighbouring router to a label that it did not allocate.
-
-The desgin choice to make with an MPLS FIB table is therefore:
-- 20 bit key: label only. When the EOS and non-EOS actions differ the result is a 'EOS-choice' object.
-- 21 bit key: label and EOS-bit. The result is then the specific action based on EOS-bit.
-
-20 bit key
- - Advantages:lower memory overhead, since there are few DB entries.
- - Disadvantages: slower DP performance in the case the path-lists
- differ, as more objects are encounterd in the switch path
-
-21 bit key
- - Advantages: faster DP performance
- Disadvantages: increased memory footprint.
-
-Switching between schemes based on observed/measured action similarity
-is not considered on the grounds of complexity and flip-flopping.
-
-VPP mantra - favour performance over memory. We choose a 21 bit key.
-
-Basics
-^^^^^^
-
-MPLS is not enabled by default. There are two steps to get
-started. First, create the default MPLS FIB:
-
-.. code-block:: console
-
- $ mpls table add 0
-
-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
-
- $ set interface mpls GigEthernet0/0/0 enable
-
-there is no equivalent enable for transmit, all that is required is to
-use an interface as an egress path.
-
-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 associate
-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 – 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)
-#. Swap the label and forward via a specified next-hop.
-
-These can be programmed respectively by:
-
-.. code-block:: console
-
- $ 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. 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.
-
-
-MPLS VPN
-^^^^^^^^
-
-To configure an MPLS VPN for a PE the following example can be used.
-
-Step 1; Configure routes to the iBGP peers - note these route MUST
-have out-going labels;
-
-.. code-block:: console
-
- $ 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
-
-Step 2; Configure the customer 'VRF'
-
-.. 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
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