75c276f9e3
Change-Id: I9d903db89facc916fb5dd23d564417230e1c76e0 Signed-off-by: Neale Ranns <nranns@cisco.com>
194 lines
6.0 KiB
ReStructuredText
194 lines
6.0 KiB
ReStructuredText
.. _mplsfib:
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MPLS FIB
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--------
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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
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the IP FIB. The only difference is the implementation of the
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table. Whereas for IPv4 this is an mtrie and for IPv6 a hash table,
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for MPLS it is a flat array indexed by a 21 bit key (label & EOS
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bit). This implementation is chosen to favour packet forwarding speed.
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Basics
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^^^^^^
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MPLS is not enabled by default. There are two steps to get
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started. First, create the default MPLS FIB:
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.. code-block:: console
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$ mpls table add 0
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With '0' being the magic number for the 'default' table (just like it
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is for IPv[46]). One can create other MPLS tables, but, unlike IP
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tables, one cannot 'bind' non-default MPLS tables to interfaces, in
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other words all MPLS packets received on an interface will always
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result in a lookup in the default table. One has to be more inventive
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to use the non-default tables...
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Secondly, for *each* interface on which you wish to *receive* MPLS
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packets, that interface must be MPLS 'enabled'
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.. code-block:: console
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$ set interface mpls GigEthernet0/0/0 enable
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there is no equivalent enable for transmit, all that is required is to
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use an interface as an egress path.
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Entries in the MPLS FIB can be displayed with:
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.. code-block:: console
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$ sh mpls fib [table X] [label]
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There is a tight coupling between IP and MPLS forwarding. MPLS
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forwarding equivalence classes (FECs) are often an IP prefix – that is
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to say that traffic matching a given IP prefix is routed into a MPLS
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label switch path (LSP). It is thus necessary to be able to associated
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a given prefix/route with an [out-going] MPLS label that will be
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imposed when the packet is forwarded. This is configured 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 GigEthernet0/0/0 out-labels 33
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packets matching 1.1.1.1/32 will be forwarded out GigEthernet0/0/0 and have
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MPLS label 33 imposed. More than one out-going label can be
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specified. Out-going MPLS labels can be applied to recursive and
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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-labels 34
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packets matching 2.2.2.0/24 will thus have two MPLS labels imposed; 34
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and 33. 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
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packets to that 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
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programmed with actions associated which each label value – this is
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the role of the MPLS FIB. The MPLS FIB Is a table, whose key is the
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MPLS label value and end-of-stack (EOS) bit, which stores the action
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to perform on packets with matching encapsulation. Currently supported
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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 eos ip4-lookup-in-table X
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#. mpls local-label 33 [eos] via 10.10.10.10 GigEthernet0/0/0
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#. mpls local-label 33 [eos] via 10.10.10.10 GigEthernet0/0/0 out-labels 66
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the latter is an example of an MPLS cross connect. Any description of
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a next-hop, recursive, non-recursive, labelled, non-labelled, etc,
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that is valid for an IP prefix, is also valid for an MPLS
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local-label. Note the use of the 'eos' keyword which indicates the
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programming is for the case when the label is end-of-stack. The last
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two operations can apply to both eos and non-eos packets, but the pop
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and IP lookup only to an eos packet.
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MPLS VPN
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^^^^^^^^
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To configure an MPLS VPN for a PE the follow example can be used.
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Step 1; Configure routes to the iBGP peers - note these route MUST
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have out-going labels;
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.. code-block:: console
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$ ip route add 10.0.0.1/32 via 192.168.1.2 Eth0 out-labels 33
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$ ip route add 10.0.0.2/32 via 192.168.2.2 Eth0 out-labels 34
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Step 2; Configure the customer 'VRF'
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.. code-block:: console
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$ ip table add 2
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Step 3; add a route via the iBGP peer[s] with the MPLS label
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advertised by that peer
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.. code-block:: console
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$ ip route add table 2 10.10.10.0/24 via 10.0.0.2 next-hop-table 0 out-label 122
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$ ip route add table 2 10.10.10.0/24 via 10.0.0.1 next-hop-table 0 out-label 121
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Step 4; add a route via the eBGP peer
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.. code-block:: console
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$ ip route add table 2 10.10.20.0/24 via 172.16.0.1 next-hop-table 2
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Step 5; depending on the label allocation scheme used, add routes to
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the MPLS FIB to accept incoming labelled packets:
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#. per-prefix label scheme - this command 'binds' the label to the same
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forwarding as the IP route
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.. code-block:: console
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$ mpls local-label 99 10.10.20.0/24
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#. per-CE label scheme - this pops the incoming label and forwards via
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the next-hop provided. Append config for 'out-labels' if so desired.
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.. code-block:: console
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$ mpls local-label 99 via 172.16.0.1 next-hop-table 2
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#. per-VRF label scheme
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.. code-block:: console
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$ mpls local-label 99 via ip4-lookup-in-table 2
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MPLS Tunnels
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^^^^^^^^^^^^
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MPLS tunnels are unidirectional and can impose a stack of labels. They
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are 'normal' interfaces and thus can be used, for example, as the
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target for IP routes and L2 cross-connects. To construct a tunnel:
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.. code-block:: console
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$ mpls tunnel add via 10.10.10.10 GigEthernet0/0/0 out-labels 33 44 55
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and to then have that created tunnel to perform ECMP:
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.. code-block:: console
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$ mpls tunnel add mpls-tunnel0 via 10.10.10.11 GigEthernet0/0/0 out-labels 66 77 88
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use
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.. code-block:: console
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$ sh mpls tunnel [X]
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to see the monster you have created.
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An MPLS tunnel interface is an interface like any other and now ready
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for use with the usual set of interface commands, e.g.:
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.. code-block:: console
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$ set interface state mpls-tunnel0 up
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$ set interface ip address mpls-tunnel0 192.168.1.1/30
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$ ip route 1.1.1.1/32 via mpls-tunnel0
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