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Doc: MPLS documentation

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@ -341,10 +341,9 @@ Configuration keywords are <cf/flow4/ and <cf/flow6/.
<sect1>MPLS switching rules
<label id="mpls-routes">
<p>This nettype is currently a stub before implementing more support of <rfc id="3031">.
BIRD currently does not support any label distribution protocol nor any label assignment method.
Only the Kernel, Pipe and Static protocols can use MPLS tables.
Configuration keyword is <cf/mpls/.
<p>MPLS routes control MPLS forwarding in the same way as IP routes control IP
forwarding. MPLS-aware routing protocols produce both labeled IP routes and
corresponding MPLS routes. Configuration keyword is <cf/mpls/.
<itemize>
<item>(PK) MPLS label
@ -403,6 +402,79 @@ regular <ref id="cli-down" name="down"> command. In this way routing neighbors
are notified about planned graceful restart and routes are kept in kernel table
after shutdown.
<sect>MPLS
<label id="mpls">
<p>Multiprotocol Label Switching (MPLS) is a networking technology which works
below IP routing but above the link (e.g. ethernet) layer. It is described in
<rfc id="3031">.
In regular IP forwarding, the destination address of a packet is independently
examined in each hop, a route with longest prefix match is selected from the
routing table, and packet is processed accordingly. In general, there is no
difference between border routers and internal routers w.r.t. IP forwarding.
In MPLS forwarding, when a packet enters the network, it is classified (based on
destination address, ingress interface and other factors) into one of forwarding
equivalence classes (FECs), then a header with a MPLS label identifying the FEC
is attached to it, and the packet is forwarded. In internal routers, only the
MPLS label is examined, the matching MPLS route is selected from the MPLS
routing table, and the packet is processed accordingly. The specific value of
MPLS label has local meaning only and may change between hops (that is why it is
called label switching). When the packet leaves the network, the MPLS header is
removed.
The advantage of the MPLS approach is that other factors than the destination
address can be considered and used consistently in the whole network, for
example IP traffic with multiple overlapping private address ranges could be
mixed together, or particular paths for specific flows could be defined. Another
advantage is that MPLS forwarding by internal routers can be much simpler than
IP forwarding, as instead of the longest prefix match algorithm it uses simpler
exact match for MPLS route selection. The disadvantage is additional complexity
in signalling. For further details, see <rfc id="3031">.
MPLS-aware routing protocols not only distribute IP routing information, but
they also distribute labels. Therefore, they produce labeled routes - routes
representing label switched paths (LSPs) through the MPLS domain. Such routes
have IP prefix and next hop address like regular (non-labeled) routes, but they
also have local MPLS label (in route attribute <ref id="rta-mpls-label"
name="mpls_label">) and outgoing MPLS label (as a part of the next hop). They
are stored in regular IP routing tables.
Labeled routes are used for exchange of routing information between routing
protocols and for ingress (IP -&gt; MPLS) forwarding, but they are not directly
used for MPLS forwarding. For that purpose <ref id="mpls-routes" name="MPLS
routes"> are used. These are routes that have local MPLS label as a primary key
and they are stored in the MPLS routing table.
In BIRD, the whole process generally works this way: A MPLS-aware routing
protocol (say BGP) receives routing information including remote label. It
produces a route with attribute <ref id="rta-mpls-policy" name="mpls_policy">
specifying desired <ref id="mpls-channel-label-policy" name="MPLS label policy">.
Such route then passes the import filter (which could modify the MPLS label
policy or perhaps assign a static label) and when it is accepted, a local MPLS
label is selected (according to the label policy) and attached to the route,
producing labeled route. When a new MPLS label is allocated, the MPLS-aware
protocol automatically produces corresponding MPLS route. When all labeled
routes that use specific local MPLS label are retracted, the corresponding MPLS
route is retracted too.
There are three important concepts for MPLS in BIRD: MPLS domains, MPLS tables
and MPLS channels. MPLS domain represents an independent label space, all
MPLS-aware protocols are associated with some MPLS domain. It is responsible for
label management, handling label allocation requests from MPLS-aware protocols.
MPLS table is just a routing table for MPLS routes. Routers usually have one
MPLS domain and one MPLS table, with Kernel protocol to export MPLS routes into
kernel FIB.
MPLS channels make protocols MPLS-aware, they are responsible for keeping track
of active FECs (and corresponding allocated labels), selecting FECs / local
labels for labeled routes, and maintaining correspondence between labeled routes
and MPLS routes.
Note that local labels are allocated to individual MPLS-aware protocols and
therefore it is not possible to share local labels between different protocols.
<chapt>Configuration
<label id="config">
@ -626,6 +698,12 @@ include "tablename.conf";;
defined by this option. See the <ref id="rtable-opts"
name="routing table configuration section"> for routing table options.
<tag><label id="opt-mpls-domain">mpls domain <m/name/ [ { <m/option/; [<m/.../] } ]</tag>
Define a new MPLS domain. MPLS domains represent independent label
spaces and are responsible for MPLS label management. All MPLS-aware
protocols are associated with some MPLS domain. See the <ref id="mpls-opts"
name="MPLS configuration section"> for MPLS domain options.
<tag><label id="opt-eval">eval <m/expr/</tag>
Evaluates given filter expression. It is used by the developers for testing of filters.
</descrip>
@ -1032,6 +1110,96 @@ protocol bgp from {
</code>
<sect>MPLS options
<label id="mpls-opts">
<p>The MPLS domain definition is mandatory for a MPLS router. All MPLS channels
and MPLS-aware protocols are associated with some MPLS domain (although usually
implicitly with the sole one). In the MPLS domain definition you can configure
details of MPLS label allocation. Currently, there is just one option:
<descrip>
<tag><label id="mpls-domain-label-range">label range <m/name/ { start <m/number/; length <m/number/; [<m/.../] }</tag>
Define a new label range, or redefine implicit label ranges <cf/static/
and <cf/dynamic/. MPLS channels use configured label ranges for dynamic
label allocation, while <cf/static/ label range is used for static label
allocation. The label range definition must specify the extent of the
range. By default, the range <cf/static/ is 16-1000, while the range
<cf/dynamic/ is 1000-10000.
</descrip>
<p>MPLS channel should be defined in each MPLS-aware protocol in addition to its
regular channels. It is responsible for label allocation and for announcing MPLS
routes to the MPLS routing table. Besides common <ref id="channel-opts"
name="channel options">, MPLS channels have some specific options:
<descrip>
<tag><label id="mpls-channel-domain">domain <m/name/</tag>
Specify a MPLS domain to which this channel and protocol belongs.
Default: The first defined MPLS domain.
<tag><label id="mpls-channel-label-range">label range <m/name/</tag>
Use specific label range for dynamic label allocation. Note that static
labels always use the range <cf/static/. Default: the range <cf/dynamic/.
<tag><label id="mpls-channel-label-policy">label policy static|prefix|aggregate|vrf</tag>
Label policy specifies how routes are grouped to forwarding equivalence
classes (FECs) and how labels are assigned to them.
The policy <cf/static/ means no dynamic label allocation is done, and
static labels must be set in import filters using the route attribute
<ref id="rta-mpls-label" name="mpls_label">.
The policy <cf/prefix/ means each prefix uses separate label associated
with that prefix. When a labeled route is updated, it keeps the label.
This policy is appropriate for IGPs.
The policy <cf/aggregate/ means routes are grouped to FECs according to
their next hops (including next hop labels), and one label is used for
all routes in the same FEC. When a labeled route is updated, it may
change next hop, change FEC and therefore change label. This policy is
appropriate for BGP.
The policy <cf/vrf/ is only valid in L3VPN protocols. It uses one label
for all routes from a VRF, while replacing the original next hop with
lookup in the VRF.
Default: <cf/prefix/.
</descrip>
<p>This is a trivial example of MPLS setup:
<code>
mpls domain mdom {
label range bgprange { start 2000; length 1000; };
}
mpls table mtab;
protocol static {
ipv6;
mpls;
route 2001:db8:1:1/64 mpls 100 via 2001:db8:1:2::1/64 mpls 200;
}
protocol bgp {
# regular channels
ipv6 mpls { ... };
vpn6 mpls { ... };
# MPLS channel
mpls {
# domain mdom;
# table mtab;
label range bgprange;
label policy aggregate;
};
...
}
</code>
<chapt>Remote control
<label id="remote-control">
@ -1874,6 +2042,29 @@ Common route attributes are:
network for routes that do not have a native protocol metric attribute
(like <cf/ospf_metric1/ for OSPF routes). It is used mainly by BGP to
compare internal distances to boundary routers (see below).
<tag><label id="rta-mpls-label"><m/int/ mpls_label</tag>
Local MPLS label attached to the route. This attribute is produced by
MPLS-aware protocols for labeled routes. It can also be set in import
filters to assign static labels, but that also requires static MPLS
label policy.
<tag><label id="rta-mpls-policy"><m/enum/ mpls_policy</tag>
For MPLS-aware protocols, this attribute defines which
<ref id="mpls-channel-label-policy" name="MPLS label policy"> will be
used for the route. It can be set in import filters to change it on
per-route basis. Valid values are <cf/MPLS_POLICY_NONE/ (no label),
<cf/MPLS_POLICY_STATIC/ (static label), <cf/MPLS_POLICY_PREFIX/
(per-prefix label), <cf/MPLS_POLICY_AGGREGATE/ (aggregated label),
and <cf/MPLS_POLICY_VRF/ (per-VRF label). See <ref
id="mpls-channel-label-policy" name="MPLS label policy"> for details.
<tag><label id="rta-mpls-class"><m/int/ mpls_class</tag>
When <ref id="mpls-channel-label-policy" name="MPLS label policy"> is
set to <cf/aggregate/, it may be useful to apply more fine-grained
aggregation than just one based on next hops. When routes have different
value of this attribute, they will not be aggregated under one local
label even if they have the same next hops.
</descrip>
<p>Protocol-specific route attributes are described in the corresponding
@ -3051,6 +3242,18 @@ together with their appropriate channels follows.
</tabular>
</table>
<p>The BGP protocol can be configured as MPLS-aware (by defining both AFI/SAFI
channels and the MPLS channel). In such case the BGP protocol assigns labels to
routes imported from MPLS-aware SAFIs (i.e. <cf/ipvX mpls/ and <cf/vpnX mpls/)
and automatically announces corresponding MPLS route for each labeled route. As
BGP generally processes a large amount of routes, it is suggested to set MPLS
label policy to <cf/aggregate/.
<p>Note that even BGP instances without MPLS channel and without local MPLS
configuration can still propagate third-party MPLS labels, e.g. as route
reflectors, they just will not assign local labels to imported routes and will
not announce MPLS routes for local MPLS forwarding.
<p>Due to <rfc id="8212">, external BGP protocol requires explicit configuration
of import and export policies (in contrast to other protocols, where default
policies of <cf/import all/ and <cf/export none/ are used in absence of explicit
@ -5443,6 +5646,11 @@ but only routes of the same network type are allowed, as the static protocol
has just one channel. E.g., to have both IPv4 and IPv6 static routes, define two
static protocols, each with appropriate routes and channel.
<p>The Static protocol can be configured as MPLS-aware (by defining both the
primary channel and MPLS channel). In that case the Static protocol assigns
labels to IP routes and automatically announces corresponding MPLS route for
each labeled route.
<p>Global options:
<descrip>
@ -5466,16 +5674,20 @@ static protocols, each with appropriate routes and channel.
<ref id="type-prefix" name="dependent on network type">.
<descrip>
<tag>route <m/prefix/ via <m/ip/|<m/"interface"/ [<m/per-nexthop options/] [via ...]</tag>
Regular routes may bear one or more <ref id="route-next-hop" name="next hops">.
Every next hop is preceded by <cf/via/ and configured as shown.
<tag>route <m/prefix/ [mpls <m/number>] via <m/ip/|<m/"interface"/ [<m/per-nexthop options/] [via ...]</tag>
Regular routes may bear one or more <ref id="route-next-hop" name="next
hops">. Every next hop is preceded by <cf/via/ and configured as shown.
<tag>route <m/prefix/ recursive <m/ip/ [mpls <m/num/[/<m/num/[/<m/num/[...]]]]</tag>
When the Static protocol is MPLS-aware, the optional <cf/mpls/ statement
after <m/prefix/ specifies a static label for the labeled route, instead
of using dynamically allocated label.
<tag>route <m/prefix/ [mpls <m/number>] recursive <m/ip/ [mpls <m/num/[/<m/num/[/<m/num/[...]]]]</tag>
Recursive nexthop resolves the given IP in the configured IGP table and
uses that route's next hop. The MPLS stacks are concatenated; on top is
the IGP's nexthop stack and on bottom is this route's stack.
<tag>route <m/prefix/ blackhole|unreachable|prohibit</tag>
<tag>route <m/prefix/ [mpls <m/number>] blackhole|unreachable|prohibit</tag>
Special routes specifying to silently drop the packet, return it as
unreachable or return it as administratively prohibited. First two
targets are also known as <cf/drop/ and <cf/reject/.