---
date: "2023-05-21T11:01:14Z"
title: VPP MPLS - Part 3
aliases:
- /s/articles/2023/05/21/vpp-mpls-3.html
---

{{< image width="200px" float="right" src="/assets/vpp/fdio-color.svg" alt="VPP" >}}

# About this series

**Special Thanks**: Adrian _vifino_ Pistol for writing this code and for the wonderful collaboration!

Ever since I first saw VPP - the Vector Packet Processor - I have been deeply impressed with its
performance and versatility. For those of us who have used Cisco IOS/XR devices, like the classic
_ASR_ (aggregation service router), VPP will look and feel quite familiar as many of the approaches
are shared between the two.

In the [[first article]({{< ref "2023-05-07-vpp-mpls-1" >}})] of this series, I took a look at MPLS
in general, and how setting up static _Label Switched Paths_ can be done in VPP. A few details on
special case labels (such as _Implicit Null_ which enabled the fabled _Penultimate Hop Popping_)
were missing, so I took a good look at them in the [[second article]({{< ref "2023-05-17-vpp-mpls-2" >}})] of the series.

This was all just good fun but also allowed me to buy some time for
[@vifino](https://chaos.social/@vifino) who has been implementing MPLS handling within the Linux
Control Plane plugin for VPP! This final article in the series shows the engineering considerations
that went in to writing the plugin, which is currently under review but reasonably complete.
Considering the VPP 23.06 cutoff is next week, I'm not super hopeful that we'll be able to get a
full community / committer review in time, but at this point both @vifino and I think this code is
ready for consumption - considering FRR has a good _Label Distribution Protocol_ daemon, I'll switch
out of my usual habitat of Bird and install a LAB with FRR.

Caveat empor, outside of a modest functional and load-test, this MPLS functionality
hasn't seen a lot of mileage as it's only a few weeks old at this point, so it could definitely
contain some rough edges. Use at your own risk, but if you did want to discuss issues, the
[[vpp-dev@](mailto:vpp-dev@lists.fd.io)] mailinglist is a good first stop.

## Introduction

MPLS support is fairly complete in VPP already, but programming the dataplane would require custom
integrations, while using the Linux netlink subsystem feels easier from an end-user point of view.
This is a technical deep dive into the implementation of MPLS in the Linux Control Plane plugin for
VPP. If you haven't already, now is a good time to read up on the initial implementation of LCP:

*   [[Part 1]({{< ref "2021-08-12-vpp-1" >}})]: Punting traffic through TUN/TAP interfaces into Linux
*   [[Part 2]({{< ref "2021-08-13-vpp-2" >}})]: Mirroring VPP interface configuration into Linux
*   [[Part 3]({{< ref "2021-08-15-vpp-3" >}})]: Automatically creating sub-interfaces in Linux
*   [[Part 4]({{< ref "2021-08-25-vpp-4" >}})]: Synchronize link state, MTU and addresses to Linux
*   [[Part 5]({{< ref "2021-09-02-vpp-5" >}})]: Netlink Listener, synchronizing state from Linux to VPP
*   [[Part 6]({{< ref "2021-09-10-vpp-6" >}})]: Observability with LibreNMS and VPP SNMP Agent
*   [[Part 7]({{< ref "2021-09-21-vpp-7" >}})]: Productionizing and reference Supermicro fleet at IPng

To keep this writeup focused, I'll assume the anatomy of VPP plugins and the Linux Controlplane
_Interface_ and _Netlink_ plugins are understood. That way, I can focus on the _changes_ needed for
MPLS integration, which at first glance seem reasonably straight forward.

## VPP Linux-CP: Interfaces

First off, to enable any MPLS forwarding at all in VPP, I have to create the MPLS forwarding table
and enable MPLS on one or more interfaces:

```
vpp# mpls table add 0
vpp# lcp create GigabitEthernet10/0/0 host-if e0
vpp# set int mpls GigabitEthernet10/0/0 enable
```

What happens when the Gi10/0/0 interface has a _Linux Interface Pair (LIP)_ is that there exists a
corresponding TAP interface in the dataplane (typically called `tapX`) which in turn appears on the
Linux side as `e0`. Linux will want to be able to send MPLS datagrams into `e0`, and for that, two
things must happen:

1.   Linux kernel must enable MPLS input on `e0`, typically with a sysctl.
1.   VPP must enable MPLS on the TAP, in addition to the phy Gi10/0/0.

Therefore, the first order of business is to create a hook where the Linux CP interface plugin can
be made aware if MPLS is enabled or disabled in VPP - it turns out, such a callback function
_definition_ already exists, but it was never implemented. [[Gerrit
38826](https://gerrit.fd.io/r/c/vpp/+/38826)] adds a function `mpls_interface_state_change_add_callback()`,
which implements the ability to register a callback on MPLS on/off in VPP.

Now that the callback plumbing exists, Linux CP will want to register one of these, so that it can
set MPLS to the same enabled or disabled state on the Linux interface using
`/proc/sys/net/mpls/conf/${host-if}/input` (which is the moral equivalent of running `sysctl`), and
it'll also call `mpls_sw_interface_enable_disable()` on the TAP interface. With these changes both
implemented, enabling MPLS now looks like this in the logs:

```
linux-cp/mpls-sync: sync_state_cb: called for sw_if_index 1
linux-cp/mpls-sync: sync_state_cb: mpls enabled 1 parent itf-pair: [1] GigabitEthernet10/0/0 tap2 e0 97 type tap netns dataplane
linux-cp/mpls-sync: sync_state_cb: called for sw_if_index 8
linux-cp/mpls-sync: sync_state_cb: set mpls input for e0
```

Take a look at the code that implements enable/disable semantics in `src/plugins/linux-cp/lcp_mpls_sync.c`.

## VPP Linux-CP: Netlink

When Linux installs a route with MPLS labels, it will be seen in the return value of
`rtnl_route_nh_get_encap_mpls_dst()`. One or more labels can now be read using
`nl_addr_get_binary_addr()` yielding `struct mpls_label`, which contains the label value, experiment
bits and TTL, and these can be added to the route path in VPP by casting them to `struct
fib_mpls_label_t`. The last label in the stackwill have the S-bit set, so we can continue consuming these
until we find that condition. The first patchset that plays around with these semantics is
[[38702#2](https://gerrit.fd.io/r/c/vpp/+/38702/2)]. As you can see, MPLS is going to look very much
like IPv4 and IPv6 route updates in [[previous work]({{< ref "2021-09-02-vpp-5" >}})], in that they
take the Netlink representation, rewrite them into VPP representation, and update the FIB.

Up until now, the Linux Controlplane netlink plugin understands only IPv4 and IPv6. So some
preparation work is called for:

*    ***lcp_router_proto_k2f()*** gains the ability to cast Linux `AF_*` into VPP's `FIB_PROTOCOL_*`.
*    ***lcp_router_route_mk_prefix()*** turns into a switch statement that creates a `fib_prefix_t`
     for MPLS address family, in addition to the existing IPv4 and IPv6 types. It uses the non-EOS
     type.
*    ***lcp_router_mpls_nladdr_to_path()*** implements the loop that I described above, taking the
     stack of `struct mpls_label` from Netlink and turning them into a vector of `fib_mpls_label_t`
     for the VPP FIB.
*    ***lcp_router_route_path_parse()*** becomes aware of MPLS SWAP and POP operations (the latter
     being the case if there are 0 labels in the Netlink label stack)
*    ***lcp_router_fib_route_path_dup()*** is a helper function to make a copy of a the FIB path
     for the EOS and non-EOS VPP FIB inserts.

The VPP FIB differentiates between entries that are non-EOS (S=0), and can treat them differently to
those which are EOS (end of stack, S=1). Linux does not make this destinction, so it's safest to
just install non-EOS **and** EOS entries for each route from Linux.  This is why
`lcp_router_fib_route_path_dup()` exists, otherwise Netlink route deletions for the MPLS routes
will yield a double free later on.

This prep work then allows for the following two main functions to become MPLS aware:

*    ***lcp_router_route_add()*** when Linux sends a Netlink message about a new route, and that
     route carries MPLS labels, make a copy of the path for the EOS entry and proceed to insert
     both the non-EOS and newly crated EOS entries into the FIB,
*    ***lcp_router_route_del()*** when Linux sends a Netlink message about a deleted route, we can
     remove both the EOS and non-EOS variants of the route from VPP's FIB.

## VPP Linux-CP: MPLS with FRR

{{< image src="/assets/vpp-mpls/LAB v2.svg" alt="Lab Setup" >}}

I finally get to show off @vifino's lab! It's installed based off of a Debian Bookworm build, because
there's a few Netlink Library changes that haven't made their way into Debian Bullseye yet. The LAB
image is quickly built and distributed, and for this LAB I'm choosing specifically for
[[FRR](https://frrouting.org/)] because it ships with a _Label Distribution Protocol_ daemon out of
the box.

First order of business is to enable MPLS on the correct interfaces, and create the MPLS FIB table.
On each machine, I insert the following in the startup sequence:

```
ipng@vpp0-1:~$ cat << EOF | tee -a /etc/vpp/config/manual.vpp
mpls table add 0
set interface mpls GigabitEthernet10/0/0 enable
set interface mpls GigabitEthernet10/0/1 enable
EOF
```

The lab comes with OSPF and OSPFv3 enabled on each of the Gi10/0/0 and Gi10/0/1 interfaces that go
from East to West. This extra sequence enables MPLS on those interfaces, and because they have a
_Linux Interface Pair (LIP)_, VPP will enable MPLS on the internal TAP interfaces, as well as set
the Linux `sysctl` to allow the kernel to send MPLS encapsulated packets towards VPP.

Next up, turning on _LDP_ for FRR, which is easy enough:
```
ipng@vpp0-1:~$ vtysh
vpp0-2# conf t
vpp0-2(config)# mpls ldp
 router-id 192.168.10.1
 dual-stack cisco-interop
 ordered-control
 !
 address-family ipv4
  discovery transport-address 192.168.10.1
  label local advertise explicit-null
  interface e0
  interface e1
 exit-address-family
 !
 address-family ipv6
  discovery transport-address 2001:678:d78:200::1
  label local advertise explicit-null
  ttl-security disable
  interface e0
  interface e1
 exit-address-family
exit
```

I configure _LDP_ here to prefer advertising locally connected routes as _MPLS Explicit NULL_, which I
described in detail in the [[previous post]({{< ref "2023-05-17-vpp-mpls-2" >}})]. It tells the
penultimate router to send the router a packet as MPLS with label value 0,S=1 for IPv4 and value 2,S=1
for IPv6, so that VPP knows imediately to decapsulate the packet and continue to IPv4/IPv6 forwarding.
An alternative here is setting implicit-null, which instructs the router before this one to perform
_Penultimate Hop Popping_. If this is confusing, take a look at that article for reference!

Otherwise, just giving each router a transport-address of a loopback interface, and a unique router-id,
the same as used in OSPF and OSPFv3, and we're off to the races. Just take a look at how easy this was:

```
vpp0-1# show mpls ldp discovery
AF   ID              Type     Source           Holdtime
ipv4 192.168.10.0    Link     e0                     15
ipv4 192.168.10.2    Link     e1                     15
ipv6 192.168.10.0    Link     e0                     15
ipv6 192.168.10.2    Link     e1                     15

vpp0-1# show mpls ldp neighbor
AF   ID              State       Remote Address    Uptime
ipv6 192.168.10.0    OPERATIONAL 2001:678:d78:200::
                                                 19:49:10
ipv6 192.168.10.2    OPERATIONAL 2001:678:d78:200::2
                                                 19:49:10
```

The first `show ... discovery` shows which interfaces are receiving multicast _LDP Hello Packets_,
and because I enabled discovery for both IPv4 and IPv6, I can see two pairs there. If I look at
which interfaces formed adjacencies, `show ... neighbor` reveals that LDP is preferring IPv6, and
that both adjacencies to `vpp0-0` and `vpp0-2` are operational. Awesome sauce!

I see _LDP_ neighbor adjacencies, so let me show you what label information was actually
exchanged, in three different places, **FRR**'s label distribution protocol daemon, **Linux**'s
IPv4, IPv6 and MPLS routing tables, and **VPP**'s dataplane forwarding information base.

### MPLS: FRR view

There are two things to note -- the IPv4 and IPv6 routing table, called a _Forwarding Equivalent Class
(FEC)_, and the MPLS forwarding table, called the _MPLS FIB_:

```
vpp0-1# show mpls ldp binding
AF   Destination                Nexthop         Local Label Remote Label  In Use
ipv4 192.168.10.0/32            192.168.10.0    20          exp-null         yes
ipv4 192.168.10.1/32            0.0.0.0         exp-null    -                 no
ipv4 192.168.10.2/32            192.168.10.2    16          exp-null         yes
ipv4 192.168.10.3/32            192.168.10.2    33          33               yes
ipv4 192.168.10.4/31            192.168.10.0    21          exp-null         yes
ipv4 192.168.10.6/31            192.168.10.0    exp-null    exp-null          no
ipv4 192.168.10.8/31            192.168.10.2    exp-null    exp-null          no
ipv4 192.168.10.10/31           192.168.10.2    17          exp-null         yes
ipv6 2001:678:d78:200::/128     192.168.10.0    18          exp-null         yes
ipv6 2001:678:d78:200::1/128    0.0.0.0         exp-null    -                 no
ipv6 2001:678:d78:200::2/128    192.168.10.2    31          exp-null         yes
ipv6 2001:678:d78:200::3/128    192.168.10.2    38          34               yes
ipv6 2001:678:d78:210::/60      0.0.0.0         48          -                 no
ipv6 2001:678:d78:210::/128     0.0.0.0         39          -                 no
ipv6 2001:678:d78:210::1/128    0.0.0.0         40          -                 no
ipv6 2001:678:d78:210::2/128    0.0.0.0         41          -                 no
ipv6 2001:678:d78:210::3/128    0.0.0.0         42          -                 no

vpp0-1# show mpls table
 Inbound Label  Type  Nexthop                  Outbound Label
 ------------------------------------------------------------------
 16             LDP   192.168.10.9             IPv4 Explicit Null
 17             LDP   192.168.10.9             IPv4 Explicit Null
 18             LDP   fe80::5054:ff:fe00:1001  IPv6 Explicit Null
 19             LDP   fe80::5054:ff:fe00:1001  IPv6 Explicit Null
 20             LDP   192.168.10.6             IPv4 Explicit Null
 21             LDP   192.168.10.6             IPv4 Explicit Null
 31             LDP   fe80::5054:ff:fe02:1000  IPv6 Explicit Null
 32             LDP   fe80::5054:ff:fe02:1000  IPv6 Explicit Null
 33             LDP   192.168.10.9             33
 38             LDP   fe80::5054:ff:fe02:1000  34
```

In the first table, each entry of the IPv4 and IPv6 routing table, as fed by OSPF and OSPFv3,
will get a label associated with them. The negotiation of _LDP_ will ask our peer to set a
specific label, and it'll inform the peer on which label we are intending to use for the
_Label Switched Path_ towards that destination. I'll give two examples to illustrate how
this table is used:
1.   This router (`vpp0-1`) has a peer `vpp0-0` and when this router wants to send traffic to
     it, it'll be sent with `exp-null` (because it is the last router in the _LSP_), but when
     other routers might want to use this router to reach `vpp0-0`, they should use the MPLS
     label value 20.
1.   This router (`vpp0-1`) is _not_ directly connected to `vpp0-3` and as such, its IPv4 and IPv6
     loopback addresses are going to contain labels in both directions: if `vpp0-1` itself
     wants to send a packet to `vpp0-3`, it will use label value 33 and 38 respectively.
     However, if other routers want to use this router to reach `vpp0-3`, they should use the
     MPLS label value 33 and 34 respectively.

The second table describes the MPLS _Forwarding Information Base (FIB)_. When receiving an
MPLS packet with an inbound label noted in this table, the operation applied is _SWAP_ to the
outbound label, and forward towards a nexthop -- this is the stuff that _P-Routers_ use when
transiting MPLS traffic.

### MPLS: Linux view

FRR's LDP daemon will offer both of these routing tables to the Linux kernel using Netlink
messages, so the Linux view looks similar:

```
root@vpp0-1:~# ip ro
192.168.10.0 nhid 230  encap mpls  0 via 192.168.10.6 dev e0 proto ospf src 192.168.10.1 metric 20
192.168.10.2 nhid 226  encap mpls  0 via 192.168.10.9 dev e1 proto ospf src 192.168.10.1 metric 20
192.168.10.3 nhid 227  encap mpls  33 via 192.168.10.9 dev e1 proto ospf src 192.168.10.1 metric 20
192.168.10.4/31 nhid 230  encap mpls  0 via 192.168.10.6 dev e0 proto ospf src 192.168.10.1 metric 20
192.168.10.6/31 dev e0 proto kernel scope link src 192.168.10.7
192.168.10.8/31 dev e1 proto kernel scope link src 192.168.10.8
192.168.10.10/31 nhid 226  encap mpls  0 via 192.168.10.9 dev e1 proto ospf src 192.168.10.1 metric 20

root@vpp0-1:~# ip -6 ro
2001:678:d78:200:: nhid 231  encap mpls  2 via fe80::5054:ff:fe00:1001 dev e0 proto ospf src 2001:678:d78:200::1 metric 20 pref medium
2001:678:d78:200::1 dev loop0 proto kernel metric 256 pref medium
2001:678:d78:200::2 nhid 237  encap mpls  2 via fe80::5054:ff:fe02:1000 dev e1 proto ospf src 2001:678:d78:200::1 metric 20 pref medium
2001:678:d78:200::3 nhid 239  encap mpls  34 via fe80::5054:ff:fe02:1000 dev e1 proto ospf src 2001:678:d78:200::1 metric 20 pref medium
2001:678:d78:201::/112 nhid 231  encap mpls  2 via fe80::5054:ff:fe00:1001 dev e0 proto ospf src 2001:678:d78:200::1 metric 20 pref medium
2001:678:d78:201::1:0/112 dev e0 proto kernel metric 256 pref medium
2001:678:d78:201::2:0/112 dev e1 proto kernel metric 256 pref medium
2001:678:d78:201::3:0/112 nhid 237  encap mpls  2 via fe80::5054:ff:fe02:1000 dev e1 proto ospf src 2001:678:d78:200::1 metric 20 pref medium

root@vpp0-1:~# ip -f mpls ro
16 as to 0 via inet 192.168.10.9 dev e1 proto ldp
17 as to 0 via inet 192.168.10.9 dev e1 proto ldp
18 as to 2 via inet6 fe80::5054:ff:fe00:1001 dev e0 proto ldp
19 as to 2 via inet6 fe80::5054:ff:fe00:1001 dev e0 proto ldp
20 as to 0 via inet 192.168.10.6 dev e0 proto ldp
21 as to 0 via inet 192.168.10.6 dev e0 proto ldp
31 as to 2 via inet6 fe80::5054:ff:fe02:1000 dev e1 proto ldp
32 as to 2 via inet6 fe80::5054:ff:fe02:1000 dev e1 proto ldp
33 as to 33 via inet 192.168.10.9 dev e1 proto ldp
38 as to 34 via inet6 fe80::5054:ff:fe02:1000 dev e1 proto ldp
```

The first two tabled show a 'regular' Linux routing table for IPv4 and IPv6 respectively, except there's
an `encap mpls <X>` added for all not-directly-connected prefixes. In this case, `vpp0-1` connects on
`e0` to `vpp0-0` to the West, and on interface `e1` to `vpp0-2` to the East. These connected routes do
not carry MPLS information and in fact, this is how LDP can continue to work and exchange information
naturally even when no _LSPs_ are established yet.

The third table is the _MPLS FIB_, and it shows the special case of _MPLS Explicit NULL_ clearly. All IPv4
routes for which this router is the penultimate hop carry the outbound label value 0,S=1, while the IPv6
routes carry the value 2,S=1. Booyah!

### MPLS: VPP view

The _FIB_ information in general is super densely populated in VPP. Rather than dumping the whole table,
I'll show one example, for `192.168.10.3` which we can see above will be encapsulated into an MPLS
packet with label value 33,S=0 before being fowarded:

```
root@vpp0-1:~# vppctl show ip fib 192.168.10.3
ipv4-VRF:0, fib_index:0, flow hash:[src dst sport dport proto flowlabel ] epoch:0 flags:none locks:[adjacency:1, default-route:1, lcp-rt:1, ]
192.168.10.3/32 fib:0 index:78 locks:2
  lcp-rt-dynamic refs:1 src-flags:added,contributing,active,
    path-list:[29] locks:6 flags:shared, uPRF-list:53 len:1 itfs:[2, ]
      path:[41] pl-index:29 ip4 weight=1 pref=20 attached-nexthop:  oper-flags:resolved,
        192.168.10.9 GigabitEthernet10/0/1
      [@0]: ipv4 via 192.168.10.9 GigabitEthernet10/0/1: mtu:9000 next:7 flags:[] 5254000210005254000110010800
    Extensions:
     path:41  labels:[[33 pipe ttl:0 exp:0]]
 forwarding:   unicast-ip4-chain
  [@0]: dpo-load-balance: [proto:ip4 index:81 buckets:1 uRPF:53 to:[2421:363846]]
    [0] [@13]: mpls-label[@4]:[33:64:0:eos]
        [@1]: mpls via 192.168.10.9 GigabitEthernet10/0/1: mtu:9000 next:3 flags:[] 5254000210005254000110018847
```

The trick is looking at the Extensions, which shows the _out-labels_ set to 33, with ttl=0 (which makes
VPP copy the TTL from the IPv4 packet itself), and exp=0. It can then forward the packet as MPLS onto
the nexthop at 192.168.10.9 (`vpp0-2.e0` on Gi10/0/1).

The MPLS _FIB_ is also a bit chatty, and shows a fundamental difference with Linux:

```
root@vpp0-1:~# vppctl show mpls fib 33
MPLS-VRF:0, fib_index:0 locks:[interface:6, CLI:1, lcp-rt:1, ]
33:neos/21 fib:0 index:37 locks:2
  lcp-rt-dynamic refs:1 src-flags:added,contributing,active,
    path-list:[57] locks:12 flags:shared, uPRF-list:21 len:1 itfs:[2, ]
      path:[81] pl-index:57 ip4 weight=1 pref=0 attached-nexthop:  oper-flags:resolved,
        192.168.10.9 GigabitEthernet10/0/1
      [@0]: ipv4 via 192.168.10.9 GigabitEthernet10/0/1: mtu:9000 next:7 flags:[] 5254000210005254000110010800
    Extensions:
     path:81  labels:[[33 pipe ttl:0 exp:0]]
 forwarding:   mpls-neos-chain
  [@0]: dpo-load-balance: [proto:mpls index:40 buckets:1 uRPF:21 to:[0:0]]
    [0] [@6]: mpls-label[@28]:[33:64:0:neos]
        [@1]: mpls via 192.168.10.9 GigabitEthernet10/0/1: mtu:9000 next:3 flags:[] 5254000210005254000110018847

33:eos/21 fib:0 index:64 locks:2
  lcp-rt-dynamic refs:1 src-flags:added,contributing,active,
    path-list:[57] locks:12 flags:shared, uPRF-list:21 len:1 itfs:[2, ]
      path:[81] pl-index:57 ip4 weight=1 pref=0 attached-nexthop:  oper-flags:resolved,
        192.168.10.9 GigabitEthernet10/0/1
      [@0]: ipv4 via 192.168.10.9 GigabitEthernet10/0/1: mtu:9000 next:7 flags:[] 5254000210005254000110010800
    Extensions:
     path:81  labels:[[33 pipe ttl:0 exp:0]]
 forwarding:   mpls-eos-chain
  [@0]: dpo-load-balance: [proto:mpls index:67 buckets:1 uRPF:21 to:[73347:10747680]]
    [0] [@6]: mpls-label[@29]:[33:64:0:eos]
        [@1]: mpls via 192.168.10.9 GigabitEthernet10/0/1: mtu:9000 next:3 flags:[] 5254000210005254000110018847
```

I note that there are two entries here -- I wrote about them above. The MPLS implementation in VPP
allows for a different forwarding behavior in the case that the label inspected is the last one in the
stack (S=1), which is the usual case called _End of Stack (EOS)_. But, it also has a second entry
which tells it what to do if S=0 or _Not End of Stack (NEOS)_. Linux doesn't make the destinction, so
@vifino added two identical entries using that ***lcp_router_fib_route_path_dup()*** function.

But, what the entries themselves mean is that if this `vpp0-1` router were to receive an MPLS
packet with label value 33,S=1 (or value 33,S=0), it'll perform a _SWAP_ operation and put as
new outbound label (the same) value 33, and forward the packet as MPLS onto 192.168.10.9 on Gi10/0/1.

## Results

And with that, I think we achieved a running LDP with IPv4 and IPv6 and forwarding + encapsulation
of MPLS with VPP. One cool wrapup I thought I'd leave you with, is showing how these MPLS routers
are transparent with respect to IP traffic going through them. If I look at the diagram above, `lab`
reaches `vpp0-3` via three hops: first into `vpp0-0` where it is wrapped into MPLS and forwarded
to `vpp0-1`, and then through `vpp0-2`, which sets the _Explicit NULL_ label and forwards again
as MPLS onto `vpp0-3`, which does the IPv4 and IPv6 lookup.

Check this out:

```
pim@lab:~$ for node in $(seq 0 3); do traceroute -4 -q1 vpp0-$node; done
traceroute to vpp0-0 (192.168.10.0), 30 hops max, 60 byte packets
 1  vpp0-0.lab.ipng.ch (192.168.10.0)  1.907 ms
traceroute to vpp0-1 (192.168.10.1), 30 hops max, 60 byte packets
 1  vpp0-1.lab.ipng.ch (192.168.10.1)  2.460 ms
traceroute to vpp0-1 (192.168.10.2), 30 hops max, 60 byte packets
 1  vpp0-2.lab.ipng.ch (192.168.10.2)  3.860 ms
traceroute to vpp0-1 (192.168.10.3), 30 hops max, 60 byte packets
 1  vpp0-3.lab.ipng.ch (192.168.10.3)  4.414 ms

pim@lab:~$ for node in $(seq 0 3); do traceroute -6 -q1 vpp0-$node; done
traceroute to vpp0-0 (2001:678:d78:200::), 30 hops max, 80 byte packets
 1  vpp0-0.lab.ipng.ch (2001:678:d78:200::)  3.037 ms
traceroute to vpp0-1 (2001:678:d78:200::1), 30 hops max, 80 byte packets
 1  vpp0-1.lab.ipng.ch (2001:678:d78:200::1)  5.125 ms
traceroute to vpp0-1 (2001:678:d78:200::2), 30 hops max, 80 byte packets
 1  vpp0-2.lab.ipng.ch (2001:678:d78:200::2)  7.135 ms
traceroute to vpp0-1 (2001:678:d78:200::3), 30 hops max, 80 byte packets
 1  vpp0-3.lab.ipng.ch (2001:678:d78:200::3)  8.763 ms
```

With MPLS, each of these routers appears to the naked eye to be directly connected to the
`lab` headend machine, but we know better! :)

## What's next

I joined forces with [@vifino](https://chaos.social/@vifino) who has effectively added MPLS handling
to the Linux Control Plane, so VPP can start to function as an MPLS router using FRR's label
distribution protocol implementation. Gosh, I wish Bird3 would have LDP :)

Our work is mostly complete, there's two pending Gerrit's which should be ready to review and
certainly ready to play with:

1.   [[Gerrit 38826](https://gerrit.fd.io/r/c/vpp/+/38826)]: This adds the ability to listen to internal
     state changes of an interface, so that the Linux Control Plane plugin can enable MPLS on the
     _LIP_ interfaces and Linux sysctl for MPLS input.
1.   [[Gerrit 38702](https://gerrit.fd.io/r/c/vpp/+/38702)]: This adds the ability to listen to Netlink
     messages in the Linux Control Plane plugin, and sensibly apply these routes to the IPv4, IPv6
     and MPLS FIB in the VPP dataplane.

Finally, a note from your friendly neighborhood developers: this code is brand-new and has had _very
limited_ peer-review from the VPP developer community. It adds a significant feature to the Linux
Controlplane plugin, so make sure you both understand the semantics, the differences between Linux
and VPP, and the overall implementation before attempting to use in production. We're pretty sure
we got at least some of this right, but testing and runtime experience will tell.

I will be silently porting the change into my own copy of the Linux Controlplane called lcpng on
[[GitHub](https://git.ipng.ch/ipng/lcpng.git)]. If you'd like to test this - reach out to the VPP
Developer [[mailinglist](mailto:vpp-dev@lists.fd.io)] any time!

