```{eval-rst}
.. meta::
   :description: The Babel routing protocol provides robust and efficient
                 routing for wired and wireless mesh networks.
   :keywords: babel, routing, protocol, wireless, mesh, network, metric,
              ipv4, ipv6
```

(babel)=

# Babel

The Babel protocol provides robust and efficient routing for both wired and
wireless mesh networks. By default, Babel uses hop-count metrics on wired links
and a variant of Expected Transmission Count (ETX) on wireless links.
Administrators can configure Babel to account for radio diversity,
automatically compute link latency, and include that latency in the routing
metric. {rfc}`8966` defines the Babel protocol.

Babel is a dual-stack protocol. A single Babel instance routes both IPv4 and
IPv6 traffic simultaneously.

## General configuration

VyOS does not require a specific command to start the Babel process. The system
automatically starts the routing process when you configure the first
Babel-enabled interface.

```{cfgcmd} set protocols babel interface \<interface\>

**Enable Babel routing on the specified interface.**

The system immediately begins sending and receiving Babel packets on this
interface.
```

## Optional configuration

```{cfgcmd} set protocols babel parameters diversity

**Enable radio-frequency diversity routing for the Babel process.**

Enabling this feature is highly recommended for networks with many
wireless nodes.

:::{note}
When you enable diversity routing, you should also configure the
``diversity-factor`` and ``channel`` parameters.
:::
```

```{cfgcmd} set protocols babel parameters diversity-factor \<1-256\>

**Configure the multiplicative factor for diversity routing, in units of
1/256.**

Lower multiplicative factors give greater weight to diversity in route
selection. The default value is 256, which disables diversity routing.
On nodes with multiple independent radios, configure a value of 128 or less.
```

```{cfgcmd} set protocols babel parameters resend-delay \<20-655340\>

**Configure the delay in milliseconds before the system resends an
important request or update.**

The default value is 2000 ms.
```

```{cfgcmd} set protocols babel parameters smoothing-half-life \<0-65534\>

**Configure the time constant, in seconds, for the smoothing algorithm used
to implement hysteresis.**

Higher values reduce route oscillation but slightly increase convergence
time. A value of 0 disables hysteresis and is suitable for wired networks.
The default is 4 seconds.
```

## Interfaces configuration

```{cfgcmd} set protocols babel interface \<interface\> type \<auto|wired|wireless\>

**Configure the network type for the Babel-enabled interface.**

Choose from the following:

* ``auto``: Babel automatically detects if an interface is wired or
  wireless.
* ``wired``: Babel enables optimizations for wired interfaces.
* ``wireless``: Babel disables optimizations suitable only for wired
  interfaces. Specifying wireless is always correct, but may cause slower
  convergence and increased routing traffic.
```

```{cfgcmd} set protocols babel interface \<interface\> split-horizon \<default|disable|enable\>

**Configure the split-horizon routing behavior for the specified
interface.**

Use one of the following options:

* ``default``: Babel automatically enables split-horizon on wired
  interfaces and disables it on wireless interfaces.
* ``enable``: Babel enables split-horizon on the interface. This
  optimization should be used only on symmetric, transitive (wired)
  networks.
* ``disable``: Babel disables split-horizon on the interface. Disabling
  split-horizon is always safe and correct.
```

```{cfgcmd} set protocols babel interface \<interface\> hello-interval \<20-655340\>

**Configure the interval, in milliseconds, between scheduled hello messages
on the specified interface.**

On wired links, Babel detects link failures within two hello intervals.
On wireless links, link quality is reestimated at each interval. The
default is 4000 ms.
```

```{cfgcmd} set protocols babel interface \<interface\> update-interval \<20-655340\>

**Configure the interval, in milliseconds, between scheduled routing
updates on the specified interface.**

Because Babel uses triggered updates extensively, you can increase this
value on reliable links with minimal packet loss. The default is 20000 ms.
```

```{cfgcmd} set protocols babel interface \<interface\> rxcost \<1-65534\>

**Configure the base receive cost for the specified interface.**

Babel applies this value based on the configured network type:

* ``wired``: The value is the routing cost advertised to neighboring
  routers.
* ``wireless``: The value is a multiplier used to compute the ETX
  (Expected Transmission Count) reception cost.

The default value is 256.
```

```{cfgcmd} set protocols babel interface \<interface\> rtt-decay \<1-256\>

**Configure the decay factor for the exponential moving average of RTT
samples, in units of 1/256.**

Higher values discard older samples faster. The default value is 42.
```

```{cfgcmd} set protocols babel interface \<interface\> rtt-min \<1-65535\>

**Configure the minimum RTT, in milliseconds, at which the cost to a
neighbor begins to increase.**

The additional cost is linear in (rtt - rtt-min). The default value is 10 ms.
```

```{cfgcmd} set protocols babel interface \<interface\> rtt-max \<1-65535\>

**Configure the maximum RTT, in milliseconds, above which the cost to a
neighbor stops increasing.**

The default value is 120 ms.
```

```{cfgcmd} set protocols babel interface \<interface\> max-rtt-penalty \<0-65535\>

**Configure the maximum cost added to a neighbor when RTT meets or exceeds
rtt-max.**

Setting this value to 0 disables RTT-based costs. The default value is 150.
```

```{cfgcmd} set protocols babel interface \<interface\> enable-timestamps

**Configure adding timestamps to each Hello and IHU message to calculate
RTT values.**

Enabling timestamps is recommended for tunnel interfaces.
```

```{cfgcmd} set protocols babel interface \<interface\> channel \<1-254|interfering|noninterfering\>

**Configure the channel identifier that diversity routing uses for the
specified interface.**

Interfaces interfere with each other based on the assigned channel
identifier:

* ``1–254``: The interface interferes with interfaces sharing the same
  channel number and with interfaces configured as ``interfering``.
* ``interfering``: The interface interferes with all others except those
  configured as ``noninterfering``.
* ``noninterfering``: The interface interferes only with itself.
```

## Redistribution configuration

```{cfgcmd} set protocols babel redistribute \<ipv4|ipv6\> \<route source\>

**Configure the redistribution of routing information from the specified
route source into the Babel process.**

The following route sources are available:

* **ipv4:** ``bgp``, ``connected``, ``eigrp``, ``isis``, ``kernel``,
  ``nhrp``, ``ospf``, ``rip``, ``static``
* **ipv6:** ``bgp``, ``connected``, ``eigrp``, ``isis``, ``kernel``,
  ``nhrp``, ``ospfv3``, ``ripng``, ``static``
```

```{cfgcmd} set protocols babel distribute-list \<ipv4|ipv6\> access-list \<in|out\> \<number\>

**Configure global Babel route filtering using an access list.**

Specify the direction in which the access list is applied:

* ``in``: Filters incoming routes.
* ``out``: Filters outgoing routes.
```

```{cfgcmd} set protocols babel distribute-list \<ipv4|ipv6\> interface \<interface\> access-list \<in|out\> \<number\>

**Configure Babel route filtering on the specified interface using an
access list.**

Specify the direction in which the access list is applied:

* ``in``: Filters incoming routes.
* ``out``: Filters outgoing routes.
```

```{cfgcmd} set protocols babel distribute-list \<ipv4|ipv6\> prefix-list \<in|out\> \<name\>

**Configure global Babel route filtering using a prefix list.**

Specify the direction in which the prefix list is applied:

* ``in``: Filters incoming routes.
* ``out``: Filters outgoing routes.
```

```{cfgcmd} set protocols babel distribute-list \<ipv4|ipv6\> interface \<interface\> prefix-list \<in|out\> \<name\>

**Configure Babel route filtering on the specified interface using a
prefix list.**

Specify the direction in which the prefix list is applied:

* ``in``: Filters incoming routes.
* ``out``: Filters outgoing routes.
```

## Configuration example

### Basic two-node babel network

**Goal:** The following example connects two routers (Node 1 and Node 2) via
their eth0 interfaces and uses the Babel routing protocol to advertise
(redistribute) each router's locally configured networks (represented by
loopback addresses) to one another.

**Node 1:**

```none
# Configure the loopback (local networks) and physical (eth0) addresses
set interfaces loopback lo address 10.1.1.1/32
set interfaces loopback lo address fd12:3456:dead:beef::1/128
set interfaces ethernet eth0 address 192.168.1.1/24

# Enable Babel on the physical link
set protocols babel interface eth0 type wired

# Instruct Babel to advertise (redistribute) the locally configured networks
set protocols babel redistribute ipv4 connected
set protocols babel redistribute ipv6 connected
```

**Node 2:**

```none
# Configure the loopback (local networks) and physical (eth0) addresses
set interfaces loopback lo address 10.2.2.2/32
set interfaces loopback lo address fd12:3456:beef:dead::2/128
set interfaces ethernet eth0 address 192.168.1.2/24

# Enable Babel on the physical link
set protocols babel interface eth0 type wired

# Tell Babel to advertise (redistribute) the locally configured networks
set protocols babel redistribute ipv4 connected
set protocols babel redistribute ipv6 connected
```