Babel is an interior gateway protocol that is suitable both for wired networks and for wireless mesh networks. Babel has been described as ‘RIP on speed’ – it is based on the same principles as RIP, but includes a number of refinements that make it react much faster to topology changes without ever counting to infinity, and allow it to perform reliable link quality estimation on wireless links. Babel is a double-stack routing protocol, meaning that a single Babel instance is able to perform routing for both IPv4 and IPv6.

FRR implements Babel as described in RFC 6126.

Configuring babeld

The babeld daemon can be invoked with any of the common options (Common Invocation Options).

The zebra daemon must be running before babeld is invoked. Also, if zebra is restarted then babeld must be too.

Configuration of babeld is done in its configuration file babeld.conf.

Babel configuration

router babel

Enable or disable Babel routing.

babel diversity

Enable or disable routing using radio frequency diversity. This is highly recommended in networks with many wireless nodes. If you enable this, you will probably want to set babel diversity-factor and babel channel below.

babel diversity-factor (1-256)

Sets the multiplicative factor used for diversity routing, in units of 1/256; lower values cause diversity to play a more important role in route selection. The default it 256, which means that diversity plays no role in route selection; you will probably want to set that to 128 or less on nodes with multiple independent radios.

network IFNAME

Enable or disable Babel on the given interface.

babel <wired|wireless>

Specifies whether this interface is wireless, which disables a number of optimisations that are only correct on wired interfaces. Specifying wireless (the default) is always correct, but may cause slower convergence and extra routing traffic.

babel split-horizon

Specifies whether to perform split-horizon on the interface. Specifying no babel split-horizon is always correct, while babel split-horizon is an optimisation that should only be used on symmetric and transitive (wired) networks. The default is babel split-horizon on wired interfaces, and no babel split-horizon on wireless interfaces. This flag is reset when the wired/wireless status of an interface is changed.

babel hello-interval (20-655340)

Specifies the time in milliseconds between two scheduled hellos. On wired links, Babel notices a link failure within two hello intervals; on wireless links, the link quality value is reestimated at every hello interval. The default is 4000 ms.

babel update-interval (20-655340)

Specifies the time in milliseconds between two scheduled updates. Since Babel makes extensive use of triggered updates, this can be set to fairly high values on links with little packet loss. The default is 20000 ms.

babel channel (1-254)
babel channel interfering
babel channel noninterfering

Set the channel number that diversity routing uses for this interface (see babel diversity above). Noninterfering interfaces are assumed to only interfere with themselves, interfering interfaces are assumed to interfere with all other channels except noninterfering channels, and interfaces with a channel number interfere with interfering interfaces and interfaces with the same channel number. The default is babel channel interfering for wireless interfaces, and babel channel noninterfering for wired interfaces. This is reset when the wired/wireless status of an interface is changed.

babel rxcost (1-65534)

Specifies the base receive cost for this interface. For wireless interfaces, it specifies the multiplier used for computing the ETX reception cost (default 256); for wired interfaces, it specifies the cost that will be advertised to neighbours. This value is reset when the wired/wireless attribute of the interface is changed.


Do not use this command unless you know what you are doing; in most networks, acting directly on the cost using route maps is a better technique.

babel rtt-decay (1-256)

This specifies the decay factor for the exponential moving average of RTT samples, in units of 1/256. Higher values discard old samples faster. The default is 42.

babel rtt-min (1-65535)

This specifies the minimum RTT, in milliseconds, starting from which we increase the cost to a neighbour. The additional cost is linear in (rtt - rtt-min). The default is 10 ms.

babel rtt-max (1-65535)

This specifies the maximum RTT, in milliseconds, above which we don’t increase the cost to a neighbour. The default is 120 ms.

babel max-rtt-penalty (0-65535)

This specifies the maximum cost added to a neighbour because of RTT, i.e. when the RTT is higher or equal than rtt-max. The default is 150. Setting it to 0 effectively disables the use of a RTT-based cost.

babel enable-timestamps

Enable or disable sending timestamps with each Hello and IHU message in order to compute RTT values. The default is no babel enable-timestamps.

babel resend-delay (20-655340)

Specifies the time in milliseconds after which an ‘important’ request or update will be resent. The default is 2000 ms. You probably don’t want to tweak this value.

babel smoothing-half-life (0-65534)

Specifies the time constant, in seconds, of the smoothing algorithm used for implementing hysteresis. Larger values reduce route oscillation at the cost of very slightly increasing convergence time. The value 0 disables hysteresis, and is suitable for wired networks. The default is 4 s.

Babel redistribution

redistribute <ipv4|ipv6> KIND

Specify which kind of routes should be redistributed into Babel.

Show Babel information

These commands dump various parts of babeld’s internal state.

show babel route
show babel route A.B.C.D
show babel route X:X::X:X
show babel route A.B.C.D/M
show babel route X:X::X:X/M
show babel interface
show babel interface IFNAME
show babel neighbor
show babel parameters

Babel debugging commands

simple: debug babel KIND simple: no debug babel KIND

debug babel KIND

Enable or disable debugging messages of a given kind. KIND can be one of:

  • common

  • filter

  • timeout

  • interface

  • route

  • all


If you have compiled with the NO_DEBUG flag, then these commands aren’t available.

Babel sample configuration file

debug babel common
!debug babel kernel
!debug babel filter
!debug babel timeout
!debug babel interface
!debug babel route
!debug babel all

router babel
! network wlan0
! network eth0
! redistribute ipv4 kernel
! no redistribute ipv6 static

! The defaults are fine for a wireless interface

!interface wlan0

! A few optimisation tweaks are optional but recommended on a wired interface
! Disable link quality estimation, enable split horizon processing, and
! increase the hello and update intervals.

!interface eth0
! babel wired
! babel split-horizon
! babel hello-interval 12000
! babel update-interval 36000

! log file /var/log/frr/babeld.log
log stdout