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doc: add section on dummy interfaces

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Fixes #769

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>
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Joachim Wiberg
2026-04-30 11:08:02 +02:00
parent 624bfd927c
commit 5defdfdeac
2 changed files with 46 additions and 1 deletions
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doc/iface.md
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@@ -138,6 +138,49 @@ admin@example:/config/interface/veth0a/> <b>set custom-phys-address chassis offs
</code></pre>
## Dummy Interface
A dummy interface is a virtual interface that is always administratively
and operationally UP, regardless of any physical link state. It can
hold IP addresses just like any other interface.
The two most common uses are:
- **Stable OSPF router-ID**: OSPF picks its router-ID from an interface
address. If that interface goes down, adjacencies can flap. Binding
the router-ID to a /32 address on a dummy avoids this.
- **Stable management address**: A /32 on a dummy gives the device a
permanent identity on the network, reachable as long as at least one
uplink is up and the address is redistributed into the routing domain.
> [!TIP]
> WiFi interfaces also use dummies as placeholders when the radio
> hardware is not detected at boot (e.g., a USB dongle that was
> unplugged). See [WiFi](wifi.md) for details.
### Example: Stable OSPF Router-ID
Create a dummy interface with a /32 address and use it as the OSPF
router-ID so that the ID never changes when physical ports bounce:
<pre class="cli"><code>admin@example:/> <b>configure</b>
admin@example:/config/> <b>edit interface lo0</b>
admin@example:/config/interface/lo0/> <b>set type dummy</b>
admin@example:/config/interface/lo0/> <b>set ipv4 address 192.0.2.1 prefix-length 32</b>
admin@example:/config/interface/lo0/> <b>leave</b>
admin@example:/config/> <b>edit routing control-plane-protocol ospfv2 name default ospf</b>
admin@example:/config/routing/…/ospf/> <b>set explicit-router-id 192.0.2.1</b>
admin@example:/config/routing/…/ospf/> <b>leave</b>
admin@example:/> <b>copy running-config startup-config</b>
</code></pre>
To also make the address reachable by other routers, redistribute
connected routes (or add `lo0` as an OSPF interface):
<pre class="cli"><code>admin@example:/config/routing/…/ospf/> <b>set redistribute connected</b>
</code></pre>
[^1]: A YANG deviation was previously used to make it possible to set
`phys-address`, but this has been replaced with the more flexible
`custom-phys-address`.