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Merge pull request #14502 from opensourcerouting/fix/document_on_how_to_set_the_distance_from_linux

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doc: Add an example on how to set a distance for a route from the kernel
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Donald Sharp 2023-09-28 08:15:25 -04:00 committed by GitHub
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9
doc/user/bgp.rst
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@ -461,6 +461,15 @@ Administrative Distance Metrics
Sets the administrative distance for a particular route. Sets the administrative distance for a particular route.
If the system has a static route configured from the kernel, it has a
distance of 0. In some cases, it might be useful to override the route
from the FRR. E.g.: Kernel has a statically configured default route,
and you received another default route from the BGP and want to install
it to be preferred over the static route. In such a case, you MUST set
a higher distance from the kernel.
.. seealso:: :ref:`administrative-distance`
.. _bgp-requires-policy: .. _bgp-requires-policy:
Require policy on EBGP Require policy on EBGP

28
doc/user/zebra.rst
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@ -374,6 +374,8 @@ outgoing interface
Resolve PBR nexthop via ip neigh tracking Resolve PBR nexthop via ip neigh tracking
.. _administrative-distance:
Administrative Distance Administrative Distance
======================= =======================
@ -420,16 +422,34 @@ the same distances that other routing suites have chosen.
+------------+-----------+ +------------+-----------+
An admin distance of 255 indicates to Zebra that the route should not be An admin distance of 255 indicates to Zebra that the route should not be
installed into the Data Plane. Additionally routes with an admin distance installed into the Data Plane. Additionally routes with an admin distance
of 255 will not be redistributed. of 255 will not be redistributed.
Zebra does treat Kernel routes as special case for the purposes of Admin Zebra does treat Kernel routes as special case for the purposes of Admin
Distance. Upon learning about a route that is not originated by FRR Distance. Upon learning about a route that is not originated by FRR
we read the metric value as a uint32_t. The top byte of the value we read the metric value as a uint32_t. The top byte of the value
is interpreted as the Administrative Distance and the low three bytes is interpreted as the Administrative Distance and the low three bytes
are read in as the metric. This special case is to facilitate VRF are read in as the metric. This special case is to facilitate VRF
default routes. default routes.
.. code-block:: shell
$ # Set administrative distance to 255 for Zebra
$ ip route add 192.0.2.0/24 metric $(( 2**32 - 2**24 )) dev lo
$ vtysh -c 'show ip route 192.0.2.0/24 json' | jq '."192.0.2.0/24"[] | (.distance, .metric)'
255
0
$ # Set administrative distance to 192 for Zebra
$ ip route add 192.0.2.0/24 metric $(( 2**31 + 2**30 )) dev lo
$ vtysh -c 'show ip route 192.0.2.0/24 json' | jq '."192.0.2.0/24"[] | (.distance, .metric)'
192
0
$ # Set administrative distance to 128, and metric 100 for Zebra
$ ip route add 192.0.2.0/24 metric $(( 2**31 + 100 )) dev lo
$ vtysh -c 'show ip route 192.0.2.0/24 json' | jq '."192.0.2.0/24"[] | (.distance, .metric)'
128
100
Route Replace Semantics Route Replace Semantics
======================= =======================