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doc: Slightly touch on the vrf route lookup semantics

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People keep asking about the default unreachable route
in the linux vrf table.  Add a bit of color about the
design choices and what is going on.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>
This commit is contained in:
Donald Sharp 2020-12-10 07:38:52 -05:00
parent 98e3dfaee0
commit eff8e8a2b0
1 changed files with 8 additions and 0 deletions
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doc/user/zebra.rst
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@ -407,6 +407,14 @@ If no option is chosen, then the *Linux VRF* implementation as references in
https://www.kernel.org/doc/Documentation/networking/vrf.txt will be mapped over
the *Zebra* VRF. The routing table associated to that VRF is a Linux table
identifier located in the same *Linux network namespace* where *Zebra* started.
Please note when using the *Linux VRF* routing table it is expected that a
default Kernel route will be installed that has a metric as outlined in the
www.kernel.org doc above. The Linux Kernel does table lookup via a combination
of rule application of the rule table and then route lookup of the specified
table. If no route match is found then the next applicable rule is applied
to find the next route table to use to look for a route match. As such if
your VRF table does not have a default blackhole route with a high metric
VRF route lookup will leave the table specified by the VRF, which is undesirable.
If the :option:`-n` option is chosen, then the *Linux network namespace* will
be mapped over the *Zebra* VRF. That implies that *Zebra* is able to configure