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Merge pull request #7684 from donaldsharp/metric_doc

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doc: Add some notes about RR semantics and the Linux Kernel
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Mark Stapp 2020-12-08 12:16:43 -05:00 committed by GitHub
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@ -361,6 +361,25 @@ is interpreted as the Administrative Distance and the low three bytes
are read in as the metric. This special case is to facilitate VRF
default routes.
Route Replace Semantics
=======================
When using the Linux Kernel as a forwarding plane, routes are installed
with a metric of 20 to the kernel. Please note that the kernel's metric
value bears no resemblence to FRR's RIB metric or admin distance. It
merely is a way for the Linux Kernel to decide which route to use if it
has multiple routes for the same prefix from multiple sources. An example
here would be if someone else was running another routing suite besides
FRR at the same time, the kernel must choose what route to use to forward
on. FRR choose the value of 20 because of two reasons. FRR wanted a
value small enough to be choosen but large enough that the operator could
allow route prioritization by the kernel when multiple routing suites are
being run and FRR wanted to take advantage of Route Replace semantics that
the linux kernel offers. In order for Route Replacement semantics to
work FRR must use the same metric when issuing the replace command.
Currently FRR only supports Route Replace semantics using the Linux
Kernel.
Virtual Routing and Forwarding
==============================