doc: Update VRF documentation metric

Two things:

1) Update examples to show usage of metric
2) Discuss reasoning for using such a high metric.
Signed-off-by: NDonald Sharp <sharpd@cumulusnetworks.com>
Acked-by: NDavid Ahern <dsa@cumulusnetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/vrf.txt

@@ -71,7 +71,12 @@ Setup
        ip ru add iif vrf-blue table 10
 
 3. Set the default route for the table (and hence default route for the VRF).
-       ip route add table 10 unreachable default
+       ip route add table 10 unreachable default metric 4278198272
+
+   This high metric value ensures that the default unreachable route can
+   be overridden by a routing protocol suite.  FRRouting interprets
+   kernel metrics as a combined admin distance (upper byte) and priority
+   (lower 3 bytes).  Thus the above metric translates to [255/8192].
 
 4. Enslave L3 interfaces to a VRF device.
        ip link set dev eth1 master vrf-blue
@@ -256,7 +261,7 @@ older form without it.
 
    For example:
    $ ip route show vrf red
-   prohibit default
+   unreachable default  metric 4278198272
    broadcast 10.2.1.0 dev eth1  proto kernel  scope link  src 10.2.1.2
    10.2.1.0/24 dev eth1  proto kernel  scope link  src 10.2.1.2
    local 10.2.1.2 dev eth1  proto kernel  scope host  src 10.2.1.2
@@ -282,7 +287,7 @@ older form without it.
    ff00::/8 dev red  metric 256  pref medium
    ff00::/8 dev eth1  metric 256  pref medium
    ff00::/8 dev eth2  metric 256  pref medium
-
+   unreachable default dev lo  metric 4278198272  error -101 pref medium
 
 8. Route Lookup for a VRF
 
@@ -331,7 +336,7 @@ function vrf_create
     ip link add ${VRF} type vrf table ${TBID}
 
     if [ "${VRF}" != "mgmt" ]; then
-        ip route add table ${TBID} unreachable default
+        ip route add table ${TBID} unreachable default metric 4278198272
     fi
     ip link set dev ${VRF} up
 }