doc: Update VRF documentation metric

       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

   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

   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

    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

}