IPv6 Configuration Guide K/KA/KB.15.15

in the routing table defines how many leftmost contiguous bits to use when matching a packet's
destination address to a destination network prefix. For example, a route table entry of
2001:db8:0:1ad:0:f1:7a:0/112
applies to all packets with a destination address for which the first 112 bits are
2001:db8:0:1ad::f1:7a
If a packet matches more than one routing table entry, the router uses the most specific route (the
route with the longest prefix), which is assumed to be the most accurate for that packet. For example,
for the packet destination listed below, both route table entries apply, but the route selected will
be the 72-bit entry, because it is the more specific route.
Packet destination address: 2001:db8:0:1d5:a15::f:101/64
72-bit entry in route table: 2001:db8:0:1d5:a00::/72
64-bit entry in route table: 2001:db8:0:1d5::/64
Gateway for forwarding routed traffic
The gateway to a destination network can be either of the following:
• Global unicast or link-local address of the next-hop router in the IP routing providing a path
to the packet destination.
• The IP routing interconnecting the originating router to the next-hop router.
Metric and administrative distance
The routing table contains the single best route to each destination that the router has learned.
However, a router may learn more than one route to the same destination. The router compares
the metrics and administrative distances of these routes to select the best route to add to its routing
table.
• Administrative distance
The routing switch uses this parameter to compare routes learned by different routing methods.
It indicates how reliable the router considers the method through which it discovered the route:
a lower value indicates a more trustworthy route.
Administrative distance is not a factor if you are using only static routes. However, if you are
using static routing in conjunction with a routing protocol such as OSPFv3 to provide routes
to an identical destination, the routing switch selects the route with the lowest administrative
distance. Where the default administrative distances are used, a static route normally
supersedes a dynamic route to the same destination because the former has the lowest default
administrative distance and metric.
MetricAdministrative distanceRouting method
10 (not configurable)Direct connection
1Default: 1; range: 1 to 255Static route
VariableDefault: 110 (external, inter-area, and intra-area) range
(for all three): 1 to 255
OSPFv3
178 IPv6 Routing Basics