Default Administrative Distance (continued)

TABLE 1 . 3 Default Administrative Distance (continued)
Source of Route Default Administrative Distance
Internal BGP 200
Unknown 255
If more than one route exists to a given destination, the route with lowest administrative distance
will be placed in the routing table. You may be wondering what happens if multiple routes
to a given destination have the same administrative distance. This is when the second factor—
metric—comes into play.
If you establish a static route by supplying the exit interface instead of the
next-hop address, it will have a metric of 0, just like a directly connected network
would, making it preferable to next hop-based static routes. This is useful
with the ip unnumbered command or whenever you want a static route
based not on the availability of the remote next-hop address but instead on
the availability of the local interface.
A metric is the value of a route specific to a routing protocol. If multiple routes have the same
administrative distance, then the metric is used as the tiebreaker. Here’s a simple way to think
about it: The router first looks to see which route can be trusted the most. If the router has multiple
routes that are equally trustworthy, the router will then look to see which route has the
lowest metric, which is the one it finds to be the most desirable. That is the route that will populate
the routing table. Depending on the routing protocol and its configuration, multiple routes
with the same AD and metric could be placed into the routing table simultaneously.
Let’s summarize everything you’ve learned so far about routing tables and how they are populated.
At this point, you know that routes are learned either dynamically or statically. Those routes
are then placed in the routing table based on which one is the most trusted. If multiple routes exist
that are equally trusted, the one that is the most desirable is placed in the routing table.
Let’s revisit the life of a packet. When a packet is sent to a destination, if the destination is
not on the same network as the source, the packet will be sent to a local router for the immediate
network. The router then looks in its routing table to see if it has a route to the destination network.
If the router does not have a route and a default gateway doesn’t exist, the packet is discarded
and an ICMP error message is sent to the packet’s source. In fact, any router along the
path to the destination network could run into this same problem and discard the packet, notifying
the original source device of the execution. So, if a route exists, how does the packet reach
the destination? We’re going to explore getting a packet to its destination in the next section.