24-6
Cisco ASA 5500 Series Configuration Guide using ASDM
Chapter24 Routing Overview
Information About the Routing Table
Information About the Routing Table
This section includes the following topics:
•Displaying the Routing Table, page24-6
•How the Routing Table Is Populated, page24-6
•How Forwarding Decisions Are Made, page24-8
•Dynamic Routing and Failover, page24-8

Displaying the Routing Table

To show all routes in ASDM that are in the routing table, choose Monitoring > Routing > Routes.
In this pane, each row represents one route.

How the Routing Table Is Populated

The ASA routing table can be populated by statically defined routes, directly connected routes, and
routes discovered by the RIP, EIGRP, and OSPF routing protocols. Because the ASA can run multiple
routing protocols in addition to having static and connected routes in the routing table, it is possible that
the same route is discovered or entered in more than one manner. When two routes to the same
destination are put into the routing table, the one that remains in the routing table is determined as
follows:
•If the two routes have different network prefix lengths (network masks), then both routes are
considered unique and are entered into the routing table. The packet forwarding logic then
determines which of the two to use.
For example, if the RIP and OSPF processes discovered the following routes:
–
RIP: 192.168.32.0/24
–
OSPF: 192.168.32.0/19
Even though OSPF routes have the better administrative distance, both routes are installed in the
routing table because each of these routes has a different prefix length (subnet mask). They are
considered different destinations and the packet forwarding logic determines which route to use.
•If the ASA learns about multiple paths to the same destination from a single routing protocol, such
as RIP, the route with the better metric (as determined by the routing protocol) is entered into the
routing table.
Metrics are values associated with specific routes, ranking them from most preferred to least
preferred. The parameters used to determine the metrics differ for different routing protocols. The
path with the lowest metric is selected as the optimal path and installed in the routing table. If there
are multiple paths to the same destination with equal metrics, load balancing is done on these equal
cost paths.
•If the ASA learns about a destination from more than one routing protocol, the administrative
distances of the routes are compared and the routes with lower administrative distance are entered
into the routing table.