Cisco ASA 5505 - Manual (Page 441)

21-7
Cisco ASA 5500 Series Configuration Guide using the CLI
Chapter 21 Routing Overview
Information About the Routing Table
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.
You can change the administrative distances for routes discovered by or redistributed into a routing
protocol. If two routes from two different routing protocols have the same administrative distance,
then the route with the lower default administrative distance is entered into the routing table. In the
case of EIGRP and OSPF routes, if the EIGRP route and the OSPF route have the same
administrative distance, then the EIGRP route is chosen by default.
Administrative distance is a route parameter that the ASA uses to select the best path when there are two
or more different routes to the same destination from two different routing protocols. Because the routing
protocols have metrics based on algorithms that are different from the other protocols, it is not always
possible to determine the best path for two routes to the same destination that were generated by different
routing protocols.
Each routing protocol is prioritized using an administrative distance value. Table 21-1 shows the default
administrative distance values for the routing protocols supported by the ASA.
The smaller the administrative distance value, the more preference is given to the protocol. For example,
if the ASA receives a route to a certain network from both an OSPF routing process (default
administrative distance - 110) and a RIP routing process (default administrative distance - 120), the ASA
chooses the OSPF route because OSPF has a higher preference. In this case, the router adds the OSPF
version of the route to the routing table.
In this example, if the source of the OSPF-derived route was lost (for example, due to a power shutdown),
the ASA would then use the RIP-derived route until the OSPF-derived route reappears.
Table 21-1 Default Administrative Distance for Supported Routing Protocols
Route Source Default Administrative Distance
Connected interface 0
Static route 1
EIGRP Summary Route 5
Internal EIGRP 90
OSPF 110
RIP 120
EIGRP external route 170
Unknown 255