Cisco 500 Series, Configuration Manual

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Cisco Security Appliance Command Line Configuration Guide
OL-12172-03
Chapter 9 Configuring IP Routing
The Routing Table
The smaller the administrative distance value, the more preference is given to the protocol. For example,
if the security appliance 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 - 100), the
security appliance chooses the OSPF route because OSPF has a higher preference. This means the router
adds the OSPF version of the route to the routing table.
In the above example, if the source of the OSPF-derived route was lost (for example, due to a power
shutdown), the security appliance would then use the RIP-derived route until the OSPF-derived route
reappears.
The administrative distance is a local setting. For example, if you use the distance-ospf command to
change the administrative distance of routes obtained through OSPF, that change would only affect the
routing table for the security appliance the command was entered on. The administrative distance is not
advertised in routing updates.
Administrative distance does not affect the routing process. The OSPF and RIP routing processes only
advertise the routes that have been discovered by the routing process or redistributed into the routing
process. For example, the RIP routing process advertises RIP routes, even if routes discovered by the
OSPF routing process are used in the security appliance routing table.
Backup Routes
A backup route is registered when the initial attempt to install the route in the routing table fails because
another route was installed instead. If the route that was installed in the routing table fails, the routing
table maintenance process calls each routing protocol process that has registered a backup route and
requests them to reinstall the route in the routing table. If there are multiple protocols with registered
backup routes for the failed route, the preferred route is chosen based on administrative distance.
Because of this process, you can create “floating” static routes that are installed in the routing table when
the route discovered by a dynamic routing protocol fails. A floating static route is simply a static route
configured with a greater administrative distance than the dynamic routing protocols running on the
security appliance. When the corresponding route discover by a dynamic routing process fails, the static
route is installed in the routing table.
How Forwarding Decisions are Made
Forwarding decisions are made as follows:
•
If the destination does not match an entry in the routing table, the packet is forwarded through the
interface specified for the default route. If a default route has not been configured, the packet is
discarded.
• If the destination matches a single entry in the routing table, the packet is forwarded through the
interface associated with that route.
Internal EIGRP 90
OSPF 110
RIP 120
EIGRP external route 170
Unknown 255
Table 9-1 Default Administrative Distance for Supported Routing Protocols