User guide

11-4 E
XTREME
W
ARE
S
OFTWARE
U
SER
G
UIDE
IP U
NICAST
R
OUTING
D
YNAMIC
R
OUTES
Dynamic routes are typically learned by way of RIP or OSPF. Routers that use RIP or
OSPF exchange information in their routing tables in the form of advertisements. Using
dynamic routes, the routing table contains only networks that are reachable.
Dynamic routes are aged out of the table when an update for the network is not
received for a period of time, as determined by the routing protocol.
S
TATIC
R
OUTES
Static routes are manually entered into the routing table. Static routes are used to reach
networks not advertised by routers.
Static routes can also be used for security reasons, to control which routes you want
advertised by the router. You can decide if you want all static routes to be advertised,
using one of the following commands:
[enable | disable] rip export static
[enable | disable] ospf export static
The default setting is enabled. Static routes are never aged out of the routing table.
A static route must be associated with a valid IP subnet. An IP subnet is associated with
a single VLAN by its IP address and subnet mask. If the VLAN is subsequently deleted,
the static route entries using that subnet must be deleted manually.
M
ULTIPLE ROUTES
When there are multiple, conflicting choices of a route to a particular destination, the
router picks the route with the longest matching network mask. If these are still equal,
the router picks the route using the following criteria (in the order specified):
• Directly attached network interfaces
• ICMP redirects (refer to Table 11 -6, later in this chapter)
• Static routes
• Directly attached network interfaces that are not active.
If you define multiple default routes, the route that has the lowest metric is used.
If there are multiple default routes that have the same lowest metric, the system
picks one of the routes.