Multicast and Routing Guide K/KA/KB.15.15

For the switches, the administrative distance for OSPF routes is set at 110 for all route types (external,
inter-area, and intra-area.)
The switch selects one route over another based on the source of the route information. To do so,
the switch can use the administrative distances assigned to the sources to influence route choices.
You can change the distance settings in the OSPF global context to enable preference of one route
type over another.
Adjusting performance by changing the VLAN or subnet interface settings
Optional: A setting described in this section can be configured with the same value across all
subnets in a VLAN or be configured on a per-interface basis with different values.
NOTE: Most of the OSPF interface parameters also apply to virtual link configurations. However,
when used on a virtual link configuration, the OSPF context requirement is different and the
parameters are applied only to the interfaces included in the virtual link. See “Changing the dead
interval on a virtual link” (page 161).
Configuring OSPF interface authentication
Optional: OSPF supports two methods of authentication for each VLAN or subnet—simple password
and MD5. In addition, the value can be disabled, meaning no authentication is performed. Only
one method of authentication can be active on a VLAN or subnet at a time, and if one method is
configured on an interface, configuring the alternative method on the same interface automatically
overwrites the first method used.
In the default configuration, OSPF authentication is disabled. All interfaces in the same network
or subnet must have the same authentication method (password or MD5 key chain) and credentials.
Configuring an ABR to use a virtual link to the backbone
All ABRs must have either a direct, physical or indirect, virtual link to the OSPF backbone area
(0.0.0.0 or 0.) If an ABR does not have a physical link to the area backbone, the ABR can use a
virtual link to provide a logical connection to another ABR having a direct physical connection to
the area backbone. Both ABRs must belong to the same area, and this area becomes a transit
area for traffic to and from the indirectly connected ABR.
NOTE: A backbone area can be purely virtual with no physical backbone links. Also, virtual
links can be "daisy chained." If so, the virtual link may not have one end physically connected to
the backbone.
Because both ABRs in a virtual link connection are in the same OSPF area, they use the same
transit area ID. This setting is automatically determined by the ABRs and should match the area ID
value configured on both ABRs in the virtual link.
The ABRs in a virtual link connection also identify each other with a neighbor router setting:
• On the ABR having the direct connection to the backbone area, the neighbor router is the IP
address of the router interface needing a logical connection to the backbone.
• On the opposite ABR (the one needing a logical connection to the backbone), the neighbor
router is the IP address of the ABR that is directly connected to the backbone.
NOTE: By default, the router ID is the lowest numbered IP address or (user-configured) loopback
interface configured on the device. See “Changing the router ID” (page 109).
When you establish an area virtual link, you must configure it on both of the ABRs (both ends of
the virtual link.)
202 Open Shortest Path First Protocol (OSPF)