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.
When you establish an area virtual link, you must configure it on both of the ABRs (both ends of the
virtual link.)
Adjusting virtual link performance by changing the
interface settings
Optional: The OSPF interface parameters for this process are automatically set to their default values for virtual
links. No change to the defaults is usually required unless needed for specific network conditions. These
parameters are a subset of the parameters described under
Adjusting performance by changing the VLAN or
on page 211. (The
cost
and
priority
settings are not configurable for a virtual link,
and the commands for reconfiguring the settings are accessed in the router OSPF context instead of the VLAN
context.)
NOTE:
The parameter settings for virtual links must be the same on the ABRs at both ends of a
given link.
Configuring OSPF authentication on a virtual link
OSPF supports the same two methods of authentication for virtual links as it does for VLANs and subnets in an
area—password and MD5. In the default configuration, OSPF authentication is disabled. Only one method of
authentication can be active on a virtual link at a time, and if one method is configured on a virtual link, configuring
the alternative method on the same link automatically replaces the first method with the second. Both ends of a
virtual link must use the same authentication method (none, password, or MD5 key chain) and related credentials.
(Any interfaces that share a VLAN or subnet with the interface used on an ABR for a virtual link, including
intermediate routing switches, must be configured with the same OSPF authentication.)
About OSPF passive
OSPF sends LSAs to all other routers in the same AS. To limit the flooding of LSAs throughout the AS, you can
configure OSPF to be passive. OSPF does not run in the AS, but it does advertise the interface as a stub link into
OSPF. Routing updates are accepted by a passive interface, but not sent out.
Chapter 11 Open Shortest Path First Protocol (OSPF)
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