System information

Configuring OSPF
11-11
tag-value is the 32-bit tag value for filtering externally derived routing
information.
Example
In the following example, summary address 20.1.0.0 includes address 20.1.1.0,
20.1.2.0, 20.1.3.0, and so forth. Only the address 20.1.0.0 is advertised in an external
LSA.
summary-address 20.1.0.0 255.255.0.0
Managing OSPF on the BSR
This section discusses the following optional OSPF tasks:
• Establishing a Virtual Link
• Assign a Default Route for an ASBR
• Controlling OSPF Link Cost Metrics
• Allowing Dynamic OSPF Virtual Links
• Changing OSPF Administrative Distances
• Configuring Route Calculation Timers
• Blocking OSPF LSA Flooding
Establishing a Virtual Link
In OSPF, all areas must be connected to a backbone area. If there is a break in
backbone continuity or the backbone is purposefully partitioned, you can establish a
virtual link. The two endpoints of a virtual link are Area Border Routers (ABRs). The
virtual link must be configured in both routers. The configuration information in each
router consists of the other virtual endpoint (the other ABR), and the non-backbone
area that the two routers have in common (the transit area).
Note: Virtual links cannot be configured through stub areas.