Advanced Configuration and Management Guide

Algorithm for AS External LSA Reduction

Figure 8.4 shows an example in which the normal AS External LSA reduction feature is in effect. The behavior changes under the following conditions:

•There is one ASBR advertising (originating) a route to the external destination, but one of the following happens:

•A second ASBR comes on-line

•A second ASBR that is already on-line begins advertising an equivalent route to the same destination.

In either case above, the routing switch with the higher router ID floods the AS External LSAs and the other routing switch flushes its equivalent AS External LSAs. For example, if Router D is offline, Router E is the only source for a route to the external routing domain. When Router D comes on-line, it takes over flooding of the AS External LSAs to Router F, while Router E flushes its equivalent AS External LSAs to Router F.

•One of the ASBRs starts advertising a route that is no longer equivalent to the route the other ASBR is advertising. In this case, the ASBRs each flood AS External LSAs. Since the LSAs either no longer have the same cost or no longer have the same next-hop router, the LSAs are no longer equivalent, and the LSA reduction feature no longer applies.

•The ASBR with the higher router ID becomes unavailable or is reconfigured so that it is no longer an ASBR. In this case, the other ASBR floods the AS External LSAs. For example, if Router D goes off-line, then Router E starts flooding the AS with AS External LSAs for the route to Router F.

Dynamic OSPF Activation and Configuration

OSPF is automatically activated when you enable it. The protocol does not require a software reload.

You can configure and save the following OSPF changes without resetting the system:

•all OSPF interface-related parameters (for example: area, hello timer, router dead time cost, priority, re­ transmission time, transit delay)

•all area parameters

•all area range parameters

•all virtual-link parameters

•all global parameters

•creation and deletion of an area, interface or virtual link

In addition, you can make the following changes without a system reset by first disabling and then re-enabling OSPF operation:

•changes to address ranges

•changes to global values for redistribution

•addition of new virtual links

You also can change the amount of memory allocated to various types of LSA entries. However, these changes require a system reset or reboot.

Dynamic OSPF Memory

Software release 07.1.X and later dynamically allocate memory for Link State Advertisements (LSAs) and other OSPF data structures.

In previous software releases, OSPF memory is statically allocated. If the routing switch runs out of memory for a given LSA type in releases earlier than 07.1.X, an overflow condition occurs and the software sends a message to the Syslog. To change memory allocation requires entering CLI commands and reloading the software.

Software release 07.1.X and later eliminate the overflow conditions and do not require a reload to change OSPF memory allocation. So long as the routing switch has free (unallocated) dynamic memory, OSPF can use the memory.

8 - 6