Brocade Virtual ADX Switch and Router Guide (Supporting ADX v03.1.00) - Manual (Page 120)

106 Brocade Virtual ADX Switch and Router Guide
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Overview of OSPF
6
Algorithm for AS External LSA reduction
Figure 22 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 router with the higher router ID floods the AS External LSAs and the
other router 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.