Enterasys Networks Security Router X-PeditionTM - Manual (Page 117)

IP Routing Protocols
XSR User’s Guide 5-15
• Incremental SPF is always enabled. SPF calculation can be changed with timers spf
• Hello wait intervals with ip ospf dead-interval and ip ospf hello-interval as well as
the poll timer to set up adjacencies as quickly as possible with ip ospf poll-timer
• Retransmission and link-state update intervals with ip ospf retransmit-interval and ip
ospf transmit-delay
• A host of statistical display commands including: show ip ospf border routers , show ip
ospf database , show ip ospf interface , show ip ospf neighbor , show ip ospf virtual
links , show ip protocols , and show ip route
LSA Type 3 and 5 Summarization
The XSR supports LSA Type 3 and 5 summarization using the area range advertise and
summary-address commands, respectively. Type 3 LSAs (intra routes) are summarized by an
Area Border Router then injected into other areas and furnished a unique Link-State ID (Appendix
E Processing) as well as the highest metric of the included intra-area routes. Further, the XSR
installs a discard route for any active summary range of routes, defined as including at least one
intra route being leaked into the area. Conversely, specifying the
not-advertise value causes
undesired aggregated Type 3 LSAs to be discarded, and when a summary range becomes inactive,
the discard route is dropped.
Type 5 LSA summarization groups locally sourced routes which have been redistributed from
other protocols. The XSR’s Type 5 summarization is similar to Type 3 aggregation in terms of
discard routes, Appendix E processing, overlapping, and
not-advertise behavior. Additionally:
• The XSR will not summarize Type 7 to Type 5 translations.
• The XSR produce a Type 5 LSA for active summary ranges. If a NSSA area exists, a Type 7
LSA will be produced for each NSSA area.
• The XSR should not be subjected to needless re-origination of Type 5 LSAs. For example,
importing locally sourced routes which do not alter a summary’s type/cost will not re-
originate the summary LSA.
• Type 5 LSAs generated by translation may supplant a Type 5 LSA originating from a local
source. This will not affect what is being generated into a NSSA because translations are not
advertised there.
• If for a given prefix, both a summary and a locally sourced route exist, the summary will be
considered the better route even if the summary includes only that locally sourced route.
OSPF Database Overflow
A router sometimes cannot maintain the Link-State database in its entirety, typically, because the
database has overflowed due to importing many external Type 5 LSA routes into OSPF. You can
avert this issue by properly configuring OSPF routers into stub areas or NSSAs since AS-external
LSAs are omitted from this type of Link-State database. But, with an unexpected database
overflow, there is not enough time to perform this type of isolation.
The XSR’s database-overflow command controls this problem by limiting the number of Type 5
LSAs it imports and others as well: Types 1 through 4, 7, and 10. The command also can set a
warning of a pending overflow and set an interval in which to exit overflow.
Note: Summary ranges may overlap. The most specific range activates for a locally sourced route,