OSPF Continued
Areas
An OSPF Area is a logical grouping of routers membership is set at the interface level and the ID is a part of the hello packet. An i nterface can only belong to one area. There can be issues as an OSPF area grows full SPF calculations occur with link flaps, the LSDB can increase in size and become unmanageable, more memory and CPU is consumed, no route summarization occurs.
Area Border Routers(ABR) advertise a router from one area to the next and all ABR ‘s have to participate in area 0. When looking at th e routing table O routes are intra-area routes and O IA are inter-area routes.
Link-State Announcements
There are 6 LSA types
• Type 1, Router LSA; indicates the RID for the advertising router, age ,sequence, link count, and link ID
• Type 2, Network LSA; represents a multi a multi-access segment used by a DR
• Type 3, Summary LSA; represents networks from other areas
• Type 4, ASBR Summary LSA;
• Type 5, AS External LSA
• Type 7, NSSA External LSA
LSA Sequences is used to overcome problems with delay in propagation of LSA’s. LSA Age and flooding are used to help ensure that LSDB entries stay current,
An ABR will only advertise one type 3 LSA for a link even if it is aware of multiple paths.
Path Selection
OSPF uses an algorithm to create a loop free network paths are prioritized by the following fashion intra-area, inter-area, external routes. Type 1 routes are always preferred over type 3 routes. If multiple intra-area routes exist the lowest metric is installed. If OSPF identifies multi paths those routes are installed as equal-cost multipathing(default max number of entries is 4)
Summarization of route will reduce the number of LSA’s. all routers share a copy of the LSDB but all routers must accommodate the smallest / slowest router in the environment
Route filtering with OSPF would have to be performed on the ABR router as every router with in the area contains a complete copy of the LSDB. The easiest way to do this is with the not-advertise keyword
Area 1 range 192.168.0.0 255.255.255.0 not-advertise
area filtering is done on a area border router using the following syntax
Ip prefix-list filter seq 5 deny 172.16.1.0/24
!
Router ospf 1
Area 0 filter-list prefix filter in
Local OSPF filtering is possible with the following commands
ip access-list standard ACL-OSPF-FILTER
deny 172.16.1.0
permit any
!
Router ospf 1
Distribute-list ACL-OSPF-FILTER in