1. Route redistribution
Route redistribution is typically responsible for learning routes from one AS and then configuring them on routers broadcast from another AS.
When redistributing routes, consider the capabilities and characteristics of each routing protocol. The two factors considered are metric value and administrative distance.
(1).Measurement value
Metrics represent distances, and they are used to determine the optimal route when searching for a route.
The metric of OSPF routing protocol is cost, and RIP is hop count.
(2).Management distance
Administrative distance refers to the routing credibility of a routing protocol.
Default administrative distance used by cisco IOS
Route type administrative distance
Direct port 0
static route 1
OSPF 110
RIP 120
Unknown 255
(3) Path type of OSPF redistribution routes
Type 1 external path, type 2 external path
2. NSSA: non-pure terminal area
3. Summary of OSPF protocol
（1）.LSA
LSA type description
1 Router LSA
2 Network LSA
3 Network summary LSA
4 ASBR summary LSA
5 AS external LSA
7 NSSA external LSA
(2).OSPF area type
Area type 1, 2 3 4, 5 7
Backbone area (Area 0) Allowed Allowed Allowed Not allowed
Non-backbone area, non-pure peripheral area Allowed Allowed Allowed Not allowed
Stub area Allowed Allowed Not allowed Not allowed
Completely stub area Allowed Not allowed* Not allowed Not allowed
NSSA Allowed Allowed Not Allowed Allowed
Note: In a completely stub area, the ABR router can use a type 3 LSA to advertise the default route.
(3).Priority of OSPF routing management
Inter-area path, priority 1
Inter-area path, priority 2
E1 external path, priority 3
E2 external path, priority 4
Four: Overview of virtual links
A virtual link refers to a link that connects to a backbone area through a non-backbone area.
Virtual links are mainly used in the following two situations:
(1).Connect an area to the backbone area through a non-backbone area
(2). Connect some areas on both sides of a segmented backbone area through a non-backbone area
Related rules and characteristics when configuring virtual links
(1). The virtual link must be configured between two ABR routers
(2). The area through which the virtual link passes must have all routing information. Such an area is also called a transport area.
(3). The transmission area cannot be a stub area (including NSSA)
(4). The stability of the virtual link depends on the stability of the area it passes through.
(5).Virtual links help provide logical redundancy