1. Reasons why OPSF uses external routing.
In addition to internal communication, companies also need to communicate with external networks. There is also a need for mutual visits between different companies.
Join Now company A needs to visit company B, then how does company A get company B? Routing information
2. The calculation process of
external routes 2.1. The introduction of external routes
Scenario requirements:
Company A is running OSPF, and Company A needs to visit the server of Company B.
Solution:
1. Write a static route on the RA to the server of Company B.
2. Publish the static route in the OPSF network. All OSPF routers can learn this route (this route is an external route)
2.2. Type4 LSA and Type5 LSA
ASBR-Summary-LSA (Type4 LSA)
Type4 LSA is used to describe the location of ASBR
Type: Sum-Asbr //Type of LSA
Ls id: 1.1.1.1 // Router ID of ASBR
Adv rtr: 2.2.2.2 //Announce the Router ID
Metric of the Type4 LSA router : 1 //Generate the cost from the Type4 LSA router to the ASBR
AS-External-LSA (Type5 LSA)
Type5 LSA is used to describe external routing information.
Type5 LSA will not change when the entire OPSF network is flooded, so Type4 LSA is needed to help route calculations.
Type: External //LSA type
Ls id: 10.1.60.0 //Destination network segment address
Adv rtr: 1.1.1.1 //Declare Router ID of Type5 LSA router
Metric: 1 //ASBR to the destination network segment cost value
E Type ：2 //Type of external routing
Forwording Address: //When external routing is sub-optimal,
Tag: 1 //tag
2.3. Calculation of external routes
2.3.1. Calculate external routes in the same area as ASBR
1. RA will generate Type5 LSA and give it to other routers
(Type4 LSA is not needed when it is in the same area as ASBR)
2. The route to ASBR can be calculated through Type1/2 LSA, and then the external route can be calculated through Type5 LSA
2.3.2. Calculate external routes not in the same area as ASBR
1. At this time, RB will generate Type4 LSA and forward Type5 LSA. Pass the information to RD
2. After RD receives it, first calculate the route to RB through Type1/2 LSA, then calculate the route to ASBR through Type4 LSA, and finally calculate the external route through Type5 LSA
3. Types of external routing
3.1. Generation of regional routing loops
默认情况下，外部路由的类型为Type-2
但是同一条外部路由Type1优于Type2
LSA的优选：Type1 LSA > Type3 LSA > Type5 LSA（Type1） > Type5 LSA（Type2）
4. Generation of sub-optimal external routes
4.1. Generation of sub-optimal external routes
4.2. The solution to the sub-optimal path
Forwording Address will change the next hop of this Type5 LSA to avoid suboptimal paths. Then RB will forward the data directly to
the conditions that RC Forwording Address generates:
1. Broadcast network
2. The interface of the network segment cannot be a silent interface
3. The network segment of the interface starts OSPF