One, republish, redistribute, redistribute:

effect:

Between two routing protocols, or between different processes of the same protocol, an ASBR is built to work in both protocols or processes at the same time. reach

condition:

1. There must be an ASBR (Autonomous System Boundary Router (Protocol Boundary Router) working in two protocols or two processes of the same protocol at the same time, and can learn all routing entries at both ends. The default protocol is different, and the routing entries generated by different processes have Independent databases, without sharing or communication, ASBR can share the routes learned by protocol A with protocol B in the language of protocol B)

2. The seed metric must be considered: because different routing protocols calculate metric values in different ways, when republishing, the metric value of the original protocol is not carried, but when publishing to the new protocol, an initial metric (seed measure)

Second, the noun

Routing Feedback -------- If the default RIP and OSPF protocols perform double-point redistribution, since the two have different priorities, after the redistribution action of the first ASBR device is completed, it will affect the routing tables of other ASBR devices . This makes it possible for routes to be transmitted back to the source protocol, and routing feedback occurs --- after the route of protocol A is republished to protocol B, it is republished back to protocol A. Routing feedback may lead to poor route selection or even routing loops.

One-way redistribution - only redistribute the route of protocol A on ASBR to protocol B

Two-way redistribution - both sides of the protocol A and B on the ASBR share routes

Single point - there is only one ASBR between the two routing protocols

Dual point - there are only two ASBRs between the two routing protocols - backup, traffic sharing

Multi-point - there are more than two ASBRs between the two routing protocols

Three, the rules

1. When re-advertising the route of protocol A to protocol B, configure protocol B on the ASBR;

2. When the A protocol is published to the B protocol , all the routes on the ASBR learned through the A protocol and the routes directly connected to the ASBR working in the A protocol are all shared in the B protocol; --- Default routing needs to pay attention to specific rules

Fourth, the configuration command

one-way redistribution

3 configurations

A--->B publishes the route generated by one dynamic routing protocol to another dynamic routing protocol

Static--->B Redistribute the static route written by ASBR to the dynamic routing protocol

Direct connection ---> B redistributes the direct connection route on ASBR that has not been announced to protocol A to protocol A (protocol A is a dynamic routing protocol)

[1] Import other routes into the RIP protocol

A-->B

[r2]rip 1

[r2-rip-1]import-route ospf 1 The default starting metric can be understood as 0

[r2-rip-1]import-route ospf 1 cost 2 modify starting metric while republishing

Static routing --> B

[r2]rip 1

[r2-rip-1]import-route static

[r2-rip-1]import-route static cost 2

Note: The default static route will not be imported into the RIP protocol;

Direct connection -->B

[r2-rip-1]import-route direct

[r2-rip-1]import-route direct cost 4

Note: While republishing the direct connection, if the A protocol is also published to the B protocol, then the two republishing may share the same route, and the route of republishing the direct connection is preferred;

[2] Import other routes into the OSPF protocol

A->B

[r2]ospf 1

[r2-ospf-1]import-route rip

The default incoming route is type 2, the seed metric is 1; the redistributed incoming LSA is type 5/7, and the priority is 150;

Type 1: display total metric = seed metric + accumulation along the way

Type 2: Only display the seed metric, but compare the total metric = seed metric + accumulation along the route

However : type 1 is superior to type 2 in routing ;

[r2-ospf-1]import-route rip type 1 When republishing, modify the metric type to type 1 at the same time;

[r2-ospf-1] import-route rip cost 2 type 1 When republishing, both the seed metric and the metric type can be modified

Static --> B

[r2]ospf 1

[r2-ospf-1]import-route static

The default seed metric is 1, and the static default route will not be imported when the type is 2 ;

[r2-ospf-1] import-route static cost 2 type 1 When republishing, both the seed metric and the metric type can be modified

[r2-ospf-1]default-route-advertise imports the command of the default route , which defaults to type 2 and the seed metric to 1;

[r2-ospf-1] default-route-advertise cost 2 type 1 When republishing, both the seed metric and the metric type can be modified

Direct connection -->B

[r2-ospf-1]import-route direct default seed metric is 1, type is 2;

[r2-ospf-1] When import-route direct cost 2 type 1 is republished, both the seed metric and the metric type can be modified

Note: While republishing the direct connection, if the A protocol is also published to the B protocol, then the two republishing may share the same route, and the route of republishing the direct connection is preferred;

two-point two-way redistributing

When ASBR republishes routing entries, it will clear the metric carried by the original protocol and add the seed metric of the new protocol; once dual-point or multi-point redistribution is used, poor route selection may occur; -----routing strategy to solve
When two protocols with different priorities perform multipoint two-way redistribution, it is possible that after the first ASBR advertises protocol A to protocol B, these routes are learned by another ASBR. Adding a table causes the routing entries published by protocol A to protocol B to be republished by other ASBRs for protocol A, resulting in routing feedback and poor route selection;

Cisco company took the lead in adopting the method of different values of internal priority and re-distribution incoming route priority in EIGRP protocol to avoid it; this scheme is also adopted in Huawei equipment, OSPF normal internal priority is 10, re-distribution (5,7 Class routing) routing is 150; to avoid routing feedback;

But one point worth noting is that the distance vector protocol has an interface split horizon mechanism, which will still generate routing feedback due to sudden changes in the network topology --- need to use routing strategies to defend in advance

One-way two-way redistribution