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CCNA: Fast Pass phần 2

tuy nhiên, bởi vì hầu hết các nhà cung cấp dịch vụ Internet sẽ không chấp nhận các tuyến đường subnet từ khách hàng của họ. Nếu ISP1 chấp nhận các tuyến đường, nó phải giữ cho các tuyến đường subnet trong bảng của nó và chia sẻ thông tin này với ISP2 để ISP2 cũng thực hiện định tuyến tối ưu. | 1.3 Selecting an Appropriate Routing Protocol Based on User Requirements 23 DUAL provides EIGRP with possibly the fastest route convergence time among all protocols. The key to EIGRP s speedy convergence is twofold first EIGRP routers maintain a copy of all of their neighbors routes which they use to calculate their own cost to each remote network. If the best path goes down it may be as simple as examining the contents of the topology table to select the best replacement route. Secondly if there isn t a good alternative in the local topology table EIGRP routers very quickly ask their neighbors for help finding one they aren t afraid to ask directions Relying on other routers and leveraging the information they provide accounts for the diffusing character of DUAL. As I said the whole idea of the Hello messages is to enable the rapid detection of new or dead neighbors. RTP answers this call by providing a reliable mechanism for conveying and sequencing messages. Building upon this solid foundation DUAL is responsible for selecting and maintaining information about the best paths. Multiple AS EIGRP uses autonomous system numbers ASNs to identify the collection of routers that share route information. Only routers that have the same ASN share routes. In large networks you can easily end up with really complicated topology and route tables and that can markedly slow convergence during diffusing computation operations. So what s an administrator to do to mitigate the impact of managing really big networks Well it s possible to divide the network into multiple distinct EIGRP AS. Each AS is populated by a contiguous series of routers and route information can be shared among the different AS via redistribution. The use of redistribution within EIGRP leads us to another interesting feature. Normally the administrative distance of EIGRP routes is 90 but this is true only for what are known as internal EIGRP routes. These are routes originated within a specific autonomous .