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ThinkMo EDU Share – network 68.Routing Control Analysis

蒂娜 1 Comment 01/10/2023

ThinkMo EDU Share – network 68.Routing Control Analysis

Today I will take you to understand routing control. If unnecessary routes are sent to the wrong protocol, it may cause routing loops or suboptimal paths. Therefore, filtering methods can be used to filter out the corresponding routes. For filtering routes, there are the following special features: Points to note:

Route filtering can be implemented through Distribute-List. The protocols and directions that Distribute-List can apply to are:

RIP (distance vector routing protocol)
can be applied to in and out directions.

EIGRP (Distance Vector Routing Protocol)
can be used in both in and out directions.

OSPF (Link State Routing Protocol)
is only applicable to the in direction in the OSPF protocol process, and only takes effect on its own routing table, and cannot affect the neighbor’s routing table; it can be used for out in other protocol processes except OSPF The direction is to filter when OSPF is redistributed into other routing protocols. This filtering is called process filtering. It is not only applicable to redistribution of OSPF into other protocols, but also applicable to redistribution of other protocols into OSPF or redistribution between other protocols. distributed.


It can be used alone to control and filter routes during route redistribution .

In addition to filtering out routes when necessary, when it is necessary to adjust the priority of route optimization between different protocols or different AD values, it can be achieved by modifying the default AD value of the routing protocol. Modifying the AD value can be based on the entire protocol The modification will take effect for all routing entries in the protocol, and can also be modified for specific routes; if you only need to modify the AD value for a specific route, you need to call ACL or Prefix-Lix to match the specific route. In addition, you must define The next hop address of the route to the destination. Both RIP and EIGRP need to define the next hop address of the route when adjusting the AD value for a specific route. For OSPF, the LSA does not clearly indicate the next hop address to the destination. Hop address, because this address needs to be known by OSPF through LSA calculation, so the next hop address of the route defined when adjusting the AD value for a specific route under OSPF is the Router-ID that generated the LSA.

Router R1 in the RIP area redistributes the external route into RIP, and specifies a metric value of 5. Within the RIP range, the AD value for reaching is 120. When R2 redistributes the learned from RIP After being distributed into the OSPF area, when the route is passed to R4, because the AD value of OSPF is 110, which is smaller than RIP, it will eventually cause R4 to reach and circle around the farther OSPF path; if R4 continues to redistribute the route Enter RIP and set a Metric value lower than 5, which is learned by R3. As a result, R3 will pass through R4, pass through the OSPF area, and then go back to RIP to reach the target This is due to different AD values of different protocols. The resulting suboptimal path, or even a routing loop, so when configuring redistribution between protocols, the AD value needs to be considered and analyzed more.

In addition to the above filtering methods during redistribution, there is also Tag filtering technology, which is to tag the corresponding route, and reject the route by matching the Tag when redistributing it back to the original routing protocol. It should be noted that Tag filtering does not Support IGRP and RIP ver1.

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