CCIE Network Engineer The generation and solution of routing loop
Someone said that in today’s network communication traffic, about 80% of the resources are wasted, and less than one-fifth of the resources are effectively used. One of the culprits of this resource waste is the generation of network loops, which not only wastes CPU and memory resources of the device, but also the bandwidth resources of the link.
The anti-circulation mechanism is the purification mechanism produced in such a bad environment. Generally, our network loops are divided into layer 2 loops and layer 3 loops. The formation of all loops is caused by unclear destination paths leading to confusion.
The second-layer loop is what we call the switch loop. It is generated by the vicious circle of messages broadcast by the switch. Layer 3 loops are loops generated on routers.
The distance vector routing protocol simply simply advertises its own routing table periodically, and also loads the received valid routes into the routing table, and reflects the distance to the target network through the accumulated metric value, so the distance vector is run Routers of routing protocols do not understand the topology of the entire network. These characteristics make routing loops very easy in the network.
When the X network on the side of the R1 router fails, the R1 router receives the failure information and sets the X network unreachable, waiting for the update cycle to notify the neighboring R2 router. However, if the update cycle of the adjacent R2 router comes first, the R1 router learns the route to the X network from the R2 router, which is the wrong route, because the X network has been damaged at this time, but the R1 router is on its own. A route to the X network via the R2 router is added to the routing table. Then the R1 router will continue to advertise the erroneous route to the R2 router. The R2 router updates the routing table and thinks that it must go through the R1 route to the X network, and then continues to notify the neighboring routers, so that a routing loop is formed.
In summary, the problem of routing loops is very harmful to the network, so from the perspective of network design and protocol design, the hidden dangers and possibilities of loops should be fully considered and avoided.
The method to solve the three-layer loop:
1. Define the maximum hop count:
In order to prevent RIP routes from being forwarded endlessly in the network, the maximum number of hops of the route is defined as 15 hops. That is, when the metric value of a route reaches 16 hops, the route is considered unavailable and the route points to Is considered unreachable.
2. Horizontal split:
One way to eliminate routing loops and accelerate network convergence is through a technique called “horizontal splitting.” The rule is not to send routing update information again in the direction of the original routing update.
3. Route poisoning:
When a network becomes unreachable, the router that discovers the change immediately triggers a 16-hop route update to notify the router in the network that the target network is unreachable. This route is called a toxic route.
4. Reversal of toxicity:
After learning a route from an interface, it will carry these routes when it sends a Response packet from that interface, but these route metrics are set to 16 hops. You can clear useless routes in the other party’s routing table.
5. Trigger update:
Under normal circumstances, the router will periodically send routing tables to neighbor routers. The trigger update is to send routing update information immediately in response to certain changes. A router that detects a network failure immediately sends an update message to neighbor routers, and in turn generates neighbor routers that trigger updates to notify them, so that the routers on the entire network receive the update message in the shortest time, so as to quickly understand the changes of the entire network .