How do various routing protocols protected against loops

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How do various routing protocols protected against loops

1. How does RIP prevent loops

a. Define the maximum metric to prevent counting to infinity [max 16]

b. Horizontal segmentation

c. Route poisoning

d. Toxicity reversal

e. Suppression timer

f. Trigger the update

2.How OSPF prevents loops

a. The reason why the area can be acyclic is because the router can describe the complete topology of the area (including the cost of all interfaces) and the network segment information through the Class 1 and Type 2 LSAs.

b. OSPF requires that all non-zero areas must be directly connected to the backbone area. Inter-area routes need to be transited through the backbone area. This avoids the occurrence of inter-area routing loops.

3. How IS-IS prevents loops

The routing information between Level 1 and Level 2 is not interworking.

4. How BGP prevents loops

a. Within the AS: With IBGP split horizon, BGP routers do not advertise routes learned from IBGP peers to other IBGP peers.

b. Between ASs: Route update with the AS number is not accepted through the AS-PATH path attribute.

c. In special cases, split horizon causes some routers to fail to learn some routing information. To solve this problem, BGP has two mechanisms: Route reflector and federation. The anti-loop mechanism of the router reflector is configured with two attributes: ORIGINATOR-ID and CLUSTER-LIST.

ORIGINATOR-ID: When a route is reflected by the RR for the first time, the RR adds the Originator-ID attribute to the route to identify the router that initiates the route. If the Originator-ID attribute exists in a route, the RR does not create a new Originator-ID. When another BGP Speaker receives the route, it compares the received Originator-ID with the local Router ID. If the two IDs are the same, the BGP Speaker ignores the route and does not process the route.

CLUSTER-LIST: In an AS, each RR uses a unique CLUSTER-ID as the identifier. To prevent routing loops, the RR uses CLUSTER_LIST to record all CLUSTER_IDs that the reflected route passes through. When the RR receives an updated route, the RR checks the Cluster-List. If the local Cluster-ID exists in the Cluster-List, the route is discarded. If there is no local Cluster-ID, add the local Cluster-ID to the Cluster-List and reflect the updated route.





This article is reproduced from Xu Ziwen.

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