BGP originator ID & cluster list Attributes

Hi,

Loop without originator ID

1. Router R1 and router R2 are configured as RRs. Below is a common RR client, Router R3. 

2. R3 announces a route N to router R1 and R2. R1 and R2 will also advertise the route each other. 

3. On R1 there are two sources for route N (R2 & R3). R3 is preferred. 

4. When the N network announced by R3 goes down, R1 will choose the route announced by R2 and R1 will advertise route N to R3. 

5. R3 will advertise the N route learned from router R1 to R2. 

At this time, R1 reaches network N through R2, R2 reaches network N through R3 and R3 reaches network N through R1. Finally the network has a loop.

Loop without Cluster List

1. Router R1 is the RR of one cluster, and R2 and R3 are the RR of another cluster. 

2. R1 announces route N advertised by a client in the cluster to router R2 and R3. R2 and R3 will also advertise the route each other. 

3. On R2 there are two sources for route N (R1 & R3). R1 is preferred. 

4. When the N network announced by R1 goes down, R2 will choose the route announced by R3 and R2 will advertise route N to R1. 

5. R1 will advertise the N route learned from router R2 to R3. 

At this time, R2 reaches network N through R3, R3 reaches network N through R1 and R1 reaches network N through R2. Finally the network has a loop.

Both problems are solved by originator ID and Cluster List. 

https://forum.huawei.com/enterprise/en/bgp-fundamentals-22-bgp-rr-part-2/thread/561995-863

Hi,

The following two typical scenarios help you understand the RR loop prevention mechanism. 

1. Cluster_list

When a RR reflects a route, it writes its own cluster_ID (router-id by default) into the cluster_list. After receiving a route, the RR rejects the route if it finds that the cluster_IDs in the cluster_list include its own cluster_ID. As shown in the following figure, R1 rejects the received routes to prevent loops. 

2. Originator id

If the router finds that the originator_id is the same as it's router_id of the router, the router does not receive the route. Therefore, in the following figure, R4 rejects the route sent by itself and reflected by the RR.

Any further questions, let us know.