Usage Scenario
BGP-LS is a new method of collecting network topology information.
Without BGP-LS, devices use an IGP (IS-IS or OSPF) to collect topology information of each area, and the IGP reports the information to the controller. This topology information collection method has the following disadvantages:
- The upper-layer controller must have high computing capabilities and support IGP and its algorithms.
- If cross-IGP-domain topology information needs to be collected, the upper-layer controller cannot obtain the complete topology information and therefore cannot calculate the optimal E2E path.
- Different routing protocols send topology information to the upper-layer controller, and the controller's analysis and processing of the topology information is complex.
After BGP-LS is introduced, BGP summarizes and reports the topology information discovered by IGPs to an upper-layer controller. With powerful routing capabilities of BGP, BGP-LS has the following advantages:
- Lowers the requirements on the computing capability of the upper-layer controller and eliminates the requirements on the IGP capability of the controller.
- BGP summarizes the topology information of each process or AS and sends the complete topology information to the controller, facilitating path selection and calculation.
- All topology information on the network is sent to the controller through BGP.
Devices in the same IS-IS area have the same topology information. Therefore, you are advised to deploy BGP-LS only on two devices, one of which functions as the backup device. If BGP-LS needs to be deployed on more than two devices in the same IS-IS area, these devices must be configured with the same identifier, ensures the accuracy of topology information collected by BGP.
Implementation Procedure
IS-IS topology information can be flooded only within an area. As a result, IS-IS cannot report multi-area topology information to the controller. To address this problem, deploy at least one BGP node in each IS-IS area and establish a BGP-LS peer relationship between each BGP node and the controller so that BGP-LS collects and reports topology information to the controller.
Configuration Impact
- BGP-LS advertises all topology information (node, link, and prefix information), and the controller needs to save all topology information, consuming a large number of memory resources.
- Any change in the topology information collected by BGP-LS will trigger IS-IS LSP flooding, which may burden the controller. As a result, the maximum number of IS-IS neighbors supported and the topology size will be affected.
- BGP-LS requires topology synchronization between IS-IS and BGP, which further consumes CPU resources. As a result, IS-IS convergence is slowed down.