BGP Peer Tracking
BGP peer tracking provides quick detection of link or peer faults for BGP to speed up network convergence. If BGP peer tracking is enabled on a local device and a fault occurs on the link between the device and its peer, BGP peer tracking can quickly detect that routes to the peer are unreachable and notify BGP of the fault, thereby achieving rapid convergence.
Compared with BFD, BGP peer tracking is easy to configure because it needs to be configured only on the local device rather than on the entire network. Although both are fault detection mechanisms, BGP peer tracking is implemented at the network layer, whereas BFD is implemented at the link layer. For this reason, BGP peer tracking provides a slower convergence performance than BFD, making it inapplicable to services that require a rapid convergence, such as voice services.
Networking
BGP peer tracking can quickly detect link or peer faults by checking whether routes to peers exist in the IP routing table. If no route is found in the IP routing table based on the IP address of a BGP peer (or a route exists but is unreachable, for example, the outbound interface is a Null0 interface), the BGP session goes down, achieving fast BGP route convergence. If a reachable route can be found in this case, the BGP session does not go down.
On the network shown in Figure 1, IGP connections are established between DeviceA, DeviceB, and DeviceC, a BGP peer relationship is established between DeviceA and DeviceC, and BGP peer tracking is configured on DeviceA. If the link between DeviceA and DeviceB fails, the IGP performs fast convergence first. As no route is found on DeviceA based on the IP address of DeviceC, BGP peer tracking detects that no reachable route to DeviceC is available and then notifies BGP on DeviceA of the fault. As a result, DeviceA terminates the BGP connection with DeviceC.
- If a default route exists on DeviceA and the link between DeviceA and DeviceB fails, BGP peer tracking will not terminate the peer relationship between DeviceA and DeviceC. This is because DeviceA can find the default route in the IP routing table based on the peer's IP address.
- If DeviceA and DeviceC establish an IBGP peer relationship, you are advised to enable BGP peer tracking on both devices to ensure that the peer relationship can be terminated soon after a fault occurs.
- If establishing a BGP peer relationship depends on IGP routes, you need to configure how long BGP peer tracking waits after detecting peer unreachability before it terminates the BGP connection. The configured length of time should be longer than the IGP route convergence time. Otherwise, before IGP route flapping caused by intermittent disconnection is suppressed, the BGP peer relationship may have been terminated. This results in unnecessary BGP convergence.