Which of the following statements is true about BGP?

When an export routing policy applied to a BGP peer is modified, the new policy does not take effect immediately on the routes that have already been advertised. The BGP routing table and the advertisements sent to peers are not dynamically re-evaluated upon a policy change. To apply the new or updated policy, an administrator must manually trigger a re-advertisement of routes. This is typically done using a soft reset (route refresh) command, such as refresh bgp export on Huawei devices, which requests the router to resend its BGP UPDATE messages to the specified peer after re-applying the export policy, without tearing down the BGP session.

B. IGP routes can be injected into BGP using the network command, but also through redistribution using the import-route command. Therefore, the network command is not the only method.
C. Huawei VRP (Versatile Routing Platform) and other modern network operating systems support the configuration of multiple BGP processes, often used for features like BGP multi-instance or in VPN contexts.
D. A BGP OPEN message contains the standard BGP header, but it is followed by a message body containing crucial parameters for session establishment, such as BGP version, My Autonomous System number, Hold Time, and BGP Identifier.

1. For Answer A: Huawei HCIP-Datacom-Core Technology V1.0 Training Material. In the BGP route policy section, it is explained that after a policy is modified, a BGP session refresh is required for the new policy to take effect. The refresh bgp command is detailed as the mechanism for this manual intervention. (Specifically, this concept is a fundamental principle of BGP policy application covered in Huawei's official training for the H12-821 exam).

2. For Answer B: Huawei HedEx Documentation for VRP, BGP Configuration Guide. The section "Configuring BGP to Advertise Routes" describes both the network command and the import-route command as valid methods for advertising routes into BGP. This confirms that the network command is not the sole method.

3. For Answer D: IETF RFC 4271, "A Border Gateway Protocol 4 (BGP-4)". Section 4.2, "OPEN Message Format," explicitly defines the fields within an OPEN message, which include Version, My Autonomous System, Hold Time, BGP Identifier, and Optional Parameters Length, in addition to the standard 19-octet BGP header. This proves the message carries more than just the header.

4. For Answer C: Huawei HedEx Documentation for VRP, BGP Configuration Guide. The section on "BGP Multi-Instance" describes the procedure for creating and configuring multiple BGP processes on a single router, each with its own BGP instance ID. This directly contradicts the claim that a router cannot be configured with multiple BGP processes.