BGP is the only protocol designed to handle networks the size of the Internet, and the only one capable of properly managing multi-connection links between unrelated routing domains. A router’s BGP routing table maintains all BGP routes. Network reachability information includes details of the listed Autonomous Systems, which effectively constructs a topology map of AS interconnections, thereby eliminating routing loops and enabling policy decisions at the AS level.
1. BGP employs several mechanisms to prevent the aforementioned issues. One is automatic route filtering on routers. Since routers may receive unwanted routing information, we want to filter incoming IBGP routing updates at the entry point. The automatic route filtering feature meets this filtering need. When a router receives an IBGP route containing an RT value that it does not need, it discards it to reduce the amount of information stored in memory. This feature is enabled by default on all routers and requires no additional configuration to activate.
2. If this router also serves as a Route Reflector for other routers, or if it acts as an MP-EBGP peer needing to exchange inter-AS VPN information, we need to disable this feature using the command: no bgp default route-filter. BGP route refresh: Due to the automatic route filtering function enabled on routers, when a router’s configuration changes鈥攕uch as adding a new VRF or deleting an existing one鈥攖he router’s demand for VPN routes might not be satisfied in a timely manner. Therefore, BGP provides a route refresh mechanism to achieve this goal.
3. After using this feature, a short time following a configuration change, the router will request its IBGP neighbors to resend routing updates to obtain the missing VPN-IPv4 information. The router sends a route refresh request to all PE neighbors. The combination of route refresh and automatic route filtering mechanisms can reduce the amount of routing information a router needs to maintain. However, useless routing information will still propagate through the network, only being discarded once it reaches a specific router.
4. To avoid wasting network resources, BGP provides the ORF feature to advertise its inbound route filters to neighbors, so that unnecessary routing information can be filtered out before it reaches the receiving router. The ORF field is exchanged during session establishment between two routers via OPEN messages. By separating IBGP sessions among routers, each router only needs to receive routes for the