Technical Tip: Importance of advertising routes in BGP with the same prefix as in routing table.

[ahabibov]

Description	The article describes the advertisement of BGP routes with the 'network' command and the importance of making the route prefix the same as in the routing table.
Scope	All FortiGate or VDOM running in NAT mode.
Solution	The following is a simple BGP configuration with no additional settings and neighbor information: config router bgp     config neighbor         edit "10.109.49.20"             set remote-as 65001         next     end end For example, the following directly connected networks are in the routing table: get router info routing-table all Codes: K - kernel, C - connected, S - static, R - RIP, B - BGP O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area V - BGP VPNv4 * - candidate default Routing table for VRF=0 S* 0.0.0.0/0 [10/0] via 10.109.63.254, mgmt, [1/0] C 10.109.16.0/20 is directly connected, port1 C 10.109.48.0/20 is directly connected, mgmt When advertising the 10.109.16.0/20 network using a different subnet (/16 in this example), the following will be seen: config router bgp     config network         edit 1             set prefix 10.109.0.0 255.255.0.0         next     end end get router info bgp neighbors 10.109.49.20 advertised-routes % No prefix for neighbor 10.109.49.20 get router info bgp network In this case, there are two options: While using the 'network' command, the prefix should be exactly the same as the one in the routing table. It will be necessary to make the following changes for the network to be advertised:          config router bgp config network edit 1 set prefix 10.109.16.0 255.255.240.0 next end end get router info bgp neighbors 10.109.49.20 advertised-routes VRF 0 BGP table version is 1, local router ID is 10.109.49.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight RouteTag Path *>i10.109.16.0/20 10.109.49.1 100 32768 0 i <-/-> Total number of prefixes 1 get router info bgp network VRF 0 BGP table version is 1, local router ID is 10.109.49.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight RouteTag Path *> 10.109.16.0/20 0.0.0.0 100 32768 0 i <-/1> Total number of prefixes 1 If the plan is for the subnet to be advertised with /16 prefix, it will be necessary to use the 'aggregate-address' option: config router bgp config aggregate-address edit 1 set prefix 10.109.0.0 255.255.0.0 set summary-only enable next end end get router info bgp neighbors 10.109.49.20 advertised-routes VRF 0 BGP table version is 2, local router ID is 10.109.49.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight RouteTag Path *>i10.109.0.0/16 10.109.49.1 100 32768 0 i <-/-> Total number of prefixes 1 get router info bgp network VRF 0 BGP table version is 2, local router ID is 10.109.49.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight RouteTag Path *> 10.109.0.0/16 0.0.0.0 32768 0 i <-/1> s> 10.109.16.0/20 0.0.0.0 100 32768 0 i <-/1> Here, the ‘s’ in front of the /20 network indicates this route is suppressed by BGP. Related article: Technical Tip: How to implement BGP route summary (aggregation) on a FortiGate