Troubleshooting Tip: How a duplicated router id in BGP scenario can prevent installation of BGP routes in routing table

[vshahrokhkhani]

Description	This article describes how a duplicated router-id can prevent receiving routes from BGP neighbors.
Scope	FortiGate.
Solution	Consider a scenario with ADVPN and IBGP as dynamic routing protocol: Hub BGP configuration: config router bgp     set as 100     set router-id 10.253.0.191 <-----     set ibgp-multipath enable     set network-import-check disable     set scan-time 15     set graceful-restart enable     config neighbor-group         edit "remote_sites "             set capability-graceful-restart enable             set capability-default-originate enable             set next-hop-self enable             set soft-reconfiguration enable             set remote-as 100              set update-source "Hub1"             set route-reflector-client enable         next     end     config neighbor-range         edit 1             set prefix 10.253.0.0 255.255.255.0               set neighbor-group "remote_sites "         next     end Spoke-1 BGP configuration: config router bgp     set as 100     set ibgp-multipath enable     set graceful-restart enable         config neighbor             edit "10.253.0.191"                 set capability-graceful-restart enable                 set link-down-failover enable                 set soft-reconfiguration enable                 set remote-as 100             next         end         config network             edit 1                 set prefix 14.14.14.0 255.255.255.0             next         end Spoke-2 BGP configuration: config router bgp     set as 100     set ibgp-multipath enable     set network-import-check disable     set graceful-restart enable         config neighbor             edit "10.253.0.191"                 set capability-graceful-restart enable                 set link-down-failover enable                 set soft-reconfiguration enable                 set remote-as 100             next         end         config network             edit 1                 set prefix 192.168.200.140 255.255.255.255             next             edit 2                 set prefix 172.16.5.0 255.255.255.0             next         end In the example above, the router ID has not been configured for spokes. When there is no router-id configured in BGP, it will be selected automatically, based on highest IP address among its active interfaces (loopback interface addresses are preferred over physical addresses). This means, if by mistake, an interface VLAN, loopback interface or even physical interface is added to the spokes with the same IP address, it can lead to change of the router-id. In this scenario it can be observed that two spokes are not learning each others advertised routes, however the route is being learned on the hub side. If the following debug commands are used on spokes: diagnose ip router bgp all enable diagnose ip router bgp level info diagnose debug enable And if the BGP neighborship is cleared, below debug messages will be shown: BGP: 10.253.0.191-Outgoing [FSM] State: Established Event: 27 BGP: 10.253.0.191-Outgoing [RIB] Update: Prefix 172.16.5.0/24 path_id 0 denied due to originator is us BGP: 10.253.0.191-Outgoing [RIB] Update: Prefix 192.168.200.140/32 path_id 0 denied due to originator is us BGP: 10.253.0.191-Outgoing [RIB] Update: Prefix 192.168.210.0/24 path_id 0 denied due to originator is us This is an indication that the router id is the same on the spokes. This can be verified by running the following command: Spoke 1: get router info bgp summary VRF 0 BGP router identifier 192.168.210.210, local AS number 100 BGP table version is 4 1 BGP AS-PATH entries 0 BGP community entries Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 10.253.0.191 4 100 78317 78321 3 0 0 00:20:48 7 Total number of neighbors 1 Spoke 2: get router info bgp summary VRF 9 BGP router identifier 192.168.210.210, local AS number 100 BGP table version is 2 1 BGP AS-PATH entries 0 BGP community entries Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 10.253.0.191 4 100 95622 95613 1 0 0 00:36:21 7 Total number of neighbors 1 To avoid observing unexpected behaviors such as what discussed in this article, make sure to assign the router-id manually.