A route transit is defined to import traffic through a Layer 3 outside network L3extOut profile (l3extInstP), where it is to be imported. A different route transit is defined to export traffic through another l3extInstP where it is to be exported. 
                        
                        
                        Since multiple 
                           		l3extOut policies can be deployed on a single node or
                           		multiple nodes in the fabric, a variety of protocol combinations are supported.
                           		Every protocol combination can be deployed on a single node using multiple 
                           		l3extOut policies or multiple nodes using multiple 
                           		l3extOut policies. Deployments of more than two
                           		protocols in different 
                           		l3extOut policies in the fabric are supported. 
                           	 
                        
                        
                        Export route-maps are made up of prefix-list matches. Each prefix-list consists of bridge domain (BD) public subnet prefixes
                           in the VRF and the export prefixes that need to be advertised outside. 
                        
                        
                        Route control policies are defined in an l3extOut policy and controlled by properties and relations associated with the l3extOut. APIC uses the enforceRtctrl property of the l3extOut to enforce route control directions. The default is to enforce control on export and allow all on import. Imported and exported
                           routes (l3extSubnets), are defined in the l3extInstP. The default scope for every route is import. These are the routes and prefixes which form a prefix-based EPG. 
                        
                        
                        All the import routes form the import route map and are used by BGP and OSPF to control import. All the export routes form
                           the export route map used by OSPF and BGP to control export. 
                        
                        
                        Import and export route control policies are defined at different levels. All IPv4 policy levels are supported for IPv6. Extra
                           relations that are defined in the l3extInstP and l3extSubnet MOs control import. 
                        
                        
                        Default route leak is
                           		enabled by defining the 
                           		l3extDefaultRouteLeakP MO under the 
                           		l3extOut. 
                           	 
                        
                        
                        
                           l3extDefaultRouteLeakP can have Virtual Routing and Forwarding (VRF) scope or L3extOut scope per area for OSPF and per peer for BGP. 
                        
                        
                        The following set rules provide route control: 
                        
                        
                           
                           -  
                              		  
                              
                                 rtctrlSetPref 
                                 		  
                              
 
                              		
                            
                           
                           -  
                              		  
                              
                                 rtctrlSetRtMetric 
                                 		  
                              
 
                              		
                            
                           
                           -  
                              		  
                              
                                 rtctrlSetRtMetricType 
                                 		  
                              
 
                              		
                            
                           
                        
                        
                        Additional syntax for the rtctrlSetComm MO  includes the following: 
                        
                        
                        
                           
                           -  
                              		  
                              
                                 no-advertise 
                                    			  
                                 		  
                              
 
                              		
                            
                           
                           -  
                              		  
                              
                                 no-export 
                                 		  
                              
 
                              		
                            
                           
                           -  
                              		  
                              
                                 no-peer 
                                 		  
                              
 
                              		
                            
                           
                        
                        
                        
                           BGP 
                           	 
                        
                        
                        The ACI fabric
                           		supports BGP peering with external routers. BGP peers are associated with an 
                           		l3extOut policy and multiple BGP peers can be
                           		configured per 
                           		l3extOut. BGP can be enabled at the 
                           		l3extOut level by defining the 
                           		bgpExtP MO under an 
                           		l3extOut. 
                           	 
                        
                        
                        
                           
                               
 Note 
                               | 
                              
                                 
                                     Although the l3extOut policy contains the routing protocol (for example, BGP with its related VRF), the L3Out interface profile contains the necessary
                                    BGP interface configuration details. Both are needed to enable BGP. 
                                     
                                 
                               | 
                           
                        
                        
                        BGP peer reachability can be through OSPF, EIGRP, a connected interface, static routes, or a loopback. iBGP or eBGP can be
                           used for peering with external routers. The BGP route attributes from the external router are preserved since MP-BGP is used
                           for distributing the external routes in the fabric. BGP enables IPv4 and/or IPv6 address families for the VRF associated with
                           an l3extOut. The address family to enable on a switch is determined by the IP address type defined in bgpPeerP policies for the l3extOut. The policy is optional; if not defined, the default will be used. Policies can be defined for a tenant and used by a VRF
                           that is referenced by name. 
                        
                        
                        You must define at least one peer policy to enable the protocol on each border leaf (BL) switch. A peer policy can be defined
                           in two places: 
                        
                        
                        
                        
                        
                           OSPF 
                           	 
                        
                        
                        Various host types require OSPF to enable connectivity and provide redundancy. These include mainframe devices, external pods
                           and service nodes that use the ACI fabric as a Layer 3 transit within the fabric and to the WAN. Such external devices peer
                           with the fabric through a nonborder leaf switch running OSPF. Configure the OSPF area as an NSSA (stub) area to enable it
                           to receive a default route and not participate in full-area routing. Typically, existing routing deployments avoid configuration
                           changes, so a stub area configuration is not mandated. 
                        
                        
                        You enable OSPF by configuring an ospfExtP managed object under an l3extOut.  OSPF IP address family versions configured on the BL switch are determined by the address family that is configured in
                           the OSPF interface IP address. 
                        
                        
                        
                           
                               
 Note 
                               | 
                              
                                 
                                     Although the l3extOut policy contains the routing protocol (for example, OSPF with its related VRF and area ID), the Layer 3 external interface
                                    profile contains the necessary OSPF interface  details. Both are needed to enable OSPF. 
                                     
                                 
                               | 
                           
                        
                        
                        You configure OSPF policies at the VRF level by using the fvRsCtxToOspfCtxPol relation, which you can configure per address family. If you do not configured it, default parameters are used. 
                        
                        
                        You configure the OSPF area in the ospfExtP managed object, which also exposes IPv6 the required area properties.