Dell PowerSwitch S4128F-ON EMC SmartFabric OS10 User Guide Release 10.5.1 - Page 614

BGPv4 supports classless interdomain routing (CIDR) with aggregate routes and AS paths. CIDR defines a network using a prefix consisting of an IP address and mask, resulting in efficient use of the IPv4 address space. Using aggregate routes reduces the size of routing tables. Path-vector routing BGP uses a path-vector protocol that maintains dynamically updated path information. Path information updates which return to the originating node are detected and discarded. BGP does not use a traditional Internal Gateway Protocol (IGP) matrix but makes routing decisions based on path, network policies, and/or rule sets. Full-mesh topology In an AS, a BGP network must be in full mesh for routes received from an internal BGP peer to send to another IBGP peer. Each BGP router talks to all other BGP routers in a session. For example, in an AS with four BGP routers, each router has three peers; in an AS with six routers, each router has five peers. Configuration notes All Dell EMC PowerSwitches except MX-Series, S4200-Series, S5200 Series, and Z9332F-ON: ● If you use eBGP to exchange routes with switches in an SFS environment, the router must directly connect to the switch or switches present. You must use the interface IP to set up BGP peering. NOTE: This behavior is applicable only to the S4100-ON series of switches. ● By default, routes that are learned on multiple paths to eBGP peers are advertised to IBGP peers with the next-hop local IP address. This behavior allows for local repair of atomic failure of any external peers. ● Fast external failover is enabled by default. To disable or re-enable fast external failover, use the [no] fast-externalfallover command. For the fast-external-fallover command to take effect on an established BGP session, you must reset the session using the clear ip bgp {* | peer-ipv4-address | peer-ipv6-address} command. ● Enabling the BGP add-paths globally for all BGP neighbors is not supported (the add-path command in ROUTER-BGPv4-AF or ROUTER-BGPv6-AF mode). To enable the BGP add-path for one neighbor, use the add-path command in ROUTERBGPNEIGHBOR-AF mode. ● When you redistribute OSPFv3 routes to BGP, including External Type-2 routes, the multi-exit discriminator (MED) attribute is set to the OSPF route metric plus one instead of the OSPF route metric value. ● When you configure the bgp bestpath router-id ignore command, for non-best paths, the show ip bgp output displays Inactive reason: Router ID. ● Do not configure the IP address of the router as a BGP neighbor. This action causes the address being accepted as an invalid neighbor address. Sessions and peers A BGP session starts with two routers communicating using the BGP. The two end-points of the session are called peers. A peer is also called a neighbor. Events and timers determine the information exchange between peers. BGP focuses on traffic routing policies. Sessions In operations with other BGP peers, a BGP process uses a simple finite state machine consisting of six states-Idle, Connect, Active, OpenSent, OpenConfirm, and Established. For each peer-to-peer session, a BGP implementation tracks the state of the session. The BGP defines the messages that each peer exchanges to change the session from one state to another. Idle Connect Active OpenSent OpenConfirm BGP initializes all resources, refuses all inbound BGP connection attempts, and starts a TCP connection to the peer. Router waits for the TCP connection to complete and transitions to the OpenSent state if successful. If that transition is not successful, BGP resets the ConnectRetry timer and transitions to the Active state when the timer expires. Router resets the ConnectRetry timer to zero and returns to the Connect state. Router sends an Open message and waits for one in return after a successful OpenSent transition. Neighbor relation establishes and is in the OpenConfirm state after the Open message parameters are agreed on between peers. The router then receives and checks for agreement on the parameters of the open messages to establish a session. 614 Layer 3