configuration command determines how BGP handles redistributed routes. When BGP summarization is enabled (with auto-summary), all redistributed subnets are summarized to their classful boundaries in the BGP table. When it is disabled (with no auto-summary), all redistributed subnets are present in their original form in the BGP table, so only those subnets are advertised. In Cisco IOS Software Release 12.2(8)T, the default behavior of the auto-summary command was changed to disabled (no auto-summary). Prior to that, the default was enabled (auto-summary).
Content 6.3 Configuring BGP 6.3.12 BGP Synchronization The BGP synchronization rule states that a BGP router should not use, or advertise to an external neighbor, a route that is learned from IBGP unless that route is local or the router learns it from the IGP. In other words, BGP and the IGP must be synchronized before BGP can use networks that are learned from an IBGP neighbor. If an autonomous system passes traffic to another autonomous system, BGP should not advertise a route before all routers in the autonomous system have learned about the route via the IGP. A router learning a route via IBGP waits until the IGP has propagated the route within the autonomous system and then advertises it to external peers. This rule ensures that all routers in the autonomous system are synchronized and are able to route traffic that the autonomous system advertises to other autonomous systems. This approach ensures consistency of routing information (avoids “black holes”) within the autonomous system. BGP synchronization is disabled by default in Cisco IOS Software Release 12.2(8)T and later. It was on by default in earlier Cisco IOS software releases. When synchronization is disabled, BGP can use and advertise routes learned from an IBGP neighbor that are not present in the local routing table to an external BGP neighbor. BGP synchronization is unnecessary in some situations. For instance, it is safe to have BGP synchronization off if all routers in the transit path in the autonomous system are running full mesh IBGP. Having synchronization disabled allows the routers to carry fewer routes in IGP and allows BGP to converge more quickly. Use synchronization if routers in the BGP transit path in the autonomous system are not running BGP (therefore, the routers do not have full mesh IBGP within the autonomous system). Note
 In the past, the best practice was to redistribute BGP into the IGP running in an autonomous system so that IBGP was not needed in every router in the transit path. In this case, synchronization was needed to make sure that packets did not get lost; therefore, synchronization was on by default. As the Internet grew, the number of routes in the BGP table became too much for the IGPs to handle. The best practice changed to not redistribute BGP into the IGP, but instead use IBGP on all routers in the transit path. In this case, synchronization is not needed, so it is now off by default.
Content 6.3 Configuring BGP 6.3.13 BGP Synchronization Example In Figure , routers A, B, C, and D are all running IBGP and an IGP with each other. There are no matching IGP routes for the BGP routes (routers A and B are not redistributing the BGP routes into the IGP). Routers A, B, C, and D have IGP routes to the internal networks of AS 65500, but do not have routes to external networks such as 172.16.0.0. Router B advertises the route to 172.16.0.0 to the other routers in AS 65500 using IBGP. If synchronization is on, routers A, C, and D do not use the route to 172.16.0.0, nor does router A advertise that route to router E in AS 64520. Router B uses the route to 172.16.0.0 and installs it in its routing table. If router E receives traffic that is destined for network 172.16.0.0, it does not have a route for that network and cannot forward the traffic. If synchronization is off (the default) in AS 65500, routers A, C, and D can use the route to 172.16.0.0 and install the route in their routing tables, even if there are no matching IGP routes for the BGP routes (assuming that routers A, C, and D can reach the next-hop address for 172.16.0.0). Router A advertises the route to router E. Router E then has a route to 172.16.0.0 and may send traffic that is destined for that network. Router E sends the packets to router A, and router A forwards them to router C. Router C learns a route to 172.16.0.0 via IBGP; therefore, router C forwards the packets to router D. Router D forwards the packets to router B. Router B forwards the packets to router F for network 172.16.0.0. In modern autonomous systems, because the size of the Internet routing table is large, redistributing from BGP into an IGP is not scalable. Therefore, most modern autonomous systems run full mesh IBGP and do not require synchronization. Advanced BGP configuration methods, for example, using route reflectors and confederations, reduce the full mesh requirements.
Content 6.3 Configuring BGP 6.3.14 BGP Configuration Example Figure displays another BGP example. Figure displays the configuration for router B. The first two commands under the router bgp 65000 command establish that router B has the following two BGP neighbors: From the perspective of router B, router A is an EBGP neighbor, and router C is an IBGP neighbor. The neighbor statement on router B for router A is pointing at the directly connected IP address to reach the EBGP neighbor, router A. However, the neighbor statement on router B points to the loopback interface of router C, because router B has multiple paths to reach router C. If router B pointed at the 192.168.3.2 IP address of router C and that interface went down, router B would be unable to reestablish the BGP session until the link came back up. By pointing to the loopback interface of router C instead, the link stays established as long as any path to router C is available. Router C should also point to the loopback address of router B in its configuration. Line 4 notifies router B to always use its loopback 0 address, 192.168.2.1, as the source IP address when sending an update to router C, 192.168.2.2. In line 5, router B changes the next-hop address for networks that are reachable through it. The default next-hop setting for networks from AS 64520 is IP address 10.1.1.2. With this next-hop-self command, router B sets the next-hop address to the source IP address of the routing update, which is the router B loopback 0 interface, as set by the update-source command. Lines 6 and 7 notify BGP about which networks to advertise. Line 6 contains a subnet of a class B address using the mask option. Lines 7 and 8 have two network statements for the two class C networks that connect routers B and C. The default mask is 255.255.255.0, so you do not need to include it in the command. In line 9, synchronization is disabled. If router A is advertising 172.20.0.0 in BGP, router B receives that route and advertises it to router C. Since synchronization is off, router C can use this route. If router C had EBGP neighbors of its own and router B wanted to use router C as the path to those networks, synchronization on router B would also need to be off. In this network, synchronization can be off because all the routers within the autonomous system are running IBGP.
Content 6.4 Advanced BGP Configuration and Verification 6.4.1 BGP Neighbor States The BGP neighbor negotiation process proceeds through various states. These steps can be described in terms of a finite-state machine (FSM). An FSM is a set of possible states something can go through, what events causes those states, and what events result from those states. Figure presents the BGP FSM, which includes the states and some of the message events that cause them. After you have entered the neighbor command, BGP takes the IP address that is