BGP Route Protocol synchronization features

Source: Internet
Author: User

BGP is a familiar protocol. Then, let's give a brief explanation of this Protocol. It mainly targets its synchronization performance. You need to pay attention to this aspect. BGP protocol: A router running the BGP protocol will not advertise the routing information learned from the internal peer (IBGP neighbor) to the external peer (EBGP neighbor ), unless the route information can also be learned through IGP. If the BGP router can learn the route information through IGP, it can be considered that the route can be transmitted between AS and the internal access is guaranteed 。

First, RTB obtains the route information to AS100 through the EBGP neighbor relationship. For example, to in AS100, RTB will advertise the route information to RTE, RTE received the route information from the RTB announcement to the AS100 of the autonomous system. For RTE, when considering whether to advertise such route information to her EBGP neighbor RTF, synchronization needs to be considered. Synchronization here refers to: If RTE can obtain a route to through IGP (internal gateway protocol, such as OSPF Protocol, we think that EGP (external gateway protocol, such as BGP protocol) and IGP are synchronized. In the case of synchronization, RTE can advertise the route information to RTF. Otherwise, no notice. Based on the above description, I believe everyone should know what is BGP synchronization. Why do BGP designers need such a rule? Why should we introduce the concept of BGP synchronization? Next we will talk about the importance of BGP synchronization and the key issues it solves 。

Importance of BGP Route Synchronization

The preceding example is used to explain that if RTE does not consider synchronization, the route information is directly advertised to RTF, then, a route entry destined for is recorded in the RTF route table, And the next hop is directed to RTE. In this case, a packet destined for appears in the RTF route table, by viewing the route table, RTF forwards this packet to RTE. If there is no synchronization, the RTE route table also contains the table entry destined for, however, the next hop of this table item points directly to the S0 interface of RTA (for the reason, see the next hop attribute of BGP, RTE cannot directly go to the next hop of this route (because the routers RTD and RTC in the middle of the route do not go to the route, this is the case 。

So here, do smart readers understand? BGP emphasizes that the reason for synchronization is to ensure that the next hop can be reached. In the example above, when RTE wants to advertise the route information destined for to RTF, consider whether the next hop to is reachable (the next hop here refers to the S0 interface of RTA, which is the entry for AS200 and AS300 to AS100) if this portal is inaccessible, let alone access it 。

Therefore, whether the next hop is reachable is a key issue. BGP emphasizes synchronization, the IGP protocol of the autonomous system where the RTE is located can achieve the next hop. If the IGP finds the route to the next hop, then, the RTE gets two routes destined for One is obtained through BGP, And the next hop of the route cannot be reached. The other is obtained through IGP, this next hop is reachable (for more information about why the next hop of the route obtained by BGP is not reachable, but the next hop of the route obtained by IGP is reachable, you need to know more about the next hop attribute of BGP, )。

BGP Route accessibility and accessibility

Regarding the next hop reachable and inaccessibility issues, the next hop in the BGP system is based on the Autonomous System (this is described in detail in the next hop attribute of BGP) in this way, there will be such a problem. There is a route to in the RTE route table, but the next hop is far away, so that the next hop is not reachable. But for IGP, there will be no such problem. This is because of the different definitions of the next hop (for the definition of the next hop, we will not discuss the definition of BGP, which is also considered and selected by the designers). Since the BGP found that the route has the problem of the next hop being inaccessible, we need to solve this problem. BGP synchronization is the solution 。

Of course, synchronization can be selected. On the Huawei 3Com3Com router, synchronization can be canceled. After the synchronization is canceled, when the router notifies the route, the synchronization problem will not be considered, directly advertise your route information to the EBGP neighbor. However, the next hop cannot be reached, before canceling synchronization, we need to consider how to handle the possible next hop failure. For the above example: all routers in the autonomous system where RTE is located must know how to go to the S0 interface of RTA. As long as the next hop is reachable, canceling synchronization is also possible 。

After the synchronization is canceled, there are many methods to meet the next hop accessibility, depending on the network topology. In the preceding example, RTB can introduce BGP routes to IGP, of course, this method is generally not used. It should be because the BGP Route has a large amount of information, which is too heavy for IGP. In addition, we can add static routes on RTC, RTD, and RTE, at the same time, it also adds static routes to RTA. This method looks stupid, and is not the best solution. Finally, BGP can force the next hop to be modified through commands, on RTE, force the next hop destined for to the interface of RTE. At the same time, the route table of RTD and RTC must have a route destined for 。

This is not the best solution, because Static Routing may be needed. In fact, for the topology in the above example, the main reason is that the IBGP neighbor is not a direct connection relationship, but a TCP connection. The BGP routing protocol has never had an ideal solution to the next hop accessibility problem in such a topology. If the network in this situation is not very complex, but once the network is complex, it is very difficult to solve the problem, be sure to avoid this situation. IBGP neighbors should try to connect directly. The complete solution to this problem is after BGP and MPLS join hands. You will have the opportunity to learn more in the future!


The next hop attribute of BGP indicates that the next hop of a BGP Route is in the autonomous system. This leads to the problem that the next hop of a BGP Route is inaccessible, to solve the problem that the next hop of a route cannot be reached, the BGP protocol introduces the concept of synchronization. synchronization can also be canceled by command, as long as the next hop of the route can be reached 。

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