Analysis on the BGP synchronization of the Border Gateway Protocol

1. BGP Route synchronization Concept

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 is also known 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 10.1.1.1/24 in AS100, RTB will advertise the route information to RTE, RTE receives the route information from the RTB announcement to the Autonomous System AS100 10.1.1.1/24. For RTE, it needs to consider synchronization when considering whether to advertise such routing information to its EBGP neighbor RTF. Synchronization here means that if RTE can obtain a route to 10.1.1.1/24 through IGP (internal gateway protocol, such as OSPF protocol), we think EGP (external gateway protocol, such as BGP protocol) and IGP synchronization. During synchronization, RTE can advertise the route information 10.1.1.1/24 to RTF. Otherwise, no notification is allowed.

Based on the above description, I believe everyone should know what BGP synchronization is. So why do BGP designers need such a rule? Why should we introduce the concept of BGP synchronization?

The following describes the importance of BGP synchronization and the key issues it solves.

2. Importance of BGP Route Synchronization

The preceding example is used to explain that if RTE does not consider synchronization, the route information 10.1.1.1/24 is directly advertised to RTF, then, a route entry destined for 10.1.1.1/24 is recorded in the RTF routing table. The next hop is directed to RTE. In this case, there is a packet destined for 10.1.1.1/24 in the RTF. the RTF forwards the packet to the RTE by viewing the route table. If there is no synchronization, the RTE route table also has a table entry destined for 10.1.1.1/24, 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 ). Without synchronization, RTE cannot directly go to the next hop of the route (because the routers RTD and RTC in the middle of the route do not go to the route 10.1.1.1/24 ). The concept is that the next hop is not reachable.

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 preceding example, when RTE wants to advertise the route information destined for 10.1.1.1/24 to RTF, consider whether the next hop to 10.1.1.1/24 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, do not access it. Therefore, whether the next hop is reachable is a key issue. BGP emphasizes synchronization, that is, the IGP protocol of the autonomous system where the RTE is located can achieve the next hop. If IGP finds the route to the next hop, RTE gets two routes to 10.1.1.1/24. One is: obtained through BGP, the next hop of this route is not reachable. The other is: obtained through IGP, And the next hop is reachable (about why the next hop of the route obtained by BGP is not reachable, while the next hop of the route obtained by IGP is reachable, this issue requires further understanding of the next hop attribute of BGP, which will not be described here ).

3. BGP Route accessibility and accessibility

Next Hop accessibility and unaccessibility are further described below. The above example is still used. Because the next hop in the BGP system is in the Autonomous System (this is described in detail in the next hop attribute of BGP ). In this way, there will be such a problem. The route table of RTE has a route to 10.1.1.1/24, but the next hop is far away, so that the next hop is not reachable. However, this problem does not occur for IGP. This is because of the different definitions of the next hop (we will not discuss the definition of the next hop. The BGP designers have also considered and selected this definition ). Since the routes discovered by BGP have the problem that the next hop cannot be reached, we need to solve this problem. BGP synchronization is the solution.

Of course, synchronization is optional. On the Huawei 3Com3Com router, synchronization can be canceled. After the synchronization is canceled, when the router advertises the route, the synchronization problem is no longer considered, and the route information is directly advertised to the EBGP neighbor. However, the next hop cannot be reached. Therefore, before canceling synchronization, we should consider how to handle the possible next hop failures. In the preceding example, all routers in the autonomous system where the 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 achieve next hop accessibility, depending on the network topology. In the above example, the BGP Route can be introduced into IGP by RTB. 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, and also add static routes on RTA. This method seems stupid, and is not the best solution. Finally, you can use a command to forcibly modify the next hop. In RTE, the next hop destined for 10.1.1.1/24 is forcibly changed to the interface of RTE, the route table that requires RTD and RTC includes a route entry destined for 10.1.1.1/24. This is not the ideal solution. Because Static Routing is still required. In fact, for the topology in the above example, the IBGP neighbor is not a direct connection, but a TCP connection. There is no ideal solution for the next hop accessibility problem of BGP routing in such a topology. If the network is not complex in this case, you can barely solve the problem (several solutions have been introduced above ). However, once the network is complex, it is very difficult to solve the problem. Therefore, you must avoid this situation during network design. IBGP neighbors try to connect directly. The complete solution to this problem lies in the combination of BGP and MPLS. You will have the opportunity to study again later!

4. Summary

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 the BGP Route is not reachable. To solve the problem that the next hop is not reachable, the BGP protocol introduces the concept of synchronization. Synchronization can also be canceled by using commands, as long as the next hop of the route can be reached.

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