Synchronization Within an AS

BGP must be synchronized with IGP in such a way that it waits until the IGP has propagated routing information across your autonomous system before advertising transit routes to other ASs. It is important that your AS be consistent about the routes it advertises. If, for example, your BGP were to advertise a route before all routers in your AS had learned about the route through the IGP, your AS could receive traffic that some routers cannot yet route.

Whenever a router receives an update about a destination from an IBGP peer, the router tries to verify internal reachability for that destination before advertising it to other EBGP peers. The router would do so by checking for the existence of this destination in the IGP. This would give an indication whether non-BGP routers can deliver traffic to that destination. Assuming that the IGP recognizes that destination, the router will announce it to other EBGP peers. Otherwise, the router will treat the route as not being synchronized with the IGP and would not advertise it.

Consider the situation illustrated in figure 5-4; ISP1 and ISP2 are using ISP3 as a transit AS. ISP3 has multiple routers in its AS and is running BGP only on the border routers. (Even though RTB and RTD are carrying transit traffic, ISP3 has not configured BGP on these routers.) ISP3 is running some Interior Gateway Protocol inside the AS for internal connectivity.

Figure 5-4  BGP route synchronization.

Assume that ISP1 is advertising route 192.213.1.0/24 to ISP3. Because RTA and RTC are running IBGP, RTA will propagate the route to RTC. Note that other routers besides RTA and RTC are not running BGP and have no knowledge so far of the existence of route 192.213.1.0/24.

In the situation illustrated in figure 5-4, if RTC advertises the route to ISP2, traffic toward the destination 192.213.1.0/24 will start flowing toward RTC. RTC will do a recursive lookup in its IP routing table and will direct the traffic toward the next hop RTB. RTB, having no visibility to the BGP routes, will drop the traffic because it has no knowledge of the destination. This has happened because there is no synchronization between BGP and the IGP.

The BGP rule states that a BGP router should not advertise to external neighbors destinations learned from inside BGP neighbors unless those destinations are also known via IGP. If a router knows about these destinations via IGP, it assumes that the route has already been propagated inside the AS, and internal reachability is guaranteed.

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Troubleshooting:  
Synchronizing BGP and IGP to advertise routes.

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The consequence of injecting BGP routes inside an AS is costly. Redistributing routes from BGP into the IGP will result in major overhead on the internal routers, which might not be equipped to handle that many routes. Besides, carrying all external routes inside an AS is not really necessary. Routing can easily be accomplished by having internal non-BGP routers default to one of the BGP routers. Of course, this will result in routing suboptimality (there is no guarantee for shortest path for each route), but this cost is minimal compared with maintaining thousands of routes inside the AS.

Cisco offers a software knob called "no synchronization" that enables BGP to override the synchronization requirement and enables it to advertise routes learned via IBGP irrespective of an existence of an IGP route. In practice, a couple of situations exist where synchronization can be safely turned off on border routers:

•  When all transit routers inside the AS are running fully meshed IBGP. In this situation, internal reachability is guaranteed because a route that is learned via EBGP on any of the border routers will automatically be passed on via BGP to all other transit routers.

•  When the AS is not a transit AS.

Sources of Routing Updates

In networks as complex as today's Internet, route stability is a big issue. There is a close correspondence between route fluctuations and the stability of the Internet access links on one hand and how the routing information was injected into the Internet via BGP on the other hand. Information can be injected into BGP dynamically or statically. Dynamically injected routes come and go from the BGP routing table, depending on the status of the networks they identify. Statically injected routes are constantly maintained by the BGP routing tables, regardless of the status of the networks they identify. Thus, while a dynamic advertisement will cease if the network being advertised no longer exists, a static advertisement would not. Each method has its pros and cons, as you will see next.

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Troubleshooting:  
Example: Ch. 10, pp. 315-324. Sources of Routing Updates

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