CCNP ROUTE: 7. Avoiding Routing Loops when Forwarding Toward the Internet

A typical Enterprise network design uses default routes inside the Enterprise, as advertised by the IGP, to draw all Internet traffic toward one or more Internet-connected routers.
The Internet-connected routers then forward the traffic into the Internet.
Routing loops can occur when the Internet-connected routers do not have a direct connection to each other.

Two solutions exist to help these internal routers learn the routes:
- Run BGP on at least some of the routers internal to the Enterprise (iBGP full mesh).
- Redistribute BGP routes into the IGP (not recommended - this redistribution requires a relatively large amount of memory and a relatively large amount of processing by the IGP).

Although not recommended, the idea of redistributing eBGP-learned Internet routes into the Enterprise IGP needs to be discussed as a backdrop to discuss a related BGP feature called synchronization or sync.
The term refers to the idea that the iBGP-learned routes must be synchronized with IGP-learned routes for the same prefix before they can be used.
In other words, if an iBGP-learned route is to be considered to be a usable route, then that same prefix must be in the IP routing table and learned using some IGP protocol such as EIGRP or OSPF.

Note: Later IOS versions default to disable synchronization because most sites avoid redistributing routes from BGP into an IGP when using BGP for Internet routes, instead preferring iBGP meshes (or alternatives) to avoid these routing black holes.

Note: The suggestion to avoid redistribution from BGP into an IGP generally applies to cases in which BGP is used to exchange Internet routes.
However, BGP can be used for other purposes as well, including the implementation of Multiprotocol Label Switching (MPLS).
Redistribution from BGP into an IGP when using BGP for MPLS is reasonable and commonly done.