![]() Let us assume that the routers are already running BGP in your network. Routers configured as RRs are allowed to propagate routes learned via IBGP to other IBGP peers in the AS, thereby violating a typical behavior of IBGP routers. The second, more scalable option is to consider the implementation of route reflectors (RRs). In the figure above, every BGP router in the AS established 9 BGP sessions with all other BGP routers inside that AS, which means 45 IBGP sessions. IBGP sessions = n(n – 1)/2 IBGP full mesh with 10 routers = 45 IBGP sessions The formula to calculate a number of BGP sessions, considering the number of routers is as follows: Namely, a large number of IBGP sessions might consume a significant amount of bandwidth. So, the solution is not so scalable, can get pretty complex with a large number of IBGP peers, and has the potential to affect the performance of the overall network. Logically, as the number of routers increases, so does the number of BGP sessions required to establish a full mesh of IBGP peers. With only 10 routers, you would need to maintain 45 BGP sessions with 15 routers even more, 105 BGP sessions. This solution is feasible if you have a small number of IBGP routers in the AS. In order for all BGP routers internal to the AS learn all the external routes (from all external sources), they need to establish an internal BGP peering with all other internal BGP routers, that is, to form a full IBGP mesh (a network of interconnected IBGP peers). To avoid routing loops, BGP specifies that routes learned via IBGP should not be advertised to other IBGP peers ( BGP split-horizon rule). Establishing an EBGP peering with another ISP enables the exchange of routes, that is, communication between ISPs. An ISP is running IBGP within its own autonomous system. ![]() The above figure represents the idea of IBGP and EBGP from the perspective of an Internet Service Provider (ISP). When BGP peering is established between routers in different ASs, we refer to external BGP, EBGP. When BGP peering is established between two routers within the same AS, we refer to internal BGP relationship, IBGP. Routers that form neighbor relationship between each other in order to exchange routing information are referred to as BGP peers. Routers that run a BGP process are referred to as BGP speakers. It is therefore also called an interdomain routing protocol.įrom BGP’s perspective, the whole Internet is an entity of ASs, each of which is identified by a unique AS number. The role of BGP is to route information between autonomous systems (mainly ISP networks). They usually fall under the administration of a single network engineer or organization. The content of this article does not require a thorough understanding of BGP, but still, it is assumed you have a prior knowledge of routing protocols, and that you are familiar with the idea of interior/exterior routing protocols and neighbor relationships.Īs you probably remember from the CCNA level, autonomous systems (AS in further text) are routing domains with their own independent routing policies and unique routing protocols. In either case, feel free to proceed with reading. Or I might be wrong, and you have heard about this cool protocol numerous times, but just did not figure out how it works. If you decided to read the content of this post, you probably know something (or a lot) about Border Gateway Protocol (BGP) and its benefits.
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