What Is iBGP?
Interior Border Gateway Protocol (iBGP) is a part of the Border Gateway Protocol (BGP) family, which is used for exchanging routing information within a single Autonomous System (AS).
An AS is essentially a large network or group of networks under a common administration, often belonging to an organization or an internet service provider.
Here are the key points about iBGP:
Purpose: While eBGP (external BGP) is used for routing between different ASes, iBGP handles routing within an AS. It’s essential for managing complex internal network routes and ensuring that routing information is consistent across the network.
Path Information Sharing: iBGP enables routers within the same AS to share path information. This is critical for maintaining accurate and up-to-date routing information across the internal network.
Loop Prevention: Since iBGP deals with internal routing, there are specific mechanisms in place to prevent routing loops. One such mechanism is the requirement that iBGP routes received from one peer must not be advertised to another iBGP peer, unless certain conditions are met. This is different from eBGP, where routes are freely shared between peers.
Full Mesh Requirement: In an iBGP setup, a full mesh network is traditionally required, meaning each router should form a peer relationship with every other router. This can become impractical in large networks due to the sheer number of connections required. To overcome this, network designs often use route reflectors or confederations to reduce the number of necessary connections.
Uses and Applications: iBGP is widely used in large enterprise networks and by ISPs to manage internal routing efficiently. It allows for sophisticated routing policies and control over the flow of traffic within an AS.
To be able to connect to another autonomous system, it is essential to configure the following:
• The start of the routing process
• The networks to be advertised
• The BGP neighbour that the routing process will be synchronizing routing tables with over a TCP session.
How To Configure iBGP.
iBGP’s basic configuration slightly differs from that of eBGP. The configuration is not to identify and form neighborship with an eBGP peer, instead, the iBGP neighbour’s ASN is listed on the neighbour… remote-as.
The neighbor-asn command lists the same ASN as the local router’s router bgp command. However, eBGP neighbour remote-as commands lists a different ASN.
To simplify this, the remote ASN is the same ASN as the local router bgp ASN. We use the diagram below as an example.
iBGP configuration commands:
The above topology and configuration show two iBGP router peering.
Both refer to the other router’s IP address on the Serial, FastEthernet, or loopback interface between the two routers, and both refer to ASN 556.
The two routers then realize the neighbour is an iBGP neighbour because the neighbour’s ASN (556) matches the local router’s ASN, as seen on the router bgp 556 command.
In summary, iBGP plays a significant role in the internal management of routing information within an Autonomous System, ensuring efficient, reliable, and controlled distribution of routing data across an organization’s or ISP’s network.
