BGP Routing Protocol
Notice: This blog post was originally published on Indeni before its acquisition by BlueCat.
The content reflects the expertise and perspectives of the Indeni team at the time of writing. While some references may be outdated, the insights remain valuable. For the latest updates and solutions, explore the rest of our blog
Key takeawaysThis key takeaway was generated through LLMs crawling the page and coming up with an overview of the content.This article explains Border Gateway Protocol (BGP) as the Internet’s core path-vector routing protocol used to exchange routes among Autonomous Systems and enforce routing policies. It covers BGP’s operation over TCP port 179 at OSI Layer 4, distinctions between iBGP and eBGP, policy mechanisms like route maps and MED for influencing multi-homing and traffic engineering, and MP-BGP’s role in enabling MPLS-based VPN services for large enterprises. The piece emphasizes the operational need for proactive BGP monitoring and fault resolution and lists indeni’s example checks—neighbor down, neighbor state changes, overly loose summarizations, and internal devices assigned public IPs—that help maintain BGP stability and correct configuration.
What is the difference between iBGP and eBGP and when would an enterprise use each?
iBGP (internal BGP) is used to exchange routing information within a single Autonomous System, while eBGP (external BGP) exchanges routes between different Autonomous Systems. Enterprises run iBGP to control and influence internal routing metrics and policy distribution across their routers, helping to shape path selection and traffic flows inside the AS. eBGP is used when connecting to upstream ISPs or other ASes for Internet connectivity and is essential for multi-homing scenarios where MED and other attributes are applied to prefer specific entry points for load balancing or backup.
How do route maps and the MED attribute help with multi-homing and traffic engineering?
Route maps are the primary mechanism for implementing BGP routing policies; they match route attributes and set policies that influence path selection. The MED (Multi-Exit Discriminator) attribute is one example used to indicate to a neighboring AS which entry point into an AS should be preferred. In multi-homing situations where an organization connects to multiple ISPs, setting MED values and applying route maps allows operators to suggest preferred ingress paths, achieve load balancing across links, or designate backup paths, thereby shaping inbound traffic without changing overall Internet routing.
What kinds of BGP issues does indeni detect and why are those checks important?
indeni includes proactive BGP checks designed to detect operational and configuration issues that can impact topology and reachability. Examples listed are alerts for BGP neighbors appearing down, changes in BGP neighbor state, overly loose network summarizations that may contain unreachable networks, and internal devices configured with public IP addresses. These checks matter because neighbor outages and state changes can cause route loss or instability, loose summarizations can lead to incorrect aggregation and reachability problems, and misused public IPs in internal networks can cause addressing and security issues—so early detection aids faster fault resolution.
So what is BGP? In this series of posts I will be explaining the main principles of BGP. BGP–Border Gateway Protocol–is the de facto core routing protocol of the Internet. It operates by exchanging routes among Internet Autonomous Systems, and it is considered a path vector protocol. Routing is performed by shortest path possible and according to network policies within each Autonomous System. Most large service providers use BGP, and enterprises can operate BGP internally to influence metrics.
BGP operates in Layer 4 of the OSI network model and establishes TCP connections via port 179 between neighbors. BGP that is used internally is designated as iBGP and when used externally it is designated as eBGP. The implementation of routing policies are done mainly by route maps. For example, influencing traffic with BGP policies can be done with the MED (Multi-Exit Discriminator) attribute, which tells a remote AS that a specific entry into an AS is the preferred one. Therefore, BGP is great for multi-homing to different ISPs (in terms of load balancing and backup).
ISPs that run BGP also integrate Multiprotocol BGP (MP-BGP). It is a special extension of BGP that works with MPLS, which allows service providers to offer businesses VPN capabilities and secure connectivity across multiple branches.
This technology is very widespread in large-scale enterprises. Proactive monitoring and fault resolution of BGP is essential due to its importance and influence on the topology.
Smart BGP signatures are already embedded into indeni and many others are planned.
Here are several examples for checks indeni has around BGP configuration:
- BGP Neighbor Is Down – indeni will alert when neighbors appear to be down in neighbors tables.
- BGP Neighbor State Has Changed – indeni will alert when the state has changed for any BGP peers or neighbors in the peer/neighbor table.
- Some Network Summarizations Are Too Loose – indeni will verify that network summarizations are not too loose, and that they do not contain unreachable networks.
- Internal Devices Given an External IP – indeni will alert if a public IP address has been used in an internal network.
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