Networking Routing

Understanding the BGP Routing Table

A good understanding of the BGP routing table is a crucial skill for network administrators. Knowing how this table is configured can quickly respond to any misconfigurations and problems you might run into. You will also learn about common causes of disruptions and the steps you can take to avoid them.


A BGP routing table is a database of route information. It is used to determine the best routes for reaching destinations. There are many functions of a BGP routing table. For example, it can advertise courses that are in the Internet community.

The BGP process continually updates the routing table. The table is based on information from other routers. In addition, the BGP process uses a set of criteria to choose the best route.

The BGP process has three main states. They are Active, ConnectRetry, and OpenSent. When a router is in a state of Active, it is waiting for an Open message. If an error occurs, the router will return to its idle state. Once a router is in ConnectRetry, it remains for a timer to run out before switching to the OpenSent state.

The router will queue up unprocessed updates if the rate of updates exceeds its processing capacity. However, the rate of updates is expected to be small. This causes congestion and may result in a larger routing table than necessary.

Summary-only option

Summary-only is a BGP routing table feature that suppresses the advertisement of more specific routes. This includes routes that contain community names. It is a Cisco proprietary feature.

This feature’s essential function is preventing the information loop that could occur if a network with a specific prefix length is advertised to all neighbors. For example, router A might advertise four routes: two that contain the most petite prefix length, one that includes a community name, and a more specific third. However, if Router A did not enable this feature, then all four routes would be advertised to all neighbors.

In addition, the summary-only route also performs other less apparent functions. For example, it can be configured to advertise a route that only displays the BGP route after applying policies.

It can also be configured to advertise an aggregated route. A consolidated path will include all the AS numbers for all the paths contributing to the aggregated path. These paths will not be advertised outside of the BGP confederation.

Misconfiguration problems

BGP is one of the most important routing protocols on the internet, but it can also be the target of malicious attacks. It is susceptible to several vulnerabilities, including misconfiguration, hijacking, and inadvertently sending an errant message.

There are some common misconfigurations in BGP. The most common one involves advertising a route for a prefix that is not served by a router.

In addition, BGP can be prone to several other problems, from hijacking to a router or two failing to function. These incidents can be costly and can impact business operations. For instance, a failed router can cause connectivity issues, affecting employee productivity by up to 29%.

Other misconfigurations include an improperly configured router or network interface. For example, a faulty router might fail to open TCP port 179. If the underlying physical interface is not operational, implementing a soft reset enhancement might be a good idea.

BGP is only one of many protocols on the internet, but it is one of the most complex. Among the complexities is a routing table that is both vendor and user-specific.

Common causes of disruptions

The Border Gateway Protocol is an integral part of the IP network infrastructure. It provides an effective way to exchange packets between autonomous systems. However, it can cause several intricate problems, including misconfigurations and hidden routes.

Misconfigurations can occur for a variety of reasons and can affect other networks, as well. If a single router is incorrectly configured, it can profoundly affect the whole system. In some cases, BGP will refuse to allow a specific inbound connection.

Misconfigurations can also affect the Internet backbone. A misconfigured BGP session may cause outages that can take several minutes to resolve.

Route flapping is another common issue that can disrupt BGP. In this case, a neighbor may send multiple possible routes to a particular destination. When a route flap occurs, the egress edge router does not receive information from the ingress edge router. This causes the routing table to converge at a much slower rate.

Another factor that can cause BGP problems is invalid entries in the IP routing table. This happens when a threshold, such as a prefix or a route, is incorrectly inserted.


The Border Gateway Protocol is one of the most critical components of an IP network. Its main purpose is to send and receive information on Autonomous Systems. However, there are many ways that it can fail. Some of these problems can lead to significant downtime or loss of connectivity.

The best way to troubleshoot a BGP session is to start with the basics. Checking for physical connections, ARP, and ICMP pings can help you pinpoint the problem.

For the most part, you should be able to fix a BGP issue independently. However, some problems may require the help of your upstream ISP. Your ISP may have a debugging tool available or may be able to help you solve your routing issue.

Another great way to test if a route works are to use a traceroute. This can tell you if a particular route is working correctly or going offline. To do this, you need to find out the IP prefix that is being sent to and from your network.

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