BGP : Implicit Withdraw Vs Explicit Withdraw

BGP : Implicit Withdraw Vs Explicit Withdraw

BGP is used to exchange routing information between multiple autonomous systems (AS) on the Internet. An autonomous system is a group of networks that share a common administrative area. 

BGP is used to route traffic across multiple independent systems and is the protocol used by Internet Service Providers (ISPs) to communicate routing information with one another.

⭐Related : BGP Cheat Sheet for Network Engineers
⭐Related : 51 facts about BGP routing Protocol for Network Engineers 

BGP : Implicit Withdraw Vs Explicit Withdraw
Fig 1.1- BGP : Implicit Withdraw Vs Explicit Withdraw

 What is BGP Implicit withdraw ?

"Implicit withdraw" means the prefix is implicitly withdrawn by sending the same prefix with new attributes.

 Purpose of Implicit withdraw

When a BGP router discovers a new "better path" to a previously advertised prefix, it sends an update including the new path information. This update implicitly removes the previously advertised path from its neighbors' review. 

It will choose the revised route if it is better based on their routing policies.

 Benefits of Implicit withdraw

⭐1. It avoids the need for individual withdrawal messages. BGP routers can simply advertise the new "best path," and surrounding routers will no longer consider the old path. This reduces message overhead while simplifying the routing mechanism.

⭐2. When network conditions change, BGP can immediately act by announcing the new best path. Implicit withdrawal guarantees that surrounding routers adopt the new way more quickly, resulting in faster convergence to the best overall routing paths. This leads to smoother traffic flow and lower latency.

⭐3. BGP is meant to manage a large internet with many routers. Implicit withdrawal improves scalability by eliminating the requirement for individual withdrawal messages for each path modification. This reduces processing cost and message traffic, allowing BGP to function efficiently across an extensive network.

⭐4. Network administrators do not have to manually configure withdrawal messages for each path change. Implicit withdrawal simplifies this process, allowing routers to automatically adapt to changing network conditions and choose the best available pathways.

 Disadvantages of Implicit withdraw

⭐1. It can be difficult to determine exactly why a certain route was dropped. This can make identifying routing challenges more difficult, particularly when dealing with large networks that have many BGP speakers.

⭐2. BGP withdraws only the path, not the reason for removal. This absence of further information can make it difficult to identify or avoid future routing problems.

⭐3. Attackers might potentially use implicit withdrawal to disrupt or redirect traffic.  For example, by altering routing features to make a less desirable path appear more appealing, they may divert traffic away from legal destinations.

 What is BGP Explicit withdraw ?

"Explicit withdraw" means sending an actual withdraw message for a given prefix.

 Purpose of Explicit withdraw

If a BGP router detects that a previously advertised route is broken (for example, due to a configuration error or link failure), it can use explicit withdrawal to quickly inform its neighbors and prevent them from using that path.

 Benefits of Explicit withdraw

⭐1. If you have two paths (A and B) to access a particular network. If path A becomes congested, implicit withdrawal will remove both A and B from the neighbor's routing table. With explicit withdrawal, you could just remove A, allowing your neighbor to proceed on the possible less congested path of B.

⭐2. The explicit "withdraw" message clearly states the deletion of a particular route. This gives crucial data for network administrators to troubleshoot routing issues.

⭐3. If a BGP router detects that a previously advertised route is broken (due to a configuration error, link failure, etc.), explicit withdrawal informs neighbors to avoid that path.

 Disadvantages of Explicit withdraw

⭐1. The impact is greater in larger networks with many routers exchanging routing information often Additional messages can put an increase on bandwidth resources, possibly resulting in problems with performance.

⭐2. Sending and processing explicit withdrawal messages requires additional processing power on BGP routers.

⭐3. Network operators have to specify the conditions under which explicit withdrawals should occur (for example, through routing rules). This increases the complexity of BGP configuration management.

Explicit withdraw Vs Implicit Withdraw 

It is important to understand the difference between Implicit Withdraw and Explicit Withdraw and how they impact our network in case you are going to use it.

BGP Explicit withdraw Vs BGP Implicit Withdraw
Fig 1.2- BGP Explicit withdraw Vs Implicit Withdraw 

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