Guide to Internal Border Gateway Protocol (iBGP): How Is It Different From eBGP?

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Internal Border Gateway Protocol (iBGP) is a routing protocol used on the internet for exchanging routing information between two or more routers within an autonomous system (AS)—hence the use of the word “Internal.” 

iBGP based on the Border Gateway Protocol (BGP), which exchanges routing information between autonomous systems. iBGP is used to propagate routes within a single AS, while BGP is used to propagate routes between different ASs. iBGP ensures that all routers in an AS have the same view of the network topology and allows them to communicate with each other effectively.

On the other hand, External Border Gateway Protocol (eBGP) is a routing protocol used to exchange information between two autonomous systems (AS). It is also an extension of the Border Gateway Protocol (BGP) and is used to connect distinct networks, such as those belonging to different internet service providers, thus the use of the word “External.” EBGP is responsible for exchanging network prefixes between autonomous systems, allowing them to communicate with each other.

Comparing iBGP vs. eBGP

Both iBGP and eBGP are essential components of any network infrastructure and play an important role in ensuring reliable communication between autonomous systems. However, there are some key differences.

The main difference between iBGP and eBGP lies in their purpose. While iBGP is used to propagate routes within an autonomous system, eBGP is used for propagating routes between different autonomous systems. In addition, iBGP does not allow for route redistribution, while eBGP does.

The table below summarizes the differences between iBGP and eBGP.

Full nameInternal Border Gateway ProtocolExternal Border Gateway Protocol
Type of autonomous system (AS)Runs between two BGP routers in the same ASRuns between two BGP routers in different ASs
Default administrative distance20020
Route advertisementRoutes received from a peer cannot be advertised to another peer but can be advertised to an eBGP peerRoutes received from a peer can be broadcast to both eBGP and iBGP peers
Full mesh topologyRequiredNot required
UsageWithin the same organizationDistinct networks (i.e., different organizations or internet service providers)
Loop preventionUses BGP split horizonUses path for loop prevention
Default peer setupTTL = 255TTL = 1
Local preference attributesSentNot sent
Behavior on route advertisement to a peerThe next hop remains unchangedThe next hop is changed to the local router

What is the purpose of iBGP?

The purpose of iBGP is to facilitate communication between multiple routers within an AS). Some of the key functions of iBGP include exchanging routing information and ensuring its accuracy, providing redundancy and failover capabilities, and preventing routing loops.

Here are some of the primary functions of iBGP and their purposes:

  • Exchange routing information between different routers in the same AS. This allows for the efficient transfer of data within a single network.
  • Provide information to other BGP routers about available routes within the AS. This helps to ensure that traffic is routed correctly and efficiently.
  • Ensure all routers in the AS have consistent routing information to make decisions about which paths to take when forwarding packets.
  • Allow for redundancy and failover capabilities by allowing multiple paths to be used for a given route.
  • Help prevent routing loops by ensuring that only one path is used for a given route at any given time.

How iBGP works

iBGP works by exchanging routing information between two or more routers within an AS. Each router sends its own routing table to its neighbors, which contains information about the networks it knows and how they can be reached from that router. When a router receives this information from its neighbor, it updates its own routing table accordingly and then propagates this new information to its other neighbors so that they can update their tables as well. This process continues until all routers in the AS have been updated with the same routing information.

Configuring iBGP

To configure iBGP you must first configure the BGP process on each router and assign them unique AS numbers. Then, you must establish neighbor relationships between the routers by specifying which IP addresses they should use for communication. Finally, you can configure additional parameters such as route filtering and authentication. Once these steps are complete, the routers will begin exchanging routing information.

Verifying iBGP configuration

Once iBGP has been configured, it’s important to verify it’s working correctly. This can be done using several commands, such as “show ip bgp summary” and “show ip bgp neighbors.” These commands will display information about the neighbor relationships and routes being exchanged between the routers. Also, you can use tools such as traceroute or ping to test connectivity between two points in the network.

iBGP synchronization

iBGP synchronization is a rule designed to prevent traffic black holes in legacy network designs where not all devices in the BGP transit path are running iBGP. This is achieved by ensuring that a route learned through an iBGP session will not be advertised until the interior gateway protocol (IGP) has validated it.

The most common way of achieving this is to enable BGP and IGP route synchronization, which requires that upon receiving an iBGP route, a BGP router checks if the same route exists in its IGP routing table. If it does not exist, then the router will not advertise the route until it has been added to its IGP routing table. 

Another way of achieving this is by using the “no synchronization” command on iBGP routers, which tells them not to synchronize iBGP with internal routing protocols such as Enhanced Interior Gateway Routing Protocol (EIGRP) or Open Shortest Path First (OSPF).

Finally, another method of achieving iBGP synchronization is configuring all routers within a transit AS to run iBGP. This ensures that all routers have access to the same information and can validate routes before advertising them.

Bottom line: Using iBGP in your network

Internal Border Gateway Protocol is a powerful tool for exchanging routing information among multiple routers within an AS. It provides loop-free inter-router communication and ensures that all routers are aware of each other’s existence and can communicate without any loops occurring. 

By using iBGP synchronization, all routers in an AS can be updated with the same set of networks and subnets so that efficient data transmission can occur throughout the network.

Kihara Kimachia
Kihara Kimachia
Kihara Kimachia is a writer and digital marketing consultant with over a decade of experience covering issues in emerging technology and innovation. In addition to appearing regularly in Enterprise Networking Planet, his work has been published in many leading technology publications, including TechRepublic, eSecurity Planet, Server Watch, Channel Insider, IT Business Edge, and Enterprise Storage Forum.

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