The easiest way to differentiate between the two is BGP provides connectivity between different autonomous systems (ASes) -- another way to describe a network -- while EIGRP provides connectivity among networks contained within an AS. One example of an AS is an organization that has multiple independent networks that need to be connected so they can access the internet. EIGRP provides the interior data routing, while BGP establishes routing to an outside, or exterior, network, particularly the internet.
Interior networks route data using the EIGRP protocol, while communications with exterior ASes and the internet use BGP.
Figure 1 depicts how BGP and EIGRP might operate in a configuration of networks.
Border Gateway Protocol
BGP is a well-established and widely used protocol. It uses TCP port 179 to communicate with other routers, and networks are typically configured in a mesh topology, so all entities connect to each other, as shown in Figure 2.
Each independent network -- or AS -- exchanges routing data with another network when determining how to connect with specific IP ranges in that AS.
BGP differs from EIGRP in that it is an exterior gateway protocol for routing among separate networks over which there is no administrative control. BGP is also the routing protocol used by the internet, and it provides the link from an AS to an ISP, which accesses the internet.
One important aspect of BGP is it is a dynamic routing protocol. This means it can support multiple internet connections with emergency failover capabilities, as well as load balancing over the internet links. The failover capability is essential for disaster recovery, as the loss of a primary ISP link automatically triggers failover to an alternate link to ensure the best possible route to the internet is maintained.
In practice, systems that want to communicate set up a transport protocol connection. Next, the systems exchange and confirm messages that establish the parameters of the connection. They exchange messages to open and confirm the connection parameters via a BGP routing table.
Once the connection is established, BGP works to keep the link in place, even if routing table changes occur. The initial data flow uses the entire BGP routing table. Incremental updates are sent as the routing tables change. Error messages are sent when an abnormal condition occurs. BGP connections to other networks are called external links, while BGP connections within an AS are referred to as internal links.
Enhanced Interior Gateway Routing Protocol
Originally developed by Cisco in the 1990s, EIGRP was initially restricted for use only in Cisco products. Around 2013, Cisco proposed EIGRP to the IETF as a draft protocol for review and ultimate approval as an official standard. To date, it is still in the draft mode within the IETF.
In contrast to BGP, EIGRP is an interior gateway protocol, which routes data traffic dynamically across a network that an organization fully manages, such as for business or government. EIGRP is considered a hybrid protocol because it uses features of both the following protocol types:
- distance vector routing, in which routers regularly send messages that inform neighboring routers of any changes to the network topology; and
- link-state routing, which sends messages that inform each router of the entire network topology.
Network administrators often select EIGRP for campus network routing with large and small applications alike. EIGRP is also popular for use in private networks, as it balances the criteria of speed, throughput, flexibility and scalability, and ease of use.
EIGRP uses the diffusing update algorithm, which performs the following actions:
- establishes the lowest-cost links to other network destinations that can be actively reached;
- ensures that each link is consistently loop-free even during changes in network topology; and
- ensures that each router affected by a new topology establishes a new best path to maintain loop integrity.
Comparing BCP and EIGRP
Figure 3 compares the BGP and EIGRP protocols.
In the right applications, BGP and EIGRP provide cost-effective, fast and efficient data routing. BGP is optimized for larger networks that connect to other large networks, while EIGRP is ideal for smaller network infrastructures that connect to larger networks via BGP.