BGP, or Border Gateway Protocol, is the routing protocol used by autonomous networks to exchange information about how Internet traffic should reach different IP address ranges.
In simple terms, BGP helps independently operated networks tell one another:
“These IP addresses can be reached through my network.”
Internet Service Providers, cloud providers, datacenters, content networks and large enterprises use BGP to exchange routes across the global Internet.
What Does BGP Stand For?
BGP stands for:
Border Gateway Protocol
It is often described as the routing protocol of the Internet because it allows independently operated networks to exchange information about which IP prefixes they can reach.
Without BGP, the global Internet would not have a scalable way for thousands of autonomous networks to determine how traffic should move between them.
How Does BGP Work?
Imagine that a company operates the IPv4 prefix:
192.0.2.0/24
That prefix contains 256 total IPv4 addresses.
The network operating that prefix needs to tell the rest of the Internet how to reach it.
Through BGP, the network can advertise the prefix to neighboring networks.
Those networks can then pass the route to other networks.
Eventually, Internet networks around the world learn that traffic destined for 192.0.2.0/24 should follow a particular routing path.
What Is a BGP Route?
A BGP route is information describing how a network prefix can be reached.
A route can contain information including:
- IP prefix
- Originating ASN
- AS path
- Next hop
- Routing attributes
- Preference information
Network operators use this information to determine which route to use when multiple paths are available.
What Is an ASN?
ASN stands for:
Autonomous System Number
Each autonomous system participating in public BGP routing is identified by an ASN.
For example, a route effectively says:
“This IPv4 prefix is originated by Autonomous System X.”
Learn more about Autonomous System Numbers.
BGP and ASNs are therefore closely connected.
The ASN identifies the network.
BGP exchanges the routing information.
What Is an IPv4 Prefix?
A prefix represents a range of IP addresses.
Examples include:
/24 = 256 addresses
/23 = 512 addresses
/22 = 1,024 addresses
/21 = 2,048 addresses
Learn more about IPv4 prefix sizing.
When a network announces a prefix through BGP, it tells other networks that the prefix can be reached through that autonomous system.
What Is a BGP Neighbor?
BGP routers establish sessions with other routers known as BGP peers or neighbors.
These relationships can exist between:
- An ISP and its upstream carrier
- Two Internet providers
- An enterprise and an ISP
- Two datacenter networks
- A content provider and a carrier
- A customer and a transit provider
Once the BGP session is established, the networks can exchange routing information according to their policies.
What Is an AS Path?
The AS path is one of the most recognizable BGP attributes.
It shows the sequence of autonomous systems a route has passed through.
A simplified example might look like:
AS100 → AS200 → AS300
This information can help network operators determine how traffic is reaching a destination.
It also helps prevent routing loops.
If an autonomous system sees its own ASN already present in the path, it normally rejects the route.
Does BGP Always Choose the Shortest Path?
Not necessarily.
BGP is policy-based.
The AS path matters, but networks can consider multiple attributes when choosing routes.
Operators may prefer one path over another because of:
- Business relationships
- Transit cost
- Network performance
- Local preference
- Redundancy
- Routing policy
- Traffic engineering
This means the route with the fewest autonomous systems is not always the route a network chooses.
Why Would a Business Need BGP?
Most businesses do not need to operate BGP.
A normal Internet customer usually receives connectivity from an ISP and allows the provider to handle Internet routing.
BGP becomes more useful when an organization requires more control.
Common examples include:
Multiple Internet Providers
An organization connected to multiple ISPs can use BGP to improve redundancy and routing flexibility.
Provider-Independent Address Space
A company operating its own IP resources may need BGP to announce its prefixes.
Dedicated IPv4 Infrastructure
Organizations deploying significant IPv4 resources may require customized routing.
Datacenters and Colocation
Networks operating colocated infrastructure may need BGP relationships with one or more upstream providers.
Disaster Recovery
BGP can be part of a design that allows traffic to move between different locations or providers during an outage.
Learn more about dedicated IPv4 infrastructure.
BGP and Multi-Homing
Multi-homing means connecting a network to more than one upstream Internet provider.
For example:
Provider A
and
Provider B
can both provide paths to the same enterprise network.
If Provider A fails, BGP can potentially allow traffic to continue through Provider B.
Proper multi-homing requires careful network design.
Simply having two Internet circuits does not automatically provide seamless BGP failover.
BGP and IPv4 Leasing
A business leasing IPv4 resources does not necessarily need to operate BGP itself.
One option is for the network provider to originate and route the IPv4 space through its own ASN.
The customer then uses the addresses within the provider’s managed network.
This can simplify the deployment.
Learn more about IPv4 leasing.
Another arrangement may allow customer ASN integration, depending upon the provider, resources and technical requirements.
What Does “Originate a Prefix” Mean?
To originate a prefix means that an autonomous system announces itself as the network where that route begins.
If AS64500 originates:
192.0.2.0/24
other BGP networks learn that AS64500 is the origin for that prefix.
Routing security tools can also be used to validate whether that ASN is authorized to originate the prefix.
This is where RPKI becomes important.
What Can Go Wrong With BGP?
BGP is powerful, but incorrect configuration can cause serious problems.
Examples include:
Route Leaks
A network unintentionally advertises routes that should not be propagated to other networks.
Prefix Hijacking
An unauthorized network announces IP space belonging to another organization.
Incorrect Filtering
Bad filters can accidentally advertise or reject legitimate routes.
Loss of Connectivity
Improper route configuration can make an IP prefix unreachable.
Traffic Taking the Wrong Path
Incorrect preferences can send traffic through an undesirable provider or location.
For this reason, public BGP should generally be managed by experienced network engineers.
What Is Route Filtering?
Route filtering controls which prefixes a BGP router will accept or advertise.
Proper filtering can help prevent:
- Accidental route leaks
- Invalid prefix advertisements
- Excessively specific routes
- Unauthorized routes
Filtering is an important part of responsible Internet routing.
What Is RPKI?
RPKI stands for:
Resource Public Key Infrastructure
It allows network operators to publish cryptographically verifiable information about which ASN is authorized to originate an IP prefix.
A Route Origin Authorization, or ROA, can specify that authorization.
RPKI does not replace BGP.
It adds an important security mechanism around BGP route origination.
Managed BGP Services
Businesses do not necessarily need to operate complex routing themselves.
NetWest can evaluate qualified deployments requiring:
- BGP configuration
- Prefix routing
- ASN integration
- Route management
- Routing policy
- RPKI coordination
- Failover design
- Network troubleshooting
- Monitoring
Learn more about Managed BGP Services.