# Border network protocol
Border Gateway Protocol (BGP) is the standardized Exterior Gateway Protocol (EGP) used to exchange routing and reachability information between different Autonomous Systems (AS) on the Internet. It is fundamentally what makes the global Internet work by determining the best routes for data transmission.
# Terms
# AS Number (Autonomous System Number)
is a globally unique identifier assigned to an Autonomous System (AS)—a large, unified group of IP networks and routers under the control of a single administrative entity (like an Internet Service Provider, a large corporation, or a university)
| Type | Length |
|---|---|
| Public ASN | 1–64495 |
| Private ASN | 64512–65534 |
| 32-bit ASN (public) | 65536–4294967295 |
Example: Google ASN (opens new window)
# AS Path (Autonomous System Path)
is a mandatory attribute in the BGP that consists of an ordered list of AS Numbers that a routing advertisement has traversed to reach a router.
Key Functions
- Loop Prevention (Primary Function)
- Best Path Selection
- Policy and Traffic Engineering
# NLRI (Network Layer Reachability Information)
NLRI is the network prefix and its length (e.g., 192.168.1.0/24).
The Prefix (Network Address) The Prefix is the network address itself, which is 192.168.1.0.
- This is the starting address of the network and is used to identify the entire subnet.
- All devices within this network will have an IP address that starts with 192.168.1.
The Length (Subnet Mask)
The Length is the /24 part, also known as the CIDR notation (Classless Inter-Domain Routing). It tells you how many bits are used for the network portion of the address.
- An IPv4 address has 32 bits. The /24 means that the first 24 bits are reserved for the network portion.
- This length directly translates to the traditional Subnet Mask of 255.255.255.0.
| Notation | Binary Representation | Meaning |
|---|---|---|
| Prefix Length | 24 bits | Network portion (fixed) |
| Host Bits | 32−24=8 bits | Host portion (available for devices) |
| Subnet Mask | 11111111.11111111.11111111.00000000 | 255.255.255.0 |
# eBGP, iBGP
- eBGP (External BGP): Used between routers in different Autonomous Systems to exchange routing information with the global Internet. These routers are typically placed at the network boundary (the "border").
- iBGP (Internal BGP): Used between routers within the same Autonomous System to ensure that routing information learned from external peers is distributed internally.
# BGP Functions
# Route Advertisement
The mechanism where a BGP router informs its neighbors about the networks it can reach using UPDATE messages
E.g: An ISP advertises the network prefix 203.0.113.0/24 to its upstream provider, telling the provider, "Send traffic for this network to me."
# Path Vector Routing
BGP is a path-vector protocol, meaning it includes the full list of Autonomous Systems (ASes) a route has traversed.
E.g: A route to a network includes the AS path: AS64512 → AS65000 → AS12345.
# Route Selection
The multi-step algorithm used by a BGP router to choose the single "best" path when it has multiple routes to the same destination.
E.g: A router receives two routes to 10.1.1.0/24. It chooses the one with the highest Local Preference attribute.
# Policy-based Routing
The ability to influence the path selection based on business agreements, cost, and administrative rules, rather than technical metrics like speed or hop count.
E.g: A company, multihomed with two ISPs, sets a Local Preference to send all outbound traffic through its primary, cheaper ISP.
# Loop Prevention
The primary mechanism to prevent routing loops by checking the AS_PATH attribute.
A router in AS65000 receives a route with the path AS123→AS65000→AS456. It discards the route because its own ASN (65000) is in the path.
# Aggregation (Route Summarization)
Combining several specific routes into one broader, summarized route to reduce the size of the global routing table.
E.g: Instead of advertising 203.0.113.0/24, 203.0.114.0/24, and 203.0.115.0/24, a router advertises a single aggregated route: 203.0.112.0/21.
# Route Withdrawal
The process of notifying neighbors that a previously advertised route is no longer valid or reachable.
E.g: If a directly connected network goes down, the router sends an UPDATE message with a withdrawal notice for that network prefix.
# Next-Hop Attribute
Identifies the IP address of the adjacent router that should receive the traffic for the advertised network.
E.g: A BGP route for 198.51.100.0/24 specifies the next hop as 192.0.2.1, which is the neighbor's interface.
# Route Filtering
Applying rules to accept (filter in) or deny (filter out) specific routes being exchanged with a neighbor.
E.g: An ISP uses an inbound filter to reject any route advertisements from a customer that are longer than a /24 to protect the network from mistakes.
# Route Reflector / Confederation
Mechanisms used within a large single Autonomous System (iBGP) to scale the routing fabric and reduce the number of required BGP peerings.
E.g: In an AS with 50 routers, using a Route Reflector eliminates the need for 1,225 direct iBGP sessions, as each router only peers with the reflector.