CCNA BGP Configuration - An Example

Configuring BGP (Border Gateway Protocol) is a core requirement for the CCNA. Unlike IGP protocols like OSPF or EIGRP, BGP is designed to exchange routing information between different Autonomous Systems (AS).

Here is a standard configuration example for a Basic EBGP (External BGP) peering session between two routers.

Network Topology

• Router 1 (AS 100): IP 1.1.1.1 on interface G0/0

• Router 2 (AS 200): IP 1.1.1.2 on interface G0/0

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1. Router 1 Configuration (AS 100)

On Router 1, you define the local AS number and identify the neighbor's IP and their AS number.

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R1(config)# router bgp 100
R1(config-router)# neighbor 1.1.1.2 remote-as 200
R1(config-router)# network 10.10.10.0 mask 255.255.255.0

• router bgp 100: Starts the BGP process for AS 100.

• neighbor 1.1.1.2 remote-as 200: Defines the peer. Since the remote AS (200) is different from the local AS (100), this is EBGP.

• network: This command tells BGP to advertise the 10.10.10.0/24 subnet. Note: This network must already exist in the router's routing table (via static route or connected interface) for BGP to advertise it.

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2. Router 2 Configuration (AS 200)

Router 2 must have a reciprocal configuration pointing back to Router 1.

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R2(config)# router bgp 200
R2(config-router)# neighbor 1.1.1.1 remote-as 100
R2(config-router)# network 192.168.1.0 mask 255.255.255.0

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3. Verification Commands

Once configured, use these three primary commands to verify the BGP state:

• show ip bgp summary: The most important command. Look at the State/PfxRcd column. If it shows a number (e.g., 1), the session is up. If it says Active or Idle, the session is down.

• show ip bgp: Shows the BGP table (all paths learned via BGP).

• show ip route bgp: Shows only the best BGP paths that have been installed into the main routing table.

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Key CCNA Tips for BGP

• BGP Split Horizon: In IBGP (Internal BGP), a router will not advertise a route learned from an IBGP peer to another IBGP peer to prevent loops.

• Next-Hop Self: When passing EBGP routes into an IBGP network, routers often need the command neighbor [IP] next-hop-self because the next-hop IP doesn't change by default when moving between AS boundaries.

• Administrative Distance: EBGP has an AD of 20, while IBGP has an AD of 200.

Checkout CCNA labs with custom configurations.