How to configure BGP in cisco router

Configuring BGP on a Cisco router involves initiating the BGP process with router bgp <AS_number>, defining neighbors using neighbor <IP> remote-as <AS_number>, and advertising networks with the network command. For IPv4, use address-family ipv4 to activate neighbors, while ensuring necessary IP connectivity and, for EBGP, proper route policies. 

Basic BGP Configuration Steps

1. Enter Configuration Mode: configure terminal
2. Enable BGP Process: router bgp <your-AS-number>
3. Define Router ID (Optional but Recommended): bgp router-id <IP-address>
4. Configure Neighbor: neighbor <neighbor-IP> remote-as <neighbor-AS-number>
5. Activate Neighbor (IPv4):

address-family ipv4
neighbor <neighbor-IP> activate
exit-address-family
``` [1]

Advertise Networks: network <network-IP> mask <subnet-mask> 

Example: Configuring EBGP between R1 and R2

• R1 Configuration (AS 100):

router bgp 100
 bgp router-id 1.1.1.1
 neighbor 192.168.12.2 remote-as 200
 address-family ipv4
  network 10.1.1.0 mask 255.255.255.0
  neighbor 192.168.12.2 activate
 exit-address-family

R2 Configuration (AS 200):

router bgp 200
 bgp router-id 2.2.2.2
 neighbor 192.168.12.1 remote-as 100
 address-family ipv4
  network 172.16.1.0 mask 255.255.255.0
  neighbor 192.168.12.1 activate
 exit-address-family

Verification Commands

• show ip bgp summary: Checks neighbor status (look for a numeric State/PfxRcd, not Active)
• show ip bgp: Displays the BGP table
• show ip bgp neighbors <IP> advertised-routes: Shows routes sent to a neighbor 

Key Considerations

• IBGP vs EBGP: If remote-as is the same as your AS, it is IBGP; if different, it is EBGP.
• Loopback Addresses: Use Loopback interfaces for neighbor establishment to improve stability.
• Next-Hop-Self: For IBGP, use neighbor <IP> next-hop-self to ensure proper routing.
• Authentication: neighbor <IP> password <string> can be used for MD5 authentication.