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As such BGP is quite vast and complex topic.  However, we can get simple BGP configuration done on a Cisco router such as [[Cisco CSR V1000 virtual router]] for sending and receiving routes via BGP.  To do that use following steps:
# Login into router via console / ssh.  Go to enable mode and enter enable password ('<tt>enable</tt>'), if required.  Go to configure terminal
#:<pre>
#:: config t
#:</pre>
# Decide AS number to be used for BGP.  We can use AS in range 64512 to 65534 for private purposes.  Hence we can start from 65000 and use a few AS numbers as per requirement.
# Configure BGP with chosen AS number:
#:<pre>
#:: router bgp 65000
#:</pre>
#:where 65000 is the AS number to be used in current BGP configuration
# Give a router-ID (We can use unique largest IPv4 interface IP of router to avoid conflicts with other routers)
#:<pre>
#::  bgp router-id 10.1.6.1
#:</pre>
#:where 10.1.6.1 is unique router-id for this router
# Configure address family with route-redistribution and networks
#:<pre>
#::   address-family ipv4 unicast
#::       network 0.0.0.0 mask 0.0.0.0
#::       redistribute static
#::       redistribute connected
#::       exit
#:</pre>
#:Here we are configuring ipv4 unicast based BGP router.  This router will advertize default-route 0.0.0.0/0 to its peer because of 'network 0.0.0.0' line.  This router will also advertize all static and connected routes to its neighbors
# Define neighbor routers along with their AS numbers for BGP communication
#:<pre>
#::   neighbor 10.1.5.2 remote-as 65000
#::   neighbor 10.1.5.3 remote-as 65000
#::   neighbor 10.1.6.2 remote-as 65000
#::   neighbor 10.1.6.3 remote-as 65000
#:</pre>
#: where 10.1.5.2, 10.1.5.3, etc. are BGP routers in same AS 65000 where we want BGP connection to be established for routing table exchange.  
# We can check status of BGP via various show commands:
#:<pre>
#:: show bgp all
#:: show bgp ipv4 unicast summary
#:: show bgp ipv4 unicast neighbors
#:: show bgp ipv4 unicast nexthops
#:: show bgp ipv4 received-paths
#:</pre>


Refer:
* https://networklessons.com/bgp/bgp-private-and-public-as-range
* https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus6000/sw/unicast/6_x/cisco_n6k_layer3_ucast_cfg_rel_602_N2_1/l3_bgp.html
* https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-16/irg-xe-16-book/bgp-multi-paths-redistribute.html
* https://community.cisco.com/t5/networking-documents/how-to-configure-bgp-to-send-the-default-route-to-a-neighbor/ta-p/3131059



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