1. BGP first forms a neighbor relationship with peers. BGP then learns information from its neighbors , placing that information in a table--- the BGP table. Finally, BGP analyzes the BGP table to choose the best working route for each prefix in the BGP table , placing those routes into the IP touting table.
2. At a minimum , a router participating in BGP must configure the following settings:
The routers's own ASN (router bgp asn global command)
The IP address of each neighbor and that neighbor's ASN (neighbor ip-address remote-as remote-asn BGP subcommand)
3. Routers must meet several requirements to become BGP neighbors:
A local router's ASN nust match the neighboring router's reference to that ASN with its neighbor remote-as asn command.
The BGP router IDs of the two routers must not be the same.
If configured , authentication must pass.
Each router must be part of a TCP connection with the other router, with the remote router's IP address used in that TCP connection matching what the local router configures in a BGP neighbor remote-as command.
4. BGP on a router choose its RID using following steps , in order:
Configured : bgp router-id rid router subcommand.
Highest loopback
Highest other interface
5. BGP neighbors first form a TCP connection . Later , BGP messages flow over that connection , which allows BGP routers to know when the messages arrived at the neighbor and when they did not.
6. eBGP multihop
By default, when building pachets to send to an eBGP peer , Cisco IOS sets the IP TTL field in the IP header to a value of 1, With this default action , the eBGP neighborship fails to complete when using loopback interface IP address. The reason is that when the packet with TTL=1 arrives at the neighbor, the neighbor decrement the TTL value to 0 and discards the packet.
The internal Cisco IOS packet-forwarding logic decrements the TTL before giving the packet to the loopback interface, meaning that the normal IP forwarding logic discards the packet.
Configuring the routers with the neighbor ebgp-multihop 2
7. BGP Neighbor States
Idle Connect Active Opensent Openconfirm Established
8. Verifying eBGP Neighbor Status
show ip bgp summary
show ip bgp neighbors
9. BGP Message Types
Open
Keepalive
Update
Notification
10 Neighbor Wireshark Capture
