1. BGP
Overview
BGP concepts and operation

2. What Is BGP? BGP is the core routing protocol within the Internet
BGP is a path-vector protocol used for interdomain routing.
BGP views the Internet as a collection of autonomous systems.
AS 65502
BGP
BGP
AS 65501
BGP
AS 65504
BGP
BGP
AS 65503
BGP is the core routing protocol within the Internet
BGP Is an IETF standard defined in RFC 4271 (supersedes RFC 1771).

3. When Should I Use BGP?
BGP is typically used in large enterprise environments where multiple ISP connections exist, and in all service provider environments
ISP A
AS 65502
Customer A
Single-homed customers typically use a default route to the Internet.
BGP
AS 65501
BGP
Customer B
BGP
Static Routing
AS 65503
Multihomed customers use BGP to control inbound and outbound traffic.
ISP B

4. BGP Peers BGP peers can reside in different ASs or the same AS
Peers in different ASs use the external session type (EBGP)
Peers in the same AS use the internal session type (IBGP)
AS 65502
IBGP is not used because a single BGP speaker exists.
IBGP is used because multiple BGP speakers exist.
IGP
IBGP
EBGP
EBGP
IGP
EBGP
IGP
EBGP
EBGP
AS 65501
AS 65504
IGP
IBGP
AS 65503

5. Established Neighbors
BGP Peers
BGP peering sessions are manually defined and rely on TCP connections
No automatic neighbor discovery
BGP Neighbor States
TCP Connectivity
BGP Connectivity
Idle
OpenSent
Connect
OpenConfirm
Active
Established R1 R2
TCP Connectivity
BGP Connectivity
Established Neighbors

6. High-Level BGP Operation
ISP B
(AS 65002)
ISP A
(AS 65001)
ISP C
(AS 65003)
Static default route to ISP A
Static route to Customer A
Customer A is single-homed to ISP A and uses /24 subnet, which was assigned by ISP A
Customer B
(AS 65501)
Customer A

7. Static route for 172.20.21.0/24 to Customer A
ISP A’s Network
I can reach /16 R3 R2
ISP A
(AS 65001)
ISP C
(AS 65003) R4 R1
Customer A
I can reach /24
Static route for /24 to Customer A
Note: All BGP routes start as something other than BGP routes.

8. ISP A’s Aggregate
/16 is reachable through AS and AS 65001
/16 is reachable through AS 65001
/16 is reachable through AS 65003, AS and AS 65001
ISP B
(AS 65002)
ISP A
(AS 65001)
ISP C
(AS 65003)
ISP A advertises an aggregate of /16 through BGP to ISP B
Customer B
(AS 65501)
Customer A

9. Customer B’s Aggregate
/20 is reachable through AS 65002, AS and AS 65501
/20 is reachable through AS and AS 65501
ISP B
(AS 65002)
ISP A
(AS 65001)
ISP C
(AS 65003)
/20 is reachable through AS 65501
Default static route
Customer B advertises its /20 network through BGP to ISP C
Customer B
(AS 65501)
Customer A

10. Customer B Connects to ISP B
ISP B chooses the best path and advertises only that path
/20 is reachable through AS and AS 65501
/20 is reachable through AS and AS 65501
ISP B
(AS 65002)
ISP A
(AS 65001)
ISP C
(AS 65003)
/20 is reachable through AS 65501
Default static route
Customer B advertises its /20 network through BGP to ISP B and ISP C
Customer B
(AS 65501)
Customer A

11. BGP Commands 1-3 router bgp autonomous-system
Enables the BGP routing protocol

12. Establishing BGP Neighbors
neighbor ip-address remote-as autonomous-system
Activates a BGP session
Used for both external and internal neighbors

13. BGP network command network network-number
Allows BGP to advertise an IGP route if it is already in the IP table
This command differs from the network command in OSPF, RIP and EIGRP in that it does not activate the protocol on an interface