The Border Gateway Protocol (BGP) is a crucial routing protocol that enables communication between different networks on the internet by exchanging network reachability information. It operates with autonomous system (AS) numbers and employs path vector mechanisms for route selection and loop detection, facilitating efficient and resilient data traffic routing. BGP supports both external and internal peering configurations and incorporates various attributes to optimize path selection for route management within and across networks.

1. Broder Gateway Protocol (BGP)Broder Gateway Protocol (BGP)
“Border gateway protocol is how the internet routes traffic”
1969 – First Internet MSG (Login->Lo), 1984 – EGP when cisco Found, 1988 – RIP, 1990 & 1995 - BGP
K.Satheesh
CCIE R/S
Senior Network Consultant

2. At the End of this Presentation
Will be able to understand
• What is BGP?
• Why BGP?
• BGP Peer Relationships
• Configuration of BGP
• BGP attributes and Path Selection
• BGP use cases

3. Why Routing protocol ?
• Routing: Enable communication between two
different Networks.
• To Exchange network reachability information
• To have resiliency when failure occur
• Have multipath and optimize path
• Etc ..

4. What is BGP
• Broder Gateway Protocol – BGP
• Defined by IETF RFC 1771
• Work Based on Autonomous System No – AS#
– 2 Byte : 2^16 -> 0 – 65535
– 4 Byte : 2^32 -> 0 - 4294967295
• Version 4 is the latest for both IPv4 and IPv6
• It works on top of TCP

5. AS# Ranges
2 Byte AS till 2009 / 0 to 65535
• 0: reserved.
• 1-64495: public AS numbers.
• 64496 – 64511 – reserved to use in documentation.
• 64512 – 65534 – private AS numbers.
• 65535 – reserved
4 Byte AS after 2009 / 65536 to 4294967295
• Private: 4200000000 - 4294967294
4200000000 - 4294967294
4200000000 - 4294967294

6. BGP
• Path Vector Protocol
• Incremental Updates
• Many options for policy enforcement
• Classless Inter Domain Routing (CIDR)
• Widely used for Internet backbone
• Autonomous systems

7. BGP Routing Table Size

8. Autonomous System - AS
AS 100
• Collection of networks with same routing policy
• Single routing protocol
• Usually under single ownership, trust and
administrative control

9. Path Vector and AS-PATH
• Path Vector Protocol – Define a route as parting
between destination and it’s attributes.
12.6.126.0/24 207.126.96.43 1021 0 6461 7018 6337 11268 i

10. Path Vector and AS-PATH
AS 100
AS 300
AS 200
AS 500
AS 400
170.10.0.0/16 180.10.0.0/16
150.10.0.0/16
180.10.0.0/16 300 200 100
170.10.0.0/16 300 200
150.10.0.0/16 300 400
180.10.0.0/16 300 200 100
170.10.0.0/16 300 200

11. AS PATH LOOP DETECTION
AS 100
AS 300
AS 200
AS 500
170.10.0.0/16 180.10.0.0/16
180.10.0.0/16300 200 100
180.10.0.0/16100
170.10.0.0/16300 200
140.10.0.0/16300
140.10.0.0/16500 300
170.10.0.0/16500 300 200
140.10.0.0/16
180.10.0.0/16 is not announced to
AS100 as AS500 sees that it is
originated from AS100, and that
AS100 is the neighbouring AS –
loop detection in action

12. BGP General Operations
180.10.0.0/16300 200 100
180.10.0.0/16100
170.10.0.0/16300 200
140.10.0.0/16300
• Learns multiple paths via internal and external
BGP speakers
• Picks the best path and installs in
the forwarding table
• Policies applied by influencing the
best path selection

13. BGP Peering
AS
100
AS
101
AS
102
E
B D
A C
Peering
BGP speakers are called
peers

14. External BGP Peering(eBGP)
• Between BGP speakers in different AS
• Should be directly connected
• Do not run an IGP between eBGP peers
AS
100
AS
101C
A
B

15. Internal BGP Peering(iBGP)
• Topology independent
• Each iBGP speaker must peer with every
other iBGP speaker in the AS
AS
100
A
E
B
D

16. iBGP Rules
• BGP peer within the same AS
• Not required to be directly connected
• iBGP speakers need to be fully meshed
– they originate connected networks
– they do not pass on prefixes learned
from other iBGP speakers

17. BGP Messages
Type Name Functional Overview
1 OPEN Sets up and establishes BGP adjacency
2 UPDATE
Advertises, updates, or withdraws
routes
3 NOTIFICATION
Indicates an error condition to a BGP
neighbor
4 KEEPALIVE
Ensures that BGP neighbors are still
alive

18. BGP Timers
• 30s Keep Alive and 180s Hold Timer
• BGP Advertisement Timer
– 30s for eBGP
– 0s for iBGP

19. BGP States
BGP forms tcp session between routers- Peers
 Idle: Try to initiate tcp connection and listen
 Connect: tcp 3-way handshake establish
 Active: New tcp 3-wak handshake, if timer expires
 OpenSent: Send an Open message and wait for same
o Compare Ver, AS#, Authentication, IP and AUTH
 OpenConfirm : wait for Keep Alive, then move to EST
 Established: Exchange routes via Update Msgs

20. eBGP Configuration
R1(config)#router bgp 1
R1(config-router)#neighbor 11.0.0.2 remote-as 2
R2(config)#router bgp 2
R2(config-router)#neighbor 11.0.0.1 remote-as 1
192.168.1.0/24 172.16.19.0/24
R1(config)#router bgp 1
R1(config-router)#network 192.168.1.0 mask 255.255.255.0
R2(config)#router bgp 1
R2(config-router)#network 172.16.19.0 mask 255.255.255.0

21. BGP Attributes and Path Selection
• Weight : Local to Router
• Local preference : Local to AS
• AS_path
• Origin
• Multi-exit discriminator
• Community
Routes learned via BGP have associated properties that
are used to determine the best route to a destination
when multiple paths exist to a particular destination.

22. BGP Attributes and Path Selection

23. BGP Attributes and Path Selection

24. BGP Attributes and Path Selection

25. BGP Use Cases
• Multihoming / Provider redundancy
• Equipment / Port redundancy
• Peering (typically larger ASes)
• Connectivity quality (better paths)