Border Gateway Protocol BGP-4

1
Border Gateway ProtocolBGP-4

• BGP environment
• How BGP works
• BGP information
• BGP administration

2
Routing and Routers

• IGP (internal gateway protocols)
• Distance vector protocols
• Link state protocols
• EGP (external gateway protocols
• BGP (boarder gateway protocol)
• First version BGP 1 (1989)
• Current standard de facto (1993)

3
BGP - Requirements

• No restriction on the network deployment
• Scalability
• Loop-free routing
• Manageable routing policy
• CIDR support

4
CIDR Format

• Networks represented by tuple ltprefix(IP),
  length(bit)gt
• E.G. lt198.24.160.0, 19gt represents
• 198.24.160.0 255.255.224.0
• 1100110.00011000.10100000.00000000
• 1111111.11111111.11100000.00000000
• ?--------19 bit -------?

5
BGP Environment

• Autonomous system (AS) definition
• Single technical administration
• Single routing policy
• Unique identifying number

6
AS Topology

• Stub AS
• Multihomed AS
• Transit
• Non transit

7
How BGP Works ?

• Using TCP (port 179) as a transport layer
• Establish connection
• Establish BGP session
• Data base export / import
• Incremental updates
• Decision process

8
Information Sources

• EGP
• IGP
• Dynamic
• Semi-dynamic
• Static routes

9
BGP Data Base

• Input table (incoming routes)
• Local routing table lt-gt IP forwarding table
• Output table

10
BGP Update Structure

• Withdrawn list
• Path attribute
• Network layer Reachability information (NLRI)

11
Route Information(The Path Attribute)

• NEXT_HOP
• AS_PATH
• Local preference
• MULTI_EXIT_DISC
• Origin
• Community
• Atomic aggregation
• Aggregator

12
BGP Decision Process

• Next hop is inaccessible
• Largest local preference
• Locally originated path
• Shortest AS_PATH
• IGP before EGP
• Lowest MED
• Shortest internal path (IGP)
• Lowest router ID

13
Administrator Intervention (Policy)

• Identify route (match)
• Filtering (accept \ reject)
• Manipulate attributes (action)
• Local preference
• As path

14
AS_PATH Manipulation
15
Conclusion

• Topology restriction
• Scalability
• Loop-free routing
• CIDR
• Manageable

• Segregation to ASs
• Support the current internet deployment
• The path attribute
• BGP syntax
• Static routes, Filtering policy, attribute
  manipulation