© 2003, Cisco Systems, Inc. All rights reserved.

Building Scalable Cisco Internetworks (BSCI) Version 2.0 Instructor Name

Configuring Basic Border Gateway Protocol © 2003, Cisco Systems, Inc. All rights reserved.

Border Gateway Protocol Overview © 2003, Cisco Systems, Inc. All rights reserved.

Objectives Upon completing this lesson, you will be able to: Define Border Gateway Protocol (BGP) and describe the function of the Autonomous System Describe policy-based routing (PBR) using BGP path-vector functionality Define the characteristics of BGP List BGP message types

Upon completing this lesson, you will be able to:

Define Border Gateway Protocol (BGP) and describe the function of the Autonomous System

Describe policy-based routing (PBR) using BGP path-vector functionality

Define the characteristics of BGP

List BGP message types

BGP Autonomous Systems An AS is a collection of networks under a single technical administration. IGPs operate within an AS. BGP is used between Autonomous Systems. Exchange of loop-free routing information is guaranteed.

An AS is a collection of networks under a single technical administration.

IGPs operate within an AS.

BGP is used between Autonomous Systems.

Exchange of loop-free routing information is guaranteed.

BGP Path-Vector Routing IGPs announce networks and describe the cost to reach those networks. BGP announces pathways and the networks that are reachable at the end of the pathway. BGP describes the pathway by using attributes which are similar to metrics. BGP allows administrators to define policies or rules for how data will flow through the Autonomous Systems.

IGPs announce networks and describe the cost to reach those networks.

BGP announces pathways and the networks that are reachable at the end of the pathway. BGP describes the pathway by using attributes which are similar to metrics.

BGP allows administrators to define policies or rules for how data will flow through the Autonomous Systems.

BGP Policy-Based Routing BGP can support any policy conforming to the hop-by-hop ( AS-by-AS) routing paradigm.

BGP can support any policy conforming to the hop-by-hop ( AS-by-AS) routing paradigm.

BGP Characteristics BGP is most appropriate when at least one of the following conditions exists: An AS allows packets to transit through it to reach other Autonomous Systems (e.g., a service provider). An AS has multiple connections to other Autonomous Systems. Routing policy and route selection for traffic entering and leaving your AS must be manipulated. BGP is not always appropriate. Do not use BGP if you have one of the following conditions: Single connection to the Internet or other AS Lacks memory or processor power to handle constant updates on BGP routers Limited understanding of route filtering and BGP path selection process Low bandwidth between Autonomous Systems

BGP is most appropriate when at least one of the following conditions exists:

An AS allows packets to transit through it to reach other Autonomous Systems (e.g., a service provider).

An AS has multiple connections to other Autonomous Systems.

Routing policy and route selection for traffic entering and leaving your AS must be manipulated.

BGP is not always appropriate. Do not use BGP if you have one of the following conditions:

Single connection to the Internet or other AS

Lacks memory or processor power to handle constant updates on BGP routers

Limited understanding of route filtering and BGP path selection process

Low bandwidth between Autonomous Systems

BGP Characteristics BGP is a distance-vector protocol with the following enhancements: Reliable updates : BGP runs on top of TCP (port 179) Incremental, triggered updates only Periodic keepalive messages to verify TCP connectivity Rich metrics (called path vectors or attributes) Designed to scale to huge internetworks (e.g., the Internet)

BGP is a distance-vector protocol with the following enhancements:

Reliable updates : BGP runs on top of TCP (port 179)

Incremental, triggered updates only

Periodic keepalive messages to verify TCP connectivity

Rich metrics (called path vectors or attributes)

Designed to scale to huge internetworks (e.g., the Internet)

BGP Databases Neighbor table List of BGP neighbors BGP forwarding table/database List of all networks learned from each neighbor Can contain multiple pathways to destination networks Database contains BGP attributes for each pathway IP routing table List of best paths to destination networks

Neighbor table

List of BGP neighbors

BGP forwarding table/database

List of all networks learned from each neighbor

Can contain multiple pathways to destination networks

Database contains BGP attributes for each pathway

IP routing table

List of best paths to destination networks

BGP Message Types BGP defines the following message types: Open Includes holdtime and BGP router ID Keepalive Update Information for one path only (could be to multiple networks) Includes path attributes and networks Notification When error is detected BGP connection is closed after sent

BGP defines the following message types:

Open

Includes holdtime and BGP router ID

Keepalive

Update

Information for one path only (could be to multiple networks)

Includes path attributes and networks

Notification

When error is detected

BGP connection is closed after sent

Summary This lesson presented these key points: Routing protocols are categorized as either interior or exterior: IGPs exchange information within an AS. EGPs connect between Autonomous Systems, for example, BGP. BGP supports any policy that conforms to the hop-by-hop routing paradigm, which makes it highly applicable as an inter-AS routing paradigm. BGP is categorized as an advanced distance-vector protocol, but is actually a path-vector protocol that performs differently than other standard distance-vector protocols.

This lesson presented these key points:

Routing protocols are categorized as either interior or exterior:

IGPs exchange information within an AS.

EGPs connect between Autonomous Systems, for example, BGP.

BGP supports any policy that conforms to the hop-by-hop routing paradigm, which makes it highly applicable as an inter-AS routing paradigm.

BGP is categorized as an advanced distance-vector protocol, but is actually a path-vector protocol that performs differently than other standard distance-vector protocols.

Summary (Cont’d) After BGP peers initially exchange their full BGP routing table, incremental updates are sent only if network topology changes.

After BGP peers initially exchange their full BGP routing table, incremental updates are sent only if network topology changes.