1 fundamentos bgp

Border Gateway Protocol Overview

© 2003, Cisco Systems, Inc. All rights reserved. 1

© 2003, Cisco Systems, Inc. All rights reserved. BSCI 2.0—7-2

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.


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.


BGP Policy-Based Routing

• 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 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 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


BGP Message Types

BGP defines the following message types:• Open

– Includes holdtime and BGP router ID. Version number/ Hold down timer/ As Number/ Bgp Router ID/Optional Parameter.

• Keepalive• Update (withdrawn routes/ Path attributes/ NLRI)

– 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


Border Gateway Protocol Concepts and Terminology



Peers = Neighbors

• A BGP peer, also known as a BGP neighbor, is a specific term that is used for BGP speakers that have established a neighbor relationship. • Any two routers that have formed a TCP connection to exchange BGP routing information are called peers or neighbors.


External BGP

• When BGP neighbors belong to different autonomous systems they are called EBGP.

• EBGP neighbors, by default, need to be directly connected. (EBGP multihop needs to be configured)


Internal BGP

•IGBP refers to the presence of BGP neighbors within the same AS.

• The neighbors do not have to be directly connected.


IBGP and Redistribution

• Transit routers must have full knowledge of the routes for PHB destination outside the AS.• A transit AS should run IBGP on all routers because the full Internet routing table is too large to redistribute into an IGP.


IBGP Split Horizon Rule

By default, routes learned via IBGP are never propagated to other IBGP peers.In the partial mesh scenario no ibgp between B and E so no update

Partial Mesh IGP


Routing Issues without Fully Meshed IBGP

Router C will drop the packet to network 10.0.0.0. Router C is not running IBGP; therefore, it has not learned about the route to network 10.0.0.0 from router B.

In this example, router B and router E are not redistributing BGP into OSPF.


Basic Border Gateway Protocol Operations


1 fundamentos bgp

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