BORDER GATEWAY ROUTING PROTOCOL (BGP)

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.

IGP –EGP:

IGP: RIP, IGRP, EIGRP, OSPF EGPs: EBGP

IGP operates within the Same Autonomous System

EBGP operates in between Multiple Autonomous System

BGP Features:

· Open Standard

· Exterior Gateway protocol

· Designed for Inter-AS Domain Routing

· Designed to scale huge inter-network like internet.

· Classless.

o Support FLSM, VLSM, CIDR, auto and manual summary (BGP-4)

· Updates are incremental and trigger

· Path vector protocol

It sends updates to manually defined neighbor as unicast

· BGP is application layer protocol uses TCP for reliability, TCP port 179

· Metric = Attributes

· Administrative distance

o 20 External updates ( EBGP)

o 200 Internal updates (IBGP)

When to use BGP

· A.S. working as transit A.S. (Ex. ISP)

A.S. connected to multiple A.S (when the AS is multi-homed) Data traffic path entering or leaving A.S needs to be manipulated.

When not to use BGP

If it is Single-home A.S

Lack of recourses like memory and less processing power in routers.

Limited understanding of BGP route filtering and path selection processes.

Types of ISP Connections

Ø Single Homed

Ø Dual-homed site

Ø Multi-homing

Ø Dual Multi-homed

Connecting to the Internet with BGP

Ø Default route from provider(s)

Ø Some routes + default route -

Ø All routes (full table) -

Single homed site

· A site with a single ISP connection is single-homed.

· This is fine for a site that does not depend heavily on Internet or WAN connectivity.

· Either use static routes, or advertise the site routes to the ISP and receive a default route from the ISP.

Dual-Homed

· A dual-homed site has two connections to the same ISP, either from one router or two routers.

· One link might be primary and the other backup, or the site might load balance over both links.

· Either static or dynamic routing would work in this case.

Multi-homing

· Multi-homing means connecting to more than one ISP at the same time.

· It is done for redundancy and backup if one ISP fails and for better performance if one ISP provides a better path to frequently used networks.

· This also gives you an ISP-independent solution.

· BGP is typically used with multi-homed connections.

Dual Multi-homed

· You can take multi-homing a step further and be dual-multi-homed, with two connections to multiple ISPs.

· This gives the most redundancy.

· BGP is used with the ISPs and can be used internally also.

Connecting to the Internet with BGP Route reception options:

Ø Default route from provider(s)

· Easy on resources, internal traffic routed to nearest BGP router

Ø Some routes + default route -

· Allows for selection of some paths with others falling back to a default route.

Ø All routes (full table) -

· Hard on resources, but guarantees the most direct path is taken

BGP Neighbors (IBGP/EBGP)

Ø BGP neighbors are routers forming TCP connection for exchanging BGP updates.

Ø Also called as BGP Peers or BGP Speakers.

Ø Two type of BGP neighbor relationship.

IBGP (Internal BGP)

EBGP (external BGP)

BGP Databases (BGP tables)

Neighbor table

A list of all configured BGP neighbors.

Has to be manually configured using neighbor command

# show ip bgp summary

# show ip bgp neighbors

BGP forwarding table/database

A list of networks known by BGP, along with their paths and attributes.

# show ip bgp

IP routing table

List of best paths to destination networks

# Sh ip route

Configuring BGP Routing Protocol

Syntax:

Router(config)# router bgp <AS no.>

Router(config-router)# network <network ID> [mask <subnet mask>

Implementation:

Router(config) #router bgp 100

Router(config-router)#neighbor 192.168.1.2 remote-as 200