Presented By: Hanping Feng

Configuring BGP With Cisco IOS Software (Part 1)

Outline

Cisco IOS softwareCisco IOS command modesBasic BGP configuration tasksSummary

Cisco IOS software Overview

internetwork operating system runs on the majority of Cisco networking equipment provides intelligent network services:

security, multicast, load-balancing...

IOS User Interfaces the command-line interface (CLI) configuration tools

AutoInstall the setup command facility the ConfigMaker and FastStep configuration

applications

Cisco IOS Command Modes

User EXEC mode Privileged EXEC mode Global configuration mode Specific configuration modes Configuration submodes configuration subsubmodes ROM Monitor Mode

Cisco IOS Command ModesEntering CLI

User EXEC (Router>)

Log in

logout

Privileged EXEC (Router#)

enable

disable

Global configuration (Router(config)#)

configure terminal

exit/end/Ctrl+Z

Interface configuration (Router(config-if)#)

interface

Router configuration (Router(config-router)#)

router

end/Ctrl+Z

End/Ctrl+Z

exitexit

Basic BGP Configuration Tasks

Enabling BGP Routing Configuring BGP Neighbors Managing Routing Policy Changes Verifying BGP Soft Reset Configuring BGP Interactions with IGPs Configuring BGP Administrative Weights Disabling AS Path Comparison

Basic BGP Configuration Tasks

Configuring BGP Route Filtering by Neighbor Configuring BGP Filtering Using Prefix Lists Configuring BGP Path Filtering by Neighbor Disabling Next-Hop Processing on BGP

Updates Configuring the BGP Version Setting the Network Weight Configuring the Multi Exit Discriminator Metric

Enabling BGP Routing

Router(config)# router bgp autonomous-system

Router(config-router)#

Effect:

Global configuration mode Router configuration mode

Number of the autonomous system to which the router belongs.Example:

Router(config)# router bgp 200

Configuring BGP Neighbors

Router(config-router)# neighbor {ip-address | peer-group-name} remote-as number

ip-address Neighbor's IP address.

peer-group-name Name of a BGP peer group.

number Autonomous system to which the neighbor belongs.

Configuring BGP Neighbors

!Router A

router bgp 109

network 131.108.0.0

network 192.31.7.0

neighbor 131.108.200.1 remote-as

167

neighbor 131.108.234.2 remote-as

109

neighbor 150.136.64.19 remote-as

99

Managing Routing Policy Changes

BGP session must be reset to valid a policy change

reset methods: hard reset soft reset

outbound soft reset inbound soft reset

configured inbound soft reset dynamic inbound soft reset

Managing Routing Policy Changes

Router# clear ip bgp {* | address | peer-group-name} [soft [in | out]]

Router(config-router)# neighbor {ip-address | peer-group-name} soft-reconfiguration inbound Example:router bgp 100neighbor 131.108.1.1 remote-as 200neighbor 131.108.1.1 soft-reconfiguration inbound clear ip bgp 131.108.1.1 soft in

soft (Optional) Soft reset.

in | out (Optional) Triggers inbound or outbound soft reset.

Verifying BGP Soft Reset

Check information about the BGP routing table

Router# show ip bgp

Check information about BGP neighbors

Router# show ip bgp neighbors [address]

Configuring BGP Interactions with IGPs

Synchronization is enabled by default.

In some cases, you do not need synchronization.

To disable synchronization:

Router(config-router)# no synchronization

Configuring BGP Administrative Weights

a cisco-defined proprietary attribute used in the path selection process local to the router to configure this attribute:

Router(config-router)# neighbor {ip-address | peer-group-name} weight weight

Configuring BGP Administrative Weights (contd.)

!Router C router bgp 300 neighbor 1.1.1.1 remote-as 100 neighbor 1.1.1.1 weight 2000 neighbor 2.2.2.2 remote-as 200neighbor 2.2.2.2 weight 1000

Disabling AS Path Comparison

RFC1771 does not include AS-path in the path selection procedure

the Cisco implementation does by default to ignore the as-path length when selecting a

route:

Router(config-router)# bgp bestpath as-path ignore

BGP Route Filtering

Three basic BGP configuration tasks involved:

Configuring BGP Route Filtering by Neighbor Configuring BGP Filtering Using Prefix Lists Configuring BGP Path Filtering by Neighbor

access-list-number: number of an access list. 1 to 99 or 1300 to 1999.

deny: denies access if the address prefix is matched.

permit: permits access if the address prefix is matched.

address-prefix: the prefix to match.

wildcard: wildcard bits. Place ones in the bit positions you want to ignore.

To apply a distribute list on a neighbor:neighbor {ip-address | peer-group-name} distribute-list access-list-number {in | out}

Prefix Filtering To define a distribute list:

access-list access-list-number {deny | permit}

address-prefix [wildcard]

in: distribute list is applied to incoming advertisements.

out: distribute list is applied to outgoing advertisements.

Prefix Filtering

!Router C router bgp 300 network 170.10.0.0 neighbor 3.3.3.3 remote-as 200 neighbor 2.2.2.2 remote-as 100neighbor 2.2.2.2 distribute-list 1 out ! access-list 1 deny 160.10.0.0 0.0.255.255access-list 1 permit 0.0.0.0 255.255.255.255

AS_path Filtering

Router(config)# ip as-path access-list access-list-number {deny | permit} as-regular-expression

Router(config-router)# neighbor {ip-address | peer-group-name} filter-list access-list-number {in | out}

as-regular-expression: a regular expression matching a specific sequence of ASes.

AS_path Filtering

!Router C neighbor 3.3.3.3 remote-as 200 neighbor 2.2.2.2 remote-as 100neighbor 2.2.2.2 filter-list 1 out !ip as-path access-list 1 deny ^200$ip as-path access-list 1 permit .*

Using A Prefix List

Router(config-router)# ip prefix-list list-name [seq seq-

value] deny|permit network/len [ge ge-value] [le le-value]

Router(config-router)# neighbor {ip-address | peer-group-name} prefix-list prefix-listname {in | out}seq-value(Optional): Specifies the sequence number for the prefix list entry.

network/len (Mandatory): The network number and length (in bits) of the network mask.

ge-value(Optional): Specifies the lesser value of a range (the "from" portion of the range description).

le-value(Optional): Specifies the greater value of a range (the "to" portion of the range description).

Example:

ip prefix-list abc deny 128.0.0.0/8 ge 23 le 24

ip prefix-list abc permit 0.0.0.0/0

neighbor 2.2.2.2 prxfix-list abc in

Disabling Next-Hop Processing on BGP Updates

Disabling Next-Hop Processing on BGP Updates!Router C

router bgp 300 neighbor 170.10.20.1 remote-as 100 neighbor 170.10.20.1 next-hop-self

Router(config-router)# neighbor {ip-address | peer-group-name} next-hop-self

Configuring the BGP Version

To prevent negotiation and force the BGP version used

Router(config-router)# neighbor {ip-address | peer-group-name} version value

Example:

neighbor 1.1.1.1 version 4

Setting the Network Weight

The weight attribute is 32768 for the paths that the router originates by default.

To set the absolute weight for a network:

Router(config-router)# network address mask weight weight

Example:

network 128.0.0.0 weight 2000

Configuring the Multi Exit Discriminator Metric

Router(config-router)# default-metric

number

Summary

CLI commands are enough to configure BGP. Advanced BGP configuration tasks show the

flexibility of BGP. Route maps, aggregation, community, confederation,

peer groups...

Thanks and to be continued...