Configuring BGP
With Cisco IOS Software (Part 1)
Presented By: Hanping Feng
Outline
 Cisco IOS software
 Cisco IOS command modes
 Basic BGP configuration tasks
 Summary
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 Modes
Entering CLI
Log in
logout
User EXEC (Router>)
enable
end/Ctrl+Z
configure terminal
disable
Interface configuration
(Router(config-if)#)
Privileged EXEC (Router#)
exit/end/Ctrl+Z
Global configuration (Router(config)#)
interface
exit
End/Ctrl+Z
router

exit
Router configuration
(Router(config-router)#)
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
Example:
Number of the autonomous system
to which the router belongs.
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 | peergroup-name} soft-reconfiguration inbound
soft
Example:
(Optional) Soft reset.
router bgp
in | 100
out (Optional) Triggers inbound or outbound soft
neighbor 131.108.1.1 remote-as
200
reset.
neighbor 131.108.1.1 soft-reconfiguration inbound

clear ip bgp 131.108.1.1 soft in
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 200
neighbor 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
Prefix Filtering
 To define a distribute list:
access-list access-list-number {deny | permit}
address-prefix [wildcard]
 To apply a distribute list on a neighbor:
neighbor {ip-address | peer-group-name} distributelist access-list-number {in | out}
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.
in: distribute
list is applied
to incoming advertisements.
address-prefix:
the prefix
to match.
distribute
is applied
tothe
outgoing
advertisements.
wildcard:out:
wildcard
bits.list
Place
ones in
bit positions
you want to ignore.
!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 100
neighbor 2.2.2.2 distribute-list 1 out
!
access-list 1 deny 160.10.0.0 0.0.255.255
access-list 1 permit 0.0.0.0 255.255.255.255
Prefix Filtering
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.
!Router C
neighbor 3.3.3.3 remote-as 200
neighbor 2.2.2.2 remote-as 100
neighbor 2.2.2.2 filter-list 1 out
!
ip as-path access-list 1 deny ^200$
ip as-path access-list 1 permit .*
AS_path Filtering
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
Example:
length (in bits) of the network mask.
ip prefix-list abc deny 128.0.0.0/8 ge 23 le 24
ge-value(Optional): Specifies the lesser value of a
(the abc
"from"
portion
of the range description).
ip range
prefix-list
permit
0.0.0.0/0
le-value(Optional):
Specifies
neighbor
2.2.2.2 prxfix-list
abc the
in greater value of a
range (the "to" portion of the range description).
Disabling Next-Hop Processing on
BGP Updates
Disabling Next-Hop Processing on
C
BGP Updates !Router
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 | peergroup-name} next-hop-self
Configuring the BGP Version
 To prevent negotiation and force the BGP version used
Router(config-router)# neighbor {ipaddress |
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...