Configuring and
Verifying Basic
BGP Operations
Connecting an Enterprise Network to an ISP Network
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-1
Planning for BGP
 Define network requirements
 Define internal connectivity
 Define external connectivity to ISP
 Gather required parameters
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-2
Requirements for Basic BGP
Configuration
 AS numbers
 Neighbors (IP addresses)
 Networks to be advertised
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-3
Steps to Configure Basic EBGP
 Define the BGP process
 Establish a EBGP neighbor relationship
 Advertise the networks
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-4
Define BGP Process and Activate EBGP
Session
R2(config)#
router bgp 65010
 Define the BGP process locally with a local AS number.
R2(config-router)#
neighbor 10.1.1.2 remote-as 65020
Activate EBGP session to the neighbor
 Remote router IP Address and AS number
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-5
Advertise Networks
Option 1:
R2(config-router)#
network 10.2.2.0 mask 255.255.255.0
network 10.4.4.0 mask 255.255.255.0
 Configure the local networks to be advertised and include them in
BGP
Option 2:
 Redistribution from IGP to BGP
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-6
BGP network Command Details
R2(config-router)#
network 192.168.1.1 mask 255.255.255.0
 The router looks for 192.168.1.1/24 in the routing table, but
cannot find it, so it will not announce anything.
R2(config-router)#
network 192.168.0.0 mask 255.255.0.0
 The router looks for 192.168.0.0/16 in the routing table.
 If the exact route is not in the table, you can add a static route to
null0 so that the route can be announced.
R2(config-router)#
network 192.168.1.0
 The router looks for a C class 192.168.1.0 network in the routing
table.
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-7
Basic EBGP Configuration
Option 1
R2#
!
<output omitted>
!
router bgp 65010
neighbor 10.1.1.2 remote-as 65020
network 10.2.2.0 mask 255.255.255.0
network 10.4.4.0 mask 255.255.255.0
!
© 2009 Cisco Systems, Inc. All rights reserved.
Option 2
R2#
!
<output omitted>
router ospf 10
network 10.2.2.0 mask 255.255.255.0
network 10.4.4.0 mask 255.255.255.0
!
router bgp 65010
neighbor 10.1.1.2 remote-as 65020
redistribute ospf
!
ROUTE v1.0—6-8
Basic IBGP and EBGP Configuration in
the Customer A Network
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ROUTE v1.0—6-9
Shutting Down a BGP Neighbor
R2(config-router)#
neighbor 10.1.1.2 shutdown
 Administratively brings down a BGP neighbor
 Used for maintenance/policy changes to prevent route flapping
R2(config-router)#
no neighbor 10.1.1.2 shutdown
 Reenables a BGP neighbor that has been administratively shut
down
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ROUTE v1.0—6-10
IBGP Peering Issue
 An IBGP neighbor relationship is established.
 What happens if the link between R3 and R4 goes down?
 Which IP address should be used to establish an IBGP session?
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ROUTE v1.0—6-11
BGP Issues with Source IP Address
Create a BGP packet:
 The destination IP address defined by the neighbor statement
 The source IP address defined by the outbound interface
The source address of the received BGP packet is
compared to list of neighbor statements:
 If a match is found in the list of neighbors, a relationship is
established.
 If no match is found in the list of neighbors, the packet is ignored.
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-12
IBGP Using Loopback Addresses
 A loopback interface can be used as the source and destination IP
address of all BGP updates between neighbors.
 The neighbor update-source command is normally used only with IBGP
neighbors.
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ROUTE v1.0—6-13
IBGP Next-Hop Behavior
 IBGP does not modify next hop.
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ROUTE v1.0—6-14
BGP neighbor next-hop-self Command

Forces all updates for neighbor R4 to be advertised with this router as the
next hop—the same IP address as for the source of the BGP packet.
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ROUTE v1.0—6-15
BGP States
When establishing a BGP session, BGP goes through
the following states:
1. Idle: Router is searching the routing table to see whether a route
exists to reach the neighbor.
2. Connect: Router found a route to the neighbor and has
completed the three-way TCP handshake.
3. Open sent: Open message sent, with the parameters for the
BGP session.
4. Open confirm: Router received an agreement on the parameters
for establishing a session.
– Alternatively, the router goes into active state if no response
to open message
5. Established: Peering is established; routing begins.
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ROUTE v1.0—6-16
BGP Established and Idle States
 Idle: The router cannot find the address of the neighbor in the
routing table.
– Solution: Check for an IGP problem. Is the neighbor
announcing the route?
 Established: Proper state for BGP operations.
– Output of the show ip bgp summary command has a number
in the state column indicating the number of routes learned
from this neighbor.
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ROUTE v1.0—6-17
Example: show ip bgp neighbors
Command
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ROUTE v1.0—6-18
BGP Active State Verification
Active: The router has sent an open packet and is waiting
for a response.
 The state may cycle between active and idle.
 The neighbor may not know how to get back to this router
because of the following reasons:
– No route to the source IP address of the BGP open packet
– The neighbor is peering with the wrong address
– No neighbor statement for this router
– The AS number is misconfigured
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ROUTE v1.0—6-19
Example: BGP Active State Verification
AS number misconfiguration:
 At the router with the wrong remote AS number:
– %BGP-3-NOTIFICATION: sent to neighbor 172.31.1.3 2/2
(peer in wrong AS) 2 bytes FDFC
– FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF 002D 0104 FDFC
00B4 AC1F 0203 1002 0601 0400 0100 0102 0280 0002 0202 00
 At the remote router:
– %BGP-3-NOTIFICATION: received from neighbor 172.31.1.1 2/2
(peer in wrong AS) 2 bytes FDFC
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ROUTE v1.0—6-20
Example: BGP Peering
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ROUTE v1.0—6-21
BGP Neighbor Authentication
 BGP authentication uses MD5
 Configure a key—password; router generates a message digest
(is sent), or hash, of the key (is not sent) and the message
 Router generates and checks the MD5 digest of every segment
sent on the TCP connection
 Router authenticates the source of each routing update packet
that it receives
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ROUTE v1.0—6-22
Example: BGP Neighbor Authentication
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ROUTE v1.0—6-23
Example: BGP Configuration
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ROUTE v1.0—6-24
Example: show ip bgp Command
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ROUTE v1.0—6-25
Example: show ip bgp rib-failure
Command
 Displays networks that are not installed in the RIB and the reason
that they were not installed.
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ROUTE v1.0—6-26
Clearing the BGP Session
 When policies change, the change takes effect immediately.
 The next time that a prefix or path is advertised or received,
the new policy is used. This can take a long time for all networks.
 You must trigger an update for immediate action.
 Ways to trigger an update:
– Hard reset
– Soft reset
– Route refresh
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ROUTE v1.0—6-27
Hard Reset of BGP Sessions
A BGP session makes the transition from established to
idle; everything must be relearned.
R2#
clear ip bgp *
 Resets all BGP connections with this router.
 The entire BGP forwarding table is discarded.
R2#
clear ip bgp 10.1.1.2
 Resets only a single neighbor.
 Less severe than a clear ip bgp * command.
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ROUTE v1.0—6-28
Soft Reset Outbound
R2#
clear ip bgp 10.1.1.2 soft out
 Routes learned from this neighbor are not lost.
 This router resends all BGP information to the neighbor without
resetting the connection.
 This option is highly recommended when you are changing the
outbound policy.
 The soft out option does not help if you are changing an inbound
policy.
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ROUTE v1.0—6-29
Inbound Soft Reset
R2(config-router)#
neighbor 10.1.1.2 soft-reconfiguration inbound
 This router stores all updates from this neighbor in case the
inbound policy is changed.
 The command is memory intensive.
R2#
clear ip bgp 10.1.1.2 soft in
 Uses the stored information to generate new inbound updates.
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-30
Route Refresh: Dynamic Inbound Soft
Reset
R2#
clear ip bgp {*|10.1.1.2} [soft in | in]
 Routes advertised to this neighbor are not withdrawn
 Does not store update information locally
 The connection remains established
 Introduced in Cisco IOS Software Release 12.0(2)S and 12.0(6)T
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ROUTE v1.0—6-31
Monitoring Soft Reconfiguration
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ROUTE v1.0—6-32
debug ip bgp updates Command
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ROUTE v1.0—6-33
Summary
 For a BGP configuration, the following must be defined: BGP
requirements, BGP parameters, and connectivity.
 BGP is configured with the following basic BGP commands:
router bgp autonomous-system, neighbor ip-address remoteas autonomous-system, network network-number [mask
network-mask]
 The neighbor shutdown command administratively shuts down
a BGP neighbor.
 When creating a BGP packet, the neighbor statement defines the
destination IP address and the outbound interface defines the
source IP address.
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ROUTE v1.0—6-34
Summary (cont.)
 When establishing a BGP session, the BGP goes through the
following states: idle, connect, open sent, open confirm, and
established.
 You can configure MD5 authentication between two BGP peers,
which means that each segment sent on the TCP connection
between the peers is verified.
 One EBGP neighbor exists in a single-homed environment.
 The show and debug commands are used to troubleshoot the
BGP session.
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-35
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-36