1. Connecting an Enterprise Network to an ISP Network
Configuring and Verifying Basic BGP Operations
Connecting an Enterprise Network to an ISP Network

2. Planning for BGP Define network requirements
Define internal connectivity
Define external connectivity to ISP
Gather required parameters

3. Requirements for Basic BGP Configuration
AS numbers
Neighbors (IP addresses)
Networks to be advertised

4. Steps to Configure Basic EBGP
Define the BGP process
Establish a EBGP neighbor relationship
Advertise the networks

5. 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 remote-as 65020
Activate EBGP session to the neighbor
Remote router IP Address and AS number

6. Advertise Networks Option 1: Option 2:
R2(config-router)#
network mask
network mask
Configure the local networks to be advertised and include them in BGP
Option 2:
Redistribution from IGP to BGP

7. BGP network Command Details
The router looks for /24 in the routing table, but cannot find it, so it will not announce anything.
The router looks for /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.
The router looks for a C class network in the routing table.
R2(config-router)#
network mask
R2(config-router)#
network mask
R2(config-router)#
network

8. Basic EBGP Configuration
Option 1
Option 2
R2# !
<output omitted>
router bgp 65010
neighbor remote-as 65020
network mask
network mask
R2# !
<output omitted>
router ospf 10
network mask
network mask
router bgp 65010
neighbor remote-as 65020
redistribute ospf

9. Basic IBGP and EBGP Configuration in the Customer A Network

10. Shutting Down a BGP Neighbor
R2(config-router)#
neighbor shutdown
Administratively brings down a BGP neighbor
Used for maintenance/policy changes to prevent route flapping
Reenables a BGP neighbor that has been administratively shut down
R2(config-router)#
no neighbor shutdown

11. 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?

12. 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.

13. 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.

14. IBGP Next-Hop Behavior
IBGP does not modify next hop.

15. 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.

16. BGP States
When establishing a BGP session, BGP goes through the following states:
Idle: Router is searching the routing table to see whether a route exists to reach the neighbor.
Connect: Router found a route to the neighbor and has completed the three-way TCP handshake.
Open sent: Open message sent, with the parameters for the BGP session.
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
Established: Peering is established; routing begins.

17. 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.

18. Example: show ip bgp neighbors Command

19. 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

20. Example: BGP Active State Verification
AS number misconfiguration:
At the router with the wrong remote AS number:
%BGP-3-NOTIFICATION: sent to neighbor /2 (peer in wrong AS) 2 bytes FDFC
FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF 002D 0104 FDFC 00B4 AC1F
At the remote router:
%BGP-3-NOTIFICATION: received from neighbor /2 (peer in wrong AS) 2 bytes FDFC

21. Example: BGP Peering

22. 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

23. Example: BGP Neighbor Authentication

24. Example: BGP Configuration

25. Example: show ip bgp Command

26. Example: show ip bgp rib-failure Command
Displays networks that are not installed in the RIB and the reason that they were not installed.

27. 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

28. 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
Resets only a single neighbor.
Less severe than a clear ip bgp * command.

29. Soft Reset Outbound 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.
R2#
clear ip bgp soft out

30. Inbound Soft Reset
This router stores all updates from this neighbor in case the inbound policy is changed.
The command is memory intensive.
Uses the stored information to generate new inbound updates.
R2(config-router)#
neighbor soft-reconfiguration inbound
R2#
clear ip bgp soft in

31. Route Refresh: Dynamic Inbound Soft Reset
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
R2#
clear ip bgp {*| } [soft in | in]

32. Monitoring Soft Reconfiguration

33. debug ip bgp updates Command

34. 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 remote-as 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.

35. 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.