1. BGP Transit Autonomous System

2. Objectives
Upon completion of this lesson, you will be able to perform the following tasks:
Describe the functions of a transit autonomous system
List the routing protocols needed in a transit AS
Understand the differences between IBGP and EBGP
Describe the requirements of packet forwarding through transit AS
Understand common transit AS design
Configure, monitor and troubleshoot BGP in transit autonomous system
Lesson Aim
<Enter lesson aim here.>

3. Transit Autonomous System Functions
© 2001, Cisco Systems, Inc.
BGP Transit Autonomous System-3

4. Objectives
Upon completion of this section, you will be able to perform the following tasks:
List the functions of a transit autonomous system
Describe the external route propagation through transit AS
Explain the need for internal BGP inside the transit AS
Explain the need for deploying IBGP on all core routers
Lesson Aim
<Enter lesson aim here.>

5. Transit Autonomous System Tasks
Rtr-A
Rtr-B
R-12
AS 12
AS 14
Rtr-C
AS 42
Rtr-D
R-14
Propagate routes between remote Autonomous Systems
Route packets between remote networks

6. Route Propagation AS 42 AS 12 AS 14 R-14 R-12 Rtr-A Rtr-B Rtr-D Rtr-C
IBGP session must be established between transit AS border routers to propagate EBGP routes
Routes between autonomous systems are always exchanged via External BGP (EBGP)
The only protocol that can transport all BGP attributes across the backbone is BGP inside autonomous system, called Internal BGP (IBGP)

7. Packet Forwarding in an Autonomous System
AS 42
AS 12
AS 14
R-14
R-12
Rtr-A
Rtr-B
Rtr-D
Rtr-C
How will RTR-C forward the packet?
Rtr-D forwards the packet toward Rtr-A as dictated by an IBGP-learned entry in its IP routing table
Packet from AS 14 toward AS 12 is received by Rtr-D
Conclusion#1: Rtr-C needs external routes for proper packet forwarding
Conclusion#2: Rtr-C must receive BGP routes

8. Packet Forwarding in an Autonomous System
All core routers must have all external routes
Core routers must receive BGP routes
Redistribution of BGP routes into IGP is not scalable
Default routing is not applicable in transit AS core

9. Summary
After completing this section, you should be able to perform the following tasks:
List the functions of a transit autonomous system
Describe the external route propagation through transit AS
Explain the need for internal BGP inside the transit AS
Explain the need for deploying IBGP on all core routers

10. Review Questions
What are the main functions of a transit Autonomous System?
How are external routes exchanged between Autonomous Systems?
How are the BGP routes propagated across an autonomous system?
Why do you have to run BGP on all core routers in an autonomous system?
Why is the redistribution of BGP routes into IGP not advisable?

11. Internal BGP www.cisco.com © 2001, Cisco Systems, Inc.
BGP Transit Autonomous System-11

12. Objectives
Upon completion of this section, you will be able to perform the following tasks:
List the differences between internal BGP and external BGP
Describe the AS path processing in internal BGP
Explain the need for BGP split horizon and its implications
Understand the next-hop processing in internal BGP and its implications
Lesson Aim
<Enter lesson aim here.>

13. AS Path Processing in IBGP
X 12 …
Network X is announced as coming from AS 12
AS 42
AS 12
AS 14
R-14
R-12
Rtr-A
Rtr-B
Rtr-D
Rtr-C
IBGP session
X 12 …
AS path as received from external neighbor is not changed on IBGP sessions
X …
Local AS number is prepended to AS path on external BGP sessions
EBGP session

14. BGP Split Horizon
X 12 …
X 12 …
No BGP attributes are changed in IBGP updates, making loop detection impossible. Split horizon is needed to prevent BGP routing loops.
Rtr-A
Rtr-B
IBGP session
R-12
AS 12
AS 14
EBGP session
Rtr-C
AS 42
Rtr-D
R-14
To prevent loops, incoming IBGP update is not propagated to other IBGP peers
Result: Full mesh of IBGP sessions is required for proper IBGP update propagation

15. IBGP Full-mesh
AS 42
AS 12
AS 14
R-14
R-12
Rtr-A
Rtr-B
Rtr-D
Rtr-C
Full mesh of IBGP sessions has to be established between all BGP-speaking routers in the AS for proper IBGP route propagation
The IBGP full-mesh is only a logical mesh of TCP sessions, physical full mesh is not required

16. Incoming IBGP update is further propagated to next AS
IBGP Full-mesh
IBGP update
Incoming EBGP update is directly propagated from ingress router to all BGP-speaking routers in the AS
AS 42
AS 12
AS 14
R-14
R-12
Rtr-A
Rtr-B
Rtr-D
Rtr-C
EBGP update
EBGP update
Incoming IBGP update is further propagated to next AS

17. Physical connections (for example, WAN links)
IBGP Neighbors
AS 42
AS 12
AS 14
R-14
R-12
Rtr-A
Rtr-B
Rtr-D
Rtr-C
IBGP session
EBGP session
Physical connections (for example, WAN links)
Due to IBGP full-mesh requirements, IBGP neighbors are usually not directly connected
Which interfaces shall you choose as the source and destination addresses of IBGP TCP sessions

18. IBGP Neighbor Sessions
Always run your IBGP sessions
between loopback interfaces
IBGP sessions can always be established, even if some physical interfaces are down
IBGP sessions are stable - physical interface failure will not tear down IBGP session
There is no BGP recovery after a failure inside the transit AS
IGP will reestablish the path between loopback interfaces
IBGP sessions are not affected

19. IBGP Next-hop Processing
IP address
Network X is announced with the next-hop
X ( )
Next-hop is not changed on IBGP sessions
X ( )
AS 42
AS 12
AS 14
R-14
R-12
Rtr-A
Rtr-B
Rtr-D
Rtr-C
Next-hop is set to local IP address on EBGP sessions
X ( )

20. Transit Network Using External Next-hops
All EBGP peers must be reachable by all BGP-speaking routers within the AS
EBGP next hops shall be announced using IGP:
Redistribute connected interfaces into IGP at the edge routers or
Include links to EBGP neighbors into IGP and make them passive interfaces

21. Transit Network Using Edge Routers as Next-hops
Alternate design: Next-hop processing is modified at the edge routers
Edge routers announce themselves as the next-hop in IBGP updates
No redistribution of external subnets is necessary
This design might result in suboptimal routing if you have multiple paths to a neighbor AS
Use default next-hop processing if at all possible

22. Change the Next-hop Processing at Edge Routers
neighbor ip-address next-hop-self
router(config-router)#
Bypass the BGP next-hop processing and announce the local IP address as the BGP next hop in outgoing updates sent to the specified neighbor
Has to be set on all IBGP neighbor to fully bypass IBGP next-hop processing

23. Next-hop-self Example
IP address
IBGP sessions are running from loopback address
Next-hop is set to the loopback address when next-hop-self is used
X ( )
AS 42
AS 12
AS 14
R-14
R-12
Rtr-A
Rtr-B
Rtr-D
Rtr-C
X ( )
Next-hop is set to local IP address on EBGP sessions
X ( )

24. Differences Between EBGP and IBGP Sessions
No BGP attributes are changed in IBGP updates
Due to BGP split horizon, routes learned from IBGP peer are not advertised to other IBGP peers
Local-preference and MED attributes are only propagated over IBGP sessions
EBGP peers are directly connected, IBGP peers are usually distant
Route selection rules slightly prefer EBGP routes

25. Route Selection Slightly Favors EBGP Routes
Identical routes are received from internal and external peer
External route is preferred
Whenever identical routes are received from IBGP and EBGP peers, the route from EBGP peer is preferred

26. Summary
After completing this section, you should be able to perform the following tasks:
List the differences between internal BGP and external BGP
Describe the AS path processing in internal BGP
Explain the need for BGP split horizon and its implications
Understand the next-hop processing in internal BGP and its implications

27. Review Questions How does BGP split horizon work?
Why do we need BGP split horizon?
What are the implications of BGP split horizon?
Why are IBGP neighbors usually distant?
What is the recommended way to run IBGP sessions?
How is the BGP next-hop changed inside an autonomous system?
What are the implications of IBGP next-hop processing on the network design?
How can you influence IBGP next-hop processing?
List 3 major differences between EBGP and IBGP
Why are EBGP routes preferred over equivalent IBGP routes?

28. Packet Forwarding in Transit Autonomous Systems
© 2001, Cisco Systems, Inc.
BGP Transit Autonomous System-28

29. Objectives
Upon completion of this section, you will be able to perform the following tasks:
Explain the packet forwarding requirements in a transit AS
Explain recursive lookup in IOS
Explain the need for IGP in a transit backbone running BGP on all routers
Explain interactions between BGP and IGP
Lesson Aim
<Enter lesson aim here.>

30. Packet Forwarding in an Autonomous System
AS 42
AS 12
AS 14
R-14
R-12
Rtr-A
Rtr-B
Rtr-D
Rtr-C
Router on a transit path needs to know all external destinations for proper packet forwarding
All core routers must receive external routes - they must run BGP
The only scalable design, route redistribution into IGP is not scalable

31. Packet Forwarding for External Destinations
Routes learned via BGP don’t have outgoing interface associated with them in the routing table
Recursive lookup is performed to forward IP packets toward external destinations
wg3pe2#show ip route
Routing entry for /16
Known via "bgp 5", distance 200, metric 0
Tag 99, type internal
Last update from :21:25 ago
Routing Descriptor Blocks:
* , from , 01:21:25 ago
Route metric is 0, traffic share count is 1
AS Hops 5
wg3pe2#show ip route
Routing entry for /32
Known via "ospf 1", distance 110, metric 1563, type intra area
Redistributing via ospf 1, rip
Advertised by rip metric 3
Last update from on Serial1/0.121, 02:09:15 ago
* , from , 02:09:15 ago, via Serial1/0.121
Route metric is 1563, traffic share count is 1
BGP next-hop
No outgoing interface
Route toward BGP next-hop
Outgoing interface

32. Recursive Lookup in IOS
Address
Prefix
AS-Path
Next hop
Communities
Other attr.
BGP table /8
42 13
37:12
...
Entries in routing table are built from BGP table Outgoing interface is never associated with a BGP route
...
...
...
...
...
--- /8
Protocol
Address
Prefix
Next-hop
Outgoing interface
IP routing
BGP
table /8
MAC header
OSPF
/24
Ethernet 0 0c
ARP cache lookup is performed
to build layer-2 header
conn.
/24
---
Ethernet 0
Recursive lookup is performed to forward the packet toward external destination
Address
Prefix
L2 header
Switching
cache
...
...
...
Lookup result is stored in the switching cache
IP address
MAC address
ARP cache
...
...

33. Recursive Lookup in IOS
Traditional IOS switching mechanisms perform recursive lookup when forwarding the first packet
Fast switching, Optimum switching
Cisco Express Forwarding (CEF) pre-computes the forwarding table
All recursive lookups are performed while the forwarding table is built

34. Routing Protocols in a Transit Autonomous System
AS 42
AS 12
AS 14
R-14
R-12
Rtr-A
Rtr-B
Rtr-D
Rtr-C
All core routers are running IBGP
With IBGP running on all core routers, is IGP still needed in the core?
IGP is needed to resolve BGP next hops and perform fast convergence after a failure in the core network

35. Routing Protocols in a Transit Autonomous System
Core routers need to run BGP and IGP
BGP shall carry all external routes
IGP shall only propagate BGP next-hops and other core subnets
All customer routes shall also be carried in BGP
Reduces IGP topology database
Removes customer-caused route flaps from IGP: IGP becomes more stable

36. Interactions Between BGP and IGP
Ideally, there would be no interaction between BGP and IGP
BGP carries external and customer routes
IGP carries only core subnets
IGP is not affected by external route flaps
BGP is not affected by failures internal to the network as long as the BGP next-hop remains reachable
The only link between BGP and IGP should be the recursive lookup

37. Interactions Between BGP and IGP
Sometimes, BGP and IGP will propagate the same route
Usually due to bad network design
In this case, routes are believed in EBGP/IGP/IBGP order based on administrative distances of the routes

38. Caveats of BGP/IGP Interaction
If an IGP route is learned through EBGP, EBGP-route will take precedence
Potential causes: Bad network design, routing problems or denial-of-service attack
Protect your IGP routes with inbound prefix-list filters at your AS edges
You should never accept information about your subnets from an external source

39. Summary
After completing this section, you should be able to perform the following tasks:
Explain the packet forwarding requirements in a transit AS
Explain recursive lookup in IOS
Explain the need for IGP in a transit backbone running BGP on all routers
Explain interactions between BGP and IGP

40. Review Questions Why do you need to run IBGP on all core routers?
What is recursive lookup in IOS?
Why do you need IGP in a transit AS?
What are the interactions between BGP and IGP in a properly designed transit AS?
Why should you transport your customer routes in BGP, not in IGP?
How does BGP react to a failure inside a transit AS?
What happens when the same route is learned via BGP and IGP?

41. Configuring Transit Backbone with IBGP
© 2001, Cisco Systems, Inc.
BGP Transit Autonomous System-41

42. Objectives
Upon completion of this section, you will be able to perform the following tasks:
Configure IBGP neighbors
Configure IBGP sessions between loopback interfaces
Configure additional BGP parameters required for successful IBGP operation
Change the administrative distance of BGP routes
Identify the scalability limitations of IBGP-based backbones
Lesson Aim
<Enter lesson aim here.>

43. Configuring IBGP Neighbors
neighbor ip-address remote-as AS-number
router(config-router)#
Configures BGP neighbor
The AS number configured determines whether the session is EBGP session (neighbor AS is different from local AS) or IBGP session (same AS number)
neighbor ip-address description text
router(config)#
Attaches optional description to a neighbor

44. Configuring IBGP Sessions Between Loopback Interfaces
neighbor ip-address update-source interface
router(config-router)#
Configures the source interface for the TCP session that carries BGP traffic
For IBGP sessions, the source interface shall be a loopback address
Source address configured on one peering router must match the destination address configured on the other - BGP session will not start otherwise
Make sure that your loopback interfaces are announced in the backbone IGP

45. Configuring Additional BGP Parameters
no synchronization
router(config-router)#
Disables synchronization between BGP and IGP
Modern Transit Autonomous Systems do not need synchronization as they don’t rely on redistribution of BGP routes into IGP
BGP synchronization has to be disabled in modern Transit AS designs on all BGP routers

46. Change the Administrative Distance of BGP Routes
distance bgp external internal local
router(config-router)#
Sets administrative distance for EBGP, IBGP and local routes
Applies only to routes received after the command has been entered (similar to filters)
Defaults:
EBGP routes have distance 20, IBGP and local routes have distance 200
Defaults are usually OK, don’t change them

47. Scalability Limitations of IBGP-based Transit Backbone
Transit backbone requires IBGP full-mesh between all core routers
Large number of TCP sessions
Unnecessary duplicate routing traffic
Two scalability solutions:
Route reflectors
BGP confederation

48. Summary
After completing this section, you should be able to perform the following tasks:
Configure IBGP neighbors
Configure IBGP sessions between loopback interfaces
Configure additional BGP parameters required for successful IBGP operation
Change the administrative distance of BGP routes
Identify the scalability limitations of IBGP-based backbones

49. Review Questions
Which IOS command is used to configure BGP session between loopback interfaces?
Which BGP parameter needs to be disabled for proper IBGP operation?
How can you change the administrative distance of BGP routes?
What are the scalability limitations of IBGP-based transit autonomous system?
Which tools can be used to overcome scalability issues?

50. Monitoring and Troubleshooting IBGP
© 2001, Cisco Systems, Inc.
BGP Transit Autonomous System-50

51. Objectives
Upon completion of this section, you will be able to perform the following tasks:
Use IOS show commands to monitor IBGP operation
Identify common IBGP design and configuration errors
Troubleshoot IBGP-based transit backbones
Lesson Aim
<Enter lesson aim here.>

52. IBGP-related IOS Show Commands
show ip bgp neighbor
router(config)#
Displays whether a neighbor is an IBGP neighbor
show ip bgp
router(config)#
Uses a special marker (i) for IBGP routes
show ip bgp prefix
router(config)#
Displays whether the prefix is an IBGP route

53. Show ip bgp neighbor Router#show ip bgp neighbor 192.168.3.101
BGP neighbor is , remote AS 3, internal link
BGP version 4, remote router ID
BGP state = Established, up for 00:56:08
Last read 00:00:08, hold time is 180, keepalive interval is 60 seconds
Neighbor capabilities:
Route refresh: advertised and received
Address family IPv4 Unicast: advertised and received
Received 82 messages, 0 notifications, 0 in queue
Sent 97 messages, 0 notifications, 0 in queue
Route refresh request: received 0, sent 0
Minimum time between advertisement runs is 5 seconds

54. Show ip bgp prefix Router#show ip bgp 197.99.1.0
BGP routing table entry for /24, version 3
Paths: (1 available, best #1)
Advertised to non peer-group peers: 99
(metric 20) from ( )
Origin IGP, metric 0, localpref 100, valid, internal, best

55. Troubleshooting IBGP Common IBGP problems: IBGP sessions won’t start
IBGP route is in the BGP table, but is not selected
IBGP route is selected, but not entered in the routing table

56. IBGP Session Startup Issues
Symptom
IBGP session does not start
Diagnose
IBGP session is run between loopbacks and update-source keyword is missing
Verification
Use debug ip tcp transactions. You should see BGP sessions coming from unexpected IP addresses

57. IBGP Session Startup Issues
Symptom
IBGP session does not start
Diagnose
Loopback interfaces are not reachable
Verification
Do extended ping between loopback addresses to verify reachability

58. IBGP Session Startup Issues
Symptom
IBGP session does not start
Diagnose
Packet filters prevent establishment of BGP sessions
Verification
Use debug ip tcp transaction and debug ip icmp to see whether the initial TCP SYN packets are rejected

59. IBGP Route Selection Issues
Symptom
IBGP route is in the BGP table, but it is never selected as the best route
Diagnose
BGP next-hop is not reachable
Verification
Use show ip bgp prefix to find the BGP next-hop
Use show ip route to verify next-hop reachability

60. IBGP Route is not Used Symptom Diagnose Verification
IBGP route is selected as the best route, but not entered into the IP routing table
Diagnose
BGP synchronization is not disabled
Verification
Disable BGP synchronization, clear the BGP sessions and re-examine the IP routing table after the BGP table becomes stable

61. Summary
After completing this section, you should be able to perform the following tasks:
Use IOS show commands to monitor IBGP operation
Identify common IBGP design and configuration errors
Troubleshoot IBGP-based transit backbones

62. Review Questions
Which IOS show command would indicate that a BGP route is an IBGP route?
List three reasons for IBGP session startup problems
List two reasons that would prevent IBGP from being used for packet forwarding

63. Summary
After completing this lesson, you should be able to perform the following tasks:
Describe the functions of a transit autonomous system
List the routing protocols needed in a transit AS
Understand the differences between IBGP and EBGP
Describe the requirements of packet forwarding through transit AS
Understand common transit AS design
Configure, monitor and troubleshoot BGP in transit AS

64. © 2001, Cisco Systems, Inc.
BGP Transit Autonomous System-64