1. Cours BGP-MPLS-IPV6-QOS
France Telecom
Introduction to BGP

2. Outline Reviewing Routing Operations (1-3) Interdomain Routing (1-9)
BGP Characteristics (1-13)
Single-Homed and Multihomed Customers (1-18)
Selecting a BGP Path (1-25)
Working with a Transit AS (1-39)
Interacting with IBGP and EBGP in a Transit AS (1-46)
Processing BGP Routes (1-61)

3. Reviewing Routing Operations

4. Static vs. Dynamic Routes
Static Route
Uses a route that a network administrator enters into the router manually
Dynamic Route
Uses a route that a network routing protocol adjusts automatically for topology or traffic changes

5. What Is a Dynamic Routing Protocol?
Routing protocols are used between routers to determine paths to remote networks and maintain those networks in the routing tables.
After the path is determined, a router can route a routed protocol to the learned networks

6. Autonomous Systems: IGP and EGP
An autonomous system is a collection of networks within a common administrative domain.
Interior gateway protocols operate within an autonomous system.
Exterior gateway protocols connect different autonomous systems.

7. Classes of Routing Protocols

8. Selecting the Best Route Using Metrics

9. Interdomain routing

10. Interdomain Routing
An AS is a collection of networks under a single technical administration.
An IGP is run inside an AS, resulting in optimum intra-AS routing.
An EGP is run between autonomous systems to enable routing policies and improve security.

11. Design Goals for Interdomain Routing
Scalability
The Internet has more than 300,000 routes and is still growing.
Secure routing information exchange
Routers from another AS cannot be trusted.
Tight filters are required; authentication is desirable.
Support for routing policies
Routing between autonomous systems might not always follow the optimum path.

12. Why External Routing Protocols?
Q: Assuming standard IGP route selection rules, how will the traffic between AS 1 and AS 20 flow?
Q: Will AS 2 allow this traffic?
Q: How would you solve this problem with OSPF or EIGRP?

13. BGP characteristics

14. BGP Characteristics
BGP is a distance vector protocol with enhancements:
Reliable updates
Triggered updates only
Rich metrics (1-called path attributes)
Designed to scale to huge internetworks

15. BGP Characteristics (1-Cont.)
Reliable updates
TCP 179 used as transport protocol
No periodic updates
Periodic keepalives to verify TCP connectivity
Triggered updates batched and rate-limited
Every 5 seconds for internal peer
Every 30 seconds for external peer

16. BGP Characteristics (1-Cont.)
Protocol development considerations
BGP was designed to perform well in the following areas:
Interdomain routing applications
Huge internetworks with large routing tables
Environments that require complex routing policies
Some design tradeoffs were made:
BGP uses TCP for reliable transport— CPU-intensive
Scalability is the top priority—slower convergence

17. BGP Characteristics (1-Cont.)
Common BGP uses
Customers connected to more than one service provider
Service provider networks (1-transit autonomous systems)
Service providers exchanging traffic at an exchange point (1-CIX, GIX, NAP, …)
Network cores of large-enterprise customers

18. Single-Homed and Multi-Homed Customers

19. Single-Homed Customers
Large customer or small ISP connecting to the Internet

20. Use Guidelines―Single-Homed Customers
Use BGP between the customer and the service provider in these situations:
Customers multihomed to the same service provider
Customers that need dynamic routing protocol with the service provider to detect failures
Hint: Use private AS number for these customers.
Smaller ISPs that need to originate their routes in the Internet
Use static routes in all other cases:
Static routes always simpler than BGP

21. Multihomed Customers
Customer connecting to more than one service provider

22. User Guidelines―Multihomed Customers
BGP is almost mandatory for multihomed customers.
Multihomed customers have to use public AS numbers.
Multihomed customers should use a provider-independent address space.

23. Transit Autonomous Systems
Using BGP to exchange routes is mandatory for transit autonomous systems (1-provider networks carrying customer traffic).

24. BGP Limitations
BGP and associated tools cannot express all routing policies.
You cannot influence the routing policies of downstream autonomous systems.
“BGP does not enable one AS to send traffic to a neighbor AS intending that the traffic take a different route from that taken by traffic originating in the neighbor AS.”
RFC 1771

25. Selecting a BGP Path

26. BGP Path Attributes BGP metrics are called path attributes.
Characteristics of path attributes include:
Well-known versus optional
Mandatory versus discretionary
Transitive versus nontransitive
Partial

27. Well-Known Attributes
Must be recognized by all compliant BGP implementations
Are propagated to other neighbors
Well-known mandatory attributes
Must be present in all update messages
Well-known discretionary attributes
May be present in update messages

28. Optional Attributes Optional attributes
They are recognized by some implementations (1-could be private); but expected not to be recognized by all BGP routers.
Recognized optional attributes are propagated to other neighbors based on their meaning.
Optional transitive attributes
If not recognized, marked as partial and propagated to other neighbors
Optional nontransitive attributes
Discarded if not recognized

29. BGP Attributes BGP attributes include the following: AS path *
Next-hop *
Origin *
Local preference
MED
Others
* Well-known mandatory attribute

30. AS Path Attribute
A list of autonomous systems that a route has traversed:
For example, on router B, the path to is the AS sequence ( , 64520).
The AS path attribute is well-known, mandatory.

31. Next-Hop Attribute
The IP address of the next AS to reach a given network:
Router A advertises network to router B in EBGP, with a next hop of
Router B advertises in IBGP to router C, keeping as the next-hop address.
The next-hop attribute is well-known, mandatory.

32. Origin Attribute
IGP (1-i)
network command
EGP (1-e)
Redistributed from EGP
Incomplete (1-?)
Redistributed from IGP or static
The origin attribute informs all autonomous systems in the internetwork how the prefixes were introduced into BGP.
The origin attribute is well-known, mandatory.

33. Example: Origin Attribute
RouterA# show ip bgp
BGP table version is 14, local router ID is
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> / i
* i i
*> / i
*>i / i
*> / i
* i
* i i
*> / i
* i
* i i
r> / i
r i
r i i
*> / i
<output omitted>

34. Local Preference Attribute
Paths with highest local preference value are preferred:
Local preference is used to advertise to IBGP neighbors about how to leave their AS.
The local preference is sent to IBGP neighbors only (1-that is, within the AS only).
The local preference attribute is well-known and discretionary.
Default value is 100.

35. MED Attribute
The paths with the lowest MED (1-also called the metric) value are the most desirable:
MED is used to advertise to EBGP neighbors how to exit their AS to reach networks owned by this AS.
The MED attribute is optional and nontransitive.

36. Weight Attribute (1-Cisco Only)
Paths with the highest weight value are preferred
Weight not sent to any BGP neighbors; local to this router only

37. BGP Path Selection
The BGP forwarding table usually has multiple paths from which to choose for each network.
BGP is not designed to perform load balancing:
Paths are chosen because of policy.
Paths are not chosen based on bandwidth.
The BGP selection process eliminates any multiple paths through attrition until a single best path is left.
That best path is submitted to the routing table manager process and evaluated against the methods of other routing protocols for reaching that network (1-using administrative distance).
The route from the source with the lowest administrative distance is installed in the routing table.

38. Route Selection Decision Process
Consider only (1-synchronized) routes with no AS loops and a valid next hop, and then:
Prefer highest weight (1-local to router).
Prefer highest local preference (1-global within AS).
Prefer route originated by the local router (1-next hop = ).
Prefer shortest AS path.
Prefer lowest origin code (1-IGP < EGP < incomplete).
Prefer lowest MED (1-exchanged between autonomous systems).
Prefer EBGP path over IBGP path.
Prefer the path through the closest IGP neighbor.
Prefer oldest route for EBGP paths.
Prefer the path with the lowest neighbor BGP router ID.
Prefer the path with the lowest neighbor IP address.

39. Working with a transit AS

40. Working with a Transit AS
Overview
Transit AS Tasks
External Route Propagation
Internal Route Propagation
Packet Forwarding in an AS
Core Router IBGP Requirements in a Transit AS

41. Transit AS Tasks Propagate routes between remote autonomous systems
Route packets between remote networks

42. External Route Propagation

43. Internal Route Propagation

44. Packet Forwarding in an AS

45. Core Router IBGP Requirements in a Transit AS
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.

46. BGP Transit Autonomous Systems
Interacting with IBGP and EBGP in a Transit AS

47. Outline Overview AS-Path Processing in IBGP
Multipath Load Sharing in BGP
BGP Split Horizon
IBGP Full Mesh
IBGP Neighbors
IBGP Next-Hop Processing
Transit Network Using External Next Hops
Transit Network Using Edge Routers as Next Hops
Differences Between EBGP and IBGP Sessions
Summary

48. AS-Path Processing in IBGP

49. Multipath Load Sharing in BGP

50. BGP Split Horizon
Result: Full mesh of IBGP sessions is required for proper IBGP update propagation.

51. IBGP Full Mesh
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 a logical mesh of TCP sessions only; physical full mesh is not required.

52. IBGP Full Mesh Example

53. IBGP Neighbors
Because of IBGP full-mesh requirements, IBGP neighbors are usually not directly connected.
Which interfaces should be chosen as the source and destination addresses of IBGP TCP sessions?

54. IBGP Neighbors (1-Cont.)
Always run 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 sessions.
There is no BGP recovery after a failure inside the transit AS.
The configured IGP will re-establish the path between loopback interfaces.
IBGP sessions are not affected.

55. IBGP Next-Hop Processing

56. 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 the IGP.
Redistribute connected interfaces into the IGP at the edge routers. or
Include links to EBGP neighbors into the IGP and configure them as passive interfaces.

57. 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 multiple paths to a neighboring AS exist.
Use default next-hop processing if at all possible.

58. Transit Network Using Edge Routers as Next Hops Example

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

60. Differences Between EBGP and IBGP Sessions Example
Whenever identical routes are received from IBGP and EBGP peers, the route from the EBGP peer is preferred.

61. BGP Practical
Processing BGP Routes

62. Outline Overview Receiving Routing Updates Building the BGP Table
BGP Route Selection Criteria
BGP Route Propagation
Building the IP Routing Table
Advertising Local Networks
Automatic Summarization
Summary

63. Receiving Routing Updates
Small BGP Network

64. Receiving Routing Updates (1-Cont.)
Information from the BGP tables is exchanged after adjacency establishment.

65. Building the BGP Table
All inbound updates are placed into the BGP table.

66. BGP Route Selection Criteria
Exclude routes with inaccessible next hop
Prefer highest weight (1-local to router)
Prefer highest local preference (1-global within AS)
Prefer routes that the router originated
Prefer shortest AS path (1-only length is compared)
Prefer lowest origin code (1-IGP < EGP < Incomplete)
Prefer lowest MED
Prefer external (1-EBGP) paths over internal (1-IBGP)
For IBGP paths, prefer path through closest IGP neighbor
For EBGP paths, prefer oldest (1-most stable) path
Prefer paths from router with the lowest BGP router-ID

67. BGP Route Selection Criteria (1-Cont.)
The best routes to the destination networks are selected from the BGP table.

68. BGP Route Propagation
The best BGP routes are propagated to BGP neighbors.

69. Building the IP Routing Table
The best BGP routes are copied into the IP routing table based on administrative distance.

70. Advertising Local Networks
The BGP router process keeps a list of local networks (1-defined with the network command or through redistribution).
The BGP process periodically scans the IP routing table and inserts or revokes routes from the BGP routing table based on their presence in the IP routing table.

71. Advertising Local Networks (1-Cont.)
The BGP route is revoked after the network is removed from the routing table.

72. Advertising Local Networks (1-Cont.)
The BGP route is advertised after the network appears in the routing table.