1. Border Gateway Protocol (BGP4)

2. Border Gateway Protocol (BGP)
Routing/Forwarding basics
Building blocks
Exercises
BGP protocol basics
BGP path attributes
Best path computation

3. Border Gateway Protocol (BGP)...
Typical BGP topologies
Routing Policy
Exercises
Redundancy/Load sharing
Best current practices

4. Routing/Forwarding Basics

5. IP route lookup:Longest match routing
All 10/8 except
10.1/16
Packet: Destination
IP address: R1 R4 R2
10/8 -> R3
10.1/16 -> R4
20/8 -> R5
30/8 -> R6
…..
R2’s IP routing table
10.1/16

6. IP route lookup: Longest match routing
All 10/8 except
10.1/16
Packet: Destination
IP address: R1 R4 R2
10.1/16
10/8 -> R3
10.1/16 -> R4
20/8 -> R5
…..
R2’s IP routing table
& FF.0.0.0
is equal to
& FF.0.0.0
Match!

7. IP route lookup: Longest match routing
All 10/8 except
10.1/16
Packet: Destination
IP address: R1 R4 R2
10.1/16
10/8 -> R3
10.1/16 -> R4
20/8 -> R5
…..
R2’s IP routing table
& FF.FF.0.0
is equal to
& FF.FF.0.0
Match as well!

8. IP route lookup: Longest match routing
All 10/8 except
10.1/16
Packet: Destination
IP address: R1 R4 R2
10.1/16
10/8 -> R3
10.1/16 -> R4
20/8 -> R5
…..
R2’s IP routing table
& FF.0.0.0
is equal to
& FF.0.0.0
Does not match!

9. IP route lookup: Longest match routing
All 10/8 except
10.1/16
Packet: Destination
IP address: R1 R4 R2
10.1/16
10/8 -> R3
10.1/16 -> R4
20/8 -> R5
…..
R2’s IP routing table
Longest match, 16 bit netmask

10. IP route lookup: Longest match routing
default is /0
can handle it using the normal longest match algorithm
matches everything. Always the shortest match.

11. Forwarding Uses the routing table built by routing protocols
Performs the lookup to find next-hop and outgoing interface
Switches the packet with new encapsulation as per the outgoing interface

12. Building Blocks Autonomous System (AS) Types of Routes IGP/EGP DMZ
Policy
Egress
Ingress

13. Autonomous System (AS)
Collection of networks with same policy
Single routing protocol
Usually under single administrative control
IGP to provide internal connectivity

14. Autonomous System(AS)...
Identified by ‘AS number’
Public & Private AS numbers
Examples:
Service provider
Multi-homed customers
Anyone needing policy discrimination

15. Routing flow and packet flow
egress
announce
accept
AS2
AS 1
Routing flow
announce
accept
For networks in AS1 and AS2 to communicate:
AS1 must announce routes to AS2
AS2 must accept routes from AS1
AS2 must announce routes to AS1
AS1 must accept routes from AS2
ingress
packet flow

16. Egress Traffic Packets exiting the network Based on
Route availability (what others send you)
Route acceptance (what you accept from others)
Policy and tuning (what you do with routes from others)
Peering and transit agreements

17. Ingress Traffic Packets entering your network
Ingress traffic depends on:
What information you send and to who
Based on your addressing and ASes
Based on others’ policy (what they accept from you and what they do with it)

18. Types of Routes Static Routes Connected Routes Interior Routes
configured manually
Connected Routes
created automatically when an interface is ‘up’
Interior Routes
Routes within an AS
Exterior Routes
Routes exterior to AS

19. What Is an IGP? Interior Gateway Protocol Within an Autonomous System
Carries information about internal prefixes
Examples—OSPF, ISIS, EIGRP…

20. What Is an EGP? Exterior Gateway Protocol
Used to convey routing information between ASes
De-coupled from the IGP
Current EGP is BGP4

21. Why Do We Need an EGP? Scaling to large network
Hierarchy
Limit scope of failure
Define administrative boundary
Policy
Control reachability to prefixes

22. Interior vs. Exterior Routing Protocols
Automatic discovery
Generally trust your IGP routers
Routes go to all IGP routers
Exterior
Specifically configured peers
Connecting with outside networks
Set administrative boundaries

23. Hierarchy of Routing Protocols
Local NAP
FDDI
Other ISP’s
BGP4
BGP4/Static
BGP4 / OSPF
Customers
BGP4

24. Demilitarized Zone (DMZ)C
DMZ Network
AS 100
AS 101 B D E
AS 102
Shared network between ASes

25. Addressing - ISP Need to reserve address space for its network.
Need to allocate address blocks to its customers.
Need to take “growth” into consideration
Upstream link address is allocated by upstream provider

26. BGP Basics Terminology Protocol Basics Messages General Operation
Peering relationships (EBGP/IBGP)
Originating routes

27. Terminology Neighbor NLRI/Prefix Router-ID Route/Path
Configured BGP peer
NLRI/Prefix
NLRI - network layer reachability information
Reachability information for a IP address & mask
Router-ID
Highest IP address configured on the router
Route/Path
NLRI advertised by a neighbor

28. Protocol Basics Routing protocol used between ASes Runs over TCP
Peering A C
AS 100
AS 101 B D E
Routing protocol used between ASes
if you aren’t connected to multiple ASes, you don’t need BGP :)
Runs over TCP
Path vector protocol
Incremental update
AS 102

29. BGP Basics ... Each AS originates a set of NLRI
NLRI is exchanged between BGP peers
Can have multiple paths for a given prefix
Picks the best path and installs in the IP forwarding table
Policies applied (through attributes) influences BGP path selection

30. BGP Peers AS 100 AS 101 AS 102 BGP speakers are called peers
/24
/24 B D E
BGP speakers are called peers
AS 102
Peers in different AS’s are called External Peers
/24
eBGP TCP/IP
Peer Connection
Note: eBGP Peers normally should be directly connected.

31. BGP Peers AS 100 AS 101 AS 102 BGP speakers are called peers
/24
/24 B D E
BGP speakers are called peers
AS 102
Peers in the same AS are called Internal Peers
/24
iBGP TCP/IP
Peer Connection
Note: iBGP Peers don’t have to be directly connected.

32. BGP Peers A C
AS 100
AS 101
/24
/24 B D E
BGP Peers exchange Update messages containing Network Layer Reachability Information (NLRI)
AS 102
/24
BGP Update
Messages

33. Configuring BGP Peers AS 101 AS 100 A B C D
eBGP TCP Connection A B
/30 C D
interface Serial 0
ip address
router bgp 100
network mask
neighbor remote-as 101 .2
/24 .1 .2
interface Serial 0
ip address
router bgp 101
network mask
neighbor remote-as 100 .1 .2
/24 .1
BGP Peering sessions are established using the BGP “neighbor” configuration command
External (eBGP) is configured when AS numbers are different

34. Configuring BGP Peers AS 101 AS 100 A B C D
iBGP TCP Connection A B
/30 C D
interface Serial 1
ip address
router bgp 101
network mask
neighbor remote-as 101
interface Serial 1
ip address
router bgp 101
network mask
neighbor remote-as 101 .2
/24 .1 .2 .1 .2
/24 .1
BGP Peering sessions are established using the BGP “neighbor” configuration command
External (eBGP) is configured when AS numbers are different
Internal (iBGP) is configured when AS numbers are same

35. Configuring BGP Peers AS 100
iBGP TCP/IP
Peer Connection C
Each iBGP speaker must peer with every other iBGP speaker in the AS

36. Configuring BGP Peers AS 100 B A
iBGP TCP/IP
Peer Connection C
Loopback interface are normally used as peer connection end-points

37. Configuring BGP Peers AS 100 B A A C iBGP TCP/IP Peer Connection B A A
interface loopback 0
ip address
router bgp 100
network
neighbor remote-as 100
neighbor update-source loopback0
neighbor remote-as 100
neighbor update-source loopback0
iBGP TCP/IP
Peer Connection C

38. Configuring BGP Peers AS 100 B A A C iBGP TCP/IP Peer Connection B A
interface loopback 0
ip address
router bgp 100
network
neighbor remote-as 100
neighbor update-source loopback0
neighbor remote-as 100
neighbor update-source loopback0 A
iBGP TCP/IP
Peer Connection C

39. Configuring BGP Peers AS 100 B A A C iBGP TCP/IP Peer Connection B A A
iBGP TCP/IP
Peer Connection C
interface loopback 0
ip address
router bgp 100
network
neighbor remote-as 100
neighbor update-source loopback0
neighbor remote-as 100
neighbor update-source loopback0

40. BGP Updates — NLRI Network Layer Reachability Information
Used to advertise feasible routes
Composed of:
Network Prefix
Mask Length

41. BGP Updates — Attributes
Used to convey information associated with NLRI
AS path
Next hop
Local preference
Multi-Exit Discriminator (MED)
Community
Origin
Aggregator

42. AS-Path Attribute Sequence of ASes a route has traversed
Loop detection
Apply policy
AS 200
AS 100
/16
/16
Network Path / /
AS 300
AS 400
/16
Network Path / / /
AS 500

43. Next Hop Attribute Next hop to reach a network
AS 300
AS 200
/16 C
/30 D
/16 .1 .2 E B .2
Network Next-Hop Path /
/30 .1
Next hop to reach a network
Usually a local network is the next hop in eBGP session A
AS 100
/16
BGP Update
Messages

44. Next Hop Attribute Next hop to reach a network
AS 300
AS 200
/16 C
/30 D
/16 .1 .2 E
Network Next-Hop Path / / B .2
Next hop to reach a network
Usually a local network is the next hop in eBGP session
/30 .1 A
AS 100
/16
Next Hop updated between eBGP Peers
BGP Update
Messages

45. Next Hop Attribute Next hop not changed between iBGP peers AS 300
/16 C
/30 D
/16 .1 .2 E
Network Next-Hop Path / / B .2
/30
Next hop not changed between iBGP peers .1 A
AS 100
/16
BGP Update
Messages

46. Next Hop Attribute (more)
IGP should carry route to next hops
Recursive route look-up
Unlinks BGP from actual physical topology
Allows IGP to make intelligent forwarding decision

47. BGP Updates — Withdrawn Routes
Used to “withdraw” network reachability
Each Withdrawn Route is composed of:
Network Prefix
Mask Length

48. BGP Updates — Withdrawn Routes
AS 321
AS 123 .1
/24 .2
BGP Update
Message
Withdraw Routes
/24 x
Connectivity lost
/24
Network Next-Hop Path / /

49. BGP Routing Information Base
BGP RIB
Network Next-Hop Path
*>i / i
*>i / i
router bgp 100
network
no auto-summary
D /24
D /24
D /24
R /16
S /24
BGP ‘network’ commands are normally used to populate the BGP RIB with routes from the Route Table
Route Table

50. BGP Routing Information Base
BGP RIB
Network Next-Hop Path
*> / i
* i i
s> / i
s> / i
router bgp 100
network
aggregate-address summary-only
no auto-summary
D /24
D /24
D /24
R /16
S /24
BGP ‘aggregate-address’ commands may be used to install summary routes in the BGP RIB
Route Table

51. BGP Routing Information Base
BGP RIB
Network Next-Hop Path
*> / i
* i i
s> / i
s> / i
*> / ?
router bgp 100
network
redistribute static route-map foo
no auto-summary
access-list 1 permit
route-map foo permit 10 match ip address 1
D /24
D /24
D /24
R /16
S /24
BGP ‘redistribute’ commands can also be used to populate the BGP RIB with routes from the Route Table
Route Table

52. BGP Routing Information Base
IN Process
OUT Process
BGP RIB
Network Next-Hop Path
*>i / i
*>i / i
Update
Update
* /
>
Network Next-Hop Path /
BGP “in” process
receives path information from peers
results of BGP path selection placed in the BGP table
“best path” flagged (denoted by “>”)

53. BGP Routing Information Base
IN Process
OUT Process
BGP RIB
Network Next-Hop Path
*>i / i
*>i / i
Update
Update *
> /
Network Next-Hop Path / / /
Next-Hop changed
BGP “out” process
builds update using info from RIB
may modify update based on config
Sends update to peers

54. BGP Routing Information Base
BGP RIB
Network Next-Hop Path
*>i / i
*>i / i
*> /
D /24
D /24
D /24
R /16
S /24
Best paths installed in routing table if:
prefix and prefix length are unique
lowest “protocol distance”
B /16
Route Table

55. The ‘Bible’ & other resources
Route-views.oregon-ix.net
Internet Routing Architectures
Bassam Halabi
pg. 168 BGP Decision Process Summary

56. Types of BGP Messages OPEN UPDATE KEEPALIVE NOTIFICATION
To negotiate and establish peering
UPDATE
To exchange routing information
KEEPALIVE
To maintain peering session
NOTIFICATION
To report errors (results in session reset)

57. Internal BGP Peering (IBGP)
AS 100 A E B D
BGP peer within the same AS
Not required to be directly connected
Maintain full IBGP mesh or use Route Reflection

58. External BGP Peering (EBGP)
AS 100
AS 101 C B
Between BGP speakers in different AS
Directly connected or peering address is reachable

59. An Example… 35.0.0.0/8 AS3561 A AS200 F B AS21 C D AS101 AS675 E
Learns about /8 from F & D

60. Basic BGP commands Configuration commands Show commands
router bgp <AS-number>
neighbor <ip address> remote-as <as-number>
Show commands
show ip bgp summary
show ip bgp neighbors

61. Originating routes... Using network command or redistribution
network <ipaddress>
redistribute <protocol name>
Requires the route to be present in the routing table

62. Originating routes/Inserting prefixes into BGP
network command
network mask
ip route serial 0
matching route must exist in the routing table before network is announced!
Origin: IGP

63. Update message
Withdrawn routes
Path Attributes
Advertised routes

64. Stable IBGP peering Unlinks IBGP peering from physical topology.
Carry loopback address in IGP
router ospf <ID>
passive-interface loopback0
Unlink peering from physical topology
router bgp <AS1>
neighbor <x.x.x.x> remote-as <AS1>
neighbor <x.x.x.x> update-source loopback0

65. BGP4 continued...

66. BGP Path Attributes: Why ?
Encoded as Type, Length & Value (TLV)
Transitive/Non-Transitive attributes
Some are mandatory
Used in path selection
To apply policy for steering traffic

67. BGP Path Attributes... Origin AS-path Next-hop
Multi-Exit Discriminator (MED)
Local preference
BGP Community
Others...

68. AS-PATH Updated by the sending router with its AS number
Contains the list of AS numbers the update traverses.
Used to detect routing loops
Each time the router receives an update, if it finds its AS number, it discards the update

69. AS-Path Sequence of ASes a route has traversed Loop detection AS 200
/16
/16
/16
dropped
AS 300
AS 400
/16 / / /
AS 500

70. Cisco Systems Confidential
Next-Hop
AS 200
AS 300
/16 A B / /
AS 100
Next hop router to reach a network
Advertising router/Third party in EBGP
Unmodified in IBGP
/16
Cisco Systems Confidential
0799_04F7_c2 20

71. Third Party Next Hop More efficient, but bad idea! AS 200 AS 201 C A B/ C
peering A B
/24
AS 201
More efficient, but bad idea!

72. Next Hop... IGP should carry route to next hops
Recursive route look-up
Unlinks BGP from actual physical topology
Allows IGP to make intelligent forwarding decision

73. Local Preference Not for EBGP, mandatory for IBGP
Default value is 100 on Ciscos
Local to an AS
Used to prefer one exit over another
Path with highest local preference wins

74. Local Preference AS 100 AS 200 AS 300 AS 400 160.10.0.0/16 D E A B C
500
800 E A B
AS 400 /
> / C

75. Multi-Exit Discriminator
Non-transitive
Represented as a numeric value (0-0xffffffff)
Used to convey the relative preference of entry points
Comparable if paths are from the same AS
Path with lower MED wins
IGP metric can be conveyed as MED

76. Multi-Exit Discriminator (MED)
AS 200 C
preferred / / A B
/24
AS 201

77. Origin Conveys the origin of the prefix Three values:
IGP - Generated using “network” statement
ex: network
EGP - Redistributed from EGP
Incomplete - Redistribute IGP
ex: redistribute ospf
IGP < EGP < INCOMPLETE

78. Communities Transitive, Non-mandatory
Represented as a numeric value (0-0xffffffff)
Used to group destinations
Each destination could be member of multiple communities
Flexibility to scope a set of prefixes within or across AS for applying policy

79. Community... Service Provider AS 200 Customer AS 201 C D A B
/24
Customer AS 201

80. Synchronization C D OSPF C not running BGP (non-pervasive BGP)
1880 C A D
OSPF
690
35/8
209
C not running BGP (non-pervasive BGP)
A won’t advertise 35/8 to D until the IGP is in sync
Turn synchronization off!
Run pervasive BGP
router bgp 1880
no sync B

81. BGP Route Selection (bestpath) Only one path as the bestpath !
Route has to be synchronized
Prefix in forwarding table
Next-hop has to be accessible
Next-hop in forwarding table
Largest weight
Local to the router
Largest local preference
Spread within AS
Locally sourced
Via redistribute or network statement

82. BGP Route Selection ... Shortest AS-path length Lowest origin
number of ASes in the AS-path attribute
Lowest origin
IGP < EGP < INCOMPLETE
Lowest MED
between paths from same AS
External over internal
closest exit from a router
Closest next-hop
Lower IGP metric, closer exit from as AS
Lowest router-id
Lowest IP address of neighbor

83. BGP Route Selection... AS 100 AS 200 AS 300 AS 400 D A B
Increase AS path attribute length by at least 1 A B
AS 400’s Policy to reach AS100
AS 200 preferred path
AS 300 backup
AS 400

84. Stub AS Typically no need for BGP Point default towards the ISP
ISP advertises the stub network to Internet
Policy confined within ISP policy

85. Stub AS
AS 101 B
Provider A
AS 100
Customer

86. Multi-homed AS Only border routers speak BGP
IBGP only between border routers
Exterior routes must be redistributed in a controlled fashion into IGP or use defaults

87. Multi-homed AS
AS 100
AS 200
AS 300 D C B A
provider
provider
customer

88. Service Provider Network
IBGP used to carry exterior routes
IGP keeps track of topology
Full IBGP mesh is required

89. Common Service Provider Network
AS 100
AS 200 A H B C
AS 300
provider D F E G
AS 400

90. Routing Policy Why? How ? To steer traffic through preferred paths
Inbound/Outbound prefix filtering
To enforce Customer-ISP agreements
How ?
AS based route filtering - filter list
Prefix based route filtering - distribute list
BGP attribute modification - route maps

91. Distribute list - using IP access lists
access-list 1 deny
access-list 1 permit any
access-list 2 permit
… more access-lists as prefixes are added ...
router bgp 100
neighbor remote-as 33
neighbor distribute-list 1 in
neighbor distribute-list 2 out

92. Filter list rules Regular Expressions
RE is a pattern to match against an input string
Used to match against AS-path attribute
ex: ^3561.*100.*1$
Flexible enough to generate complex filter list rules

93. Filter list - using as-path access list
ip as-path access-list 1 permit 3561
ip as-path access-list 2 deny 35
ip as-path access-list 2 permit .*
router bgp 100
neighbor remote-as 33
neighbor filter-list 1 in
neighbor filter-list 2 out

94. Route Maps router bgp 300 neighbor 2.2.2.2 remote-as 100
neighbor route-map SETCOMMUNITY out !
route-map SETCOMMUNITY permit 10
match ip address 1
match community 1
set community 300:100
access-list 1 permit
ip community-list 1 permit 100:200

95. Route-map match & set clauses Match Clauses Set Clauses
AS-path
Community
IP address
AS-path prepend
Community
Local-Preference
MED
Origin
Weight
Others...

96. Route-map Configuration Example
neighbor <y.y.y.y> route-map AS200_IN in !
route-map AS200_IN permit 10
match community 1
set local-preference 200
ip community-list 1 permit 100:200
ISP2 H
eth
C21
C22
ISP3
neighbor <x.x.x.x> route-map AS100_IN in !
route-map AS100_IN permit 10
set community 100:200
Inbound route-map
to set community H
eth
C32
C31

97. Load Sharing & Redundancy using BGP

98. Load-sharing - single path
Router A:
interface loopback 0
ip address !
router bgp 100
neighbor remote-as 200
neighbor update-source loopback0
neighbor ebgp-multi-hop 2
ip route <DMZ-link1, link2>
Loopback 0 A
AS100
AS200
Loopback 0

99. Load Sharing - Multiple paths from the same AS
Router A:
router bgp 100
neighbor remote-as 200
neighbor remote-as 200
maximum-paths 2 A
100
200
Note:A still only advertises one “best” path to ibgp peers

100. Redundancy - Multi-homing
Reliable connection to Internet
3 common cases of multi-homing:
- default from all providers
- customer + default routes from all
- full routes from all

101. Default from all providers
Low memory/CPU solution
Provider sends BGP default
provider is selected based on IGP metric
Inbound traffic decided by providers’ policy
Can influence using outbound policy, example: AS-path prepend

102. Default from all providers
AS 200
Provider
AS 300 D E A B
AS 400 C

103. Customer + default from all providers
Medium memory and CPU solution
Granular routing for customer routes and default for the rest
Inbound traffic decided by providers’ policy
Can influence using outbound policy

104. Customer routes from all providers
AS /16
Provider
AS 200
Provider
AS 300 D E A B
C chooses shortest AS path
AS 400 C

105. Full routes from all providers
More memory/CPU
Full granular routing
Usually transit ASes take full routes
Usually pervasive BGP

106. Full routes from all providers
AS 100
AS 500
AS 200
AS 300 D E A B
C chooses shortest AS path
AS 400 C

107. Best Practices IGP in Backbone
IGP connects your backbone together, not your client’s routes
IGP must converge quickly
IGP should carry netmask information - OSPF, IS-IS, EIGRP

108. Best Practices... Connecting to a customer
Static routes
You control directly
No route flaps
Shared routing protocol or leaking
You must filter your customers info
Route flaps
BGP for multi-homed customers

109. Best Practices... Connecting to other ISPs
Use BGP4
Advertise only what you serve
Take back as little as you can
Take the shortest exit

110. Best Practices... The Internet Exchange
Long distance connectivity is expensive
Connect to several providers at a single point

111. Q & A