slides bgp basics...4271 external BGP are commonly abbreviated as IBGP and EBGP. 29 30 v1.0 Internal...

1

5/22/20

1 v1.0

BGP BasicsW E B I N A R C O U R S E

1

2 v1.0

Overview

• What is BGP?

• BGP Features

• BGP General Operation

• BGP Attributes Overview

• Inserting Prefixes into BGP

• “Deploying BGP” Lab Demo

• Homework !

2

2

5/22/20

3 v1.0

What is BGP?

• Border Gateway Protocol• A Routing Protocol used to exchange routing information between different

networks• Described in RFC4271

• RFC4276 gives an implementation report on BGP• RFC4277 describes operational experiences using BGP• Many updates in other RFCs, i.e. RFC8212 defining the default behavior of a BGP speaker

without policies

Interior Gateway Protocol Exterior Gateway Protocol

OSPF, IS-IS … BGP

3

4 v1.0

BGP Features

• Path Vector Protocol

• Autonomous systems

• Runs on TCP (Port 179)

• Incremental updates

• Many options for policy enforcement

• Widely used for Internet backbone

• Classless Inter Domain Routing (CIDR)

4

3

5/22/20

5 v1.0

What is Path Vector Routing Protocol

• A path vector routing protocol is used to span different autonomous systems.

5

6 v1.0



6

4

5/22/20

7 v1.0

What is an Autonomous System?

RFC1930:

• An AS is a connected group of one or more IP prefixes run by one or more

network operators which has a SINGLE and CLEARLY DEFINED routing policy.

7

8 v1.0


RFC1930:



Autonomous System Autonomous System

8

5

5/22/20

9 v1.0

Autonomous System Number (ASN)

• An AS has a globally unique number (sometimes referred to as an ASN, or

Autonomous System Number) associated with it; this number is used in both the

exchange of exterior routing information (between neighboring ASes), and as an

identifier of the AS itself.

• 2-byte only AS number range : 0 – 65535

• 4-byte only AS number range : 65,536 - 4,294,967,295

9

10 v1.0


RFC1930:



AS 64502 AS 64503

10

6

5/22/20

11 v1.0



• It defines a route as a collection of a number of AS that it passes through from

source AS to destination AS, i.e.

64500 64501 64503

• This list of AS numbers is called AS path

AS Path example:

11

12 v1.0

AS64500 AS64501

AS64502AS64503

172.16.0.0/16

172.16.0.0/16

Path Vector Routing Protocol

12

7

5/22/20

13 v1.0

AS64500 AS64501

AS64502AS64503

172.16.0.0/16

172.16.0.0/16 64503


172.16.0.0/16

13

14 v1.0

AS64500 AS64501

AS64502AS64503

172.16.0.0/16

172.16.0.0/16 64501 64503


172.16.0.0/16 64503

172.16.0.0/16

14

8

5/22/20

15 v1.0

AS64500 AS64501

AS64502AS64503

172.16.0.0/16

172.16.0.0/16 64501 64503


172.16.0.0/16 64503

172.16.0.0/16

172.16.0.0/16 64500 64501 64503

15

16 v1.0

AS64500 AS64501

AS64502AS64503

172.16.0.0/16

172.16.0.0/16 64501 64503


172.16.0.0/16 64503

172.16.0.0/16

172.16.0.0/16 64500 64501 64503 Drop route

Avoid routing loop

16

9

5/22/20

17 v1.0

AS64500 AS64501

AS64502AS64503

172.16.0.0/16

172.16.0.0/16 64501 64503


172.16.0.0/16 64503

172.16.0.0/16

172.16.0.0/16 64500 64501 64503 Drop route

Avoid routing loop

17

18 v1.0



• It defines a route as a collection of a number of AS that it passes through from

source AS to destination AS, i.e.

64500 64501 64503

• This list of AS numbers is called AS path

o used to avoid routing loop

o also used to select path to destination

AS Path example:

18

10

5/22/20

19 v1.0

BGP Message Types

• Between BGP speakers, BGP messages are sent over TCP connections, port 179.

Router1 Router2

BGP Messages

19

20 v1.0

BGP Message Types

• BGP messages are sent over TCP connections, port 179.

Message Function

OPEN Message After a TCP connection is established, the first message sent by each side is an OPEN message.

KEEPALIVE Message KEEPALIVE messages are exchanged between peers often enough not to cause the Hold Timer to expire

UPDATE MessageAn UPDATE message is used to advertise feasible routes that share common path attributes to a peer, or to withdraw multiple unfeasible routes from service

NOTIFICATION Message A NOTIFICATION message is sent when an error condition is detected. The BGP connection is closed immediately after it is sent.

20

11

5/22/20

21 v1.0

BGP Message Types


Message Function





21

22 v1.0

BGP Message Types


Message Function





22

12

5/22/20

23 v1.0

BGP Message Types


Message Function





23

24 v1.0

BGP Message Types


Message Function





24

13

5/22/20

25 v1.0

BGP Finite State Machine

Idle

Connect Active

Open Sent

Open ConfirmEstablished

25

26 v1.0


Idle

• The router is looking for a route to its neighbor

Connect

• BGP router moves from Idle to Connect state if it has found a route to its neighbor, and has started the TCP handshake

• If the TCP session successful, sends an Open message (and transitions to OpenSent)

• Else, move to Active state

Active

• A router transitions to Active state if the initial TCP connection was not successful (in Connect state)

• Restarts the TCP connection

• If successful, sends an Open message

• Else, falls back to Idle state

OpenSent

• An Open message has been sent to the neighbour

• Waiting for Open message from neighbour

• If it receives an Open message and there are no mismatches (version, source addr same as TCP addr, ASN, router-ID, TTL, md5), sends KeepAlive, moves to OpenConfirm

• Else (if mismatches/errors), sent Notification and falls back to Idle

OpenConfirm

• waiting for the initial KeepAlive

• If received, transitions to Established

• If holdtimer expires or Notification received, moves to Idle

Established

• The BGP neighbor relationship (session) is established!

• Routing information can now be exchanged

• If holdtimer expires/error, moves back to Idle

26

14

5/22/20

27 v1.0


Idle

Connect Active

Open Sent

Open ConfirmEstablished

Figure 1: The BGP Finite State Machine

27

28 v1.0

Example of BGP Neighbors

• Here is an example of one router’s IPv4 BGP neighbors:

Example on Cisco IOS

28

15

5/22/20

29 v1.0

Internal & External BGP

A peer in a different AS is referred to as an external peer, while a peer in the same AS is referred to as an internal peer. Internal BGP and external BGP are commonly abbreviated as IBGP and EBGP.4271

29

30 v1.0


AS64501

R1 R2IBGPpeer

A peer in a different AS is referred to as an external peer, while a peer in the same AS is referred to as an internal peer. Internal BGP and external BGP are commonly abbreviated as IBGP and EBGP.

4271

30

16

5/22/20

31 v1.0


AS64500AS64501

R1 R2IBGPpeerR3

EBGP peer


4271

31

32 v1.0


AS64500AS64501

R1 R2IBGPpeerR3

EBGP peer


4271

AS64504

R4

EBGP peer

32

17

5/22/20

33 v1.0


IBGP is used to:

• Carry customer networks/prefixes

• Internet routes (some or all) across the AS backbone

EBGP is used to:

• Exchange networks/routes between ASes• Aggregates and sub-

aggregates• Implement routing policies

• To manipulate inbound and outbound traffic

33

34 v1.0

EBGP Peer

• EBGP peers

o Generally directly connected!

- Session established using directly connected interface IP

- Peering address must match the TCP session!

o Else, we need a static route to reach the neighbor and change the eBGP TTL value

(default 1)

34

18

5/22/20

35 v1.0

EBGP Peer

• EBGP peers example:

AS 65000 AS 65001172.20.0.4/30

.5 .6

35

36 v1.0

EBGP Peer

• EBGP peers example:

AS 65000 AS 65001

router bgp 65000bgp router-id 172.16.0.1neighbor 172.20.0.6 remote-as 65001

address-family ipv4 unicastneighbor 172.20.0.6 activate

172.20.0.4/30.5 .6

router bgp 65001bgp router-id 172.24.0.1neighbor 172.20.0.5 remote-as 65000



36

19

5/22/20

37 v1.0

IBGP Peer

• IBGP peers

o Does not need to be directly connected

▸ IGP ensure reachability (TCP connection)

o Generally using loopback addresses

37

38 v1.0

IBGP Peer

• IBGP peers example:

AS 64512

R1 R2

38

20

5/22/20

39 v1.0

IBGP Peer

• IBGP peers example:

AS 64512

R1 R2

router bgp 64512bgp router-id 10.0.0.1neighbor 10.0.0.2 update-source Loopback0neighbor 10.0.0.2 remote-as 64512





39

40 v1.0

Sourcing IGP from Loopback for IBGP

• By default, routers use the exit-interface address as the source address for locally

originated packets (updates)

o If the BGP TCP session was established using any other interface (loopbacks) addresses, the

source address for BGP updates must match!

• In Cisco IOS, the update-source loopback command achieves this




40

21

5/22/20

41 v1.0

IBGP Operation

• Carry routes learned from outside the AS to all routers within the AS

o Fully-meshed instead of redistributing!

o Advertise routes learned from eBGP peers to all iBGP peers!

• To prevent routing loops (in a fully-meshed network)

o IBGP routers are not allowed to advertise IBGP learned routes to other IBGP peers!

41

42 v1.0

IBGP Full-mesh

router bgp 64512neighbor 10.0.0.2 remote-as 64512neighbor 10.0.0.3 remote-as 64512neighbor 10.0.0.4 remote-as 64512!

AS64512

R1

R2

R3

R4

R1:

router bgp 64512neighbor 10.0.0.1 remote-as 64512neighbor 10.0.0.3 remote-as 64512neighbor 10.0.0.4 remote-as 64512

!

R2:

Example configuration on R1 and R2


42

22

5/22/20

43 v1.0

IBGP Full-mesh

AS64512

R1

R2

R3

R4

Example configuration on R1 and R2

Another solution: Route Reflection

router bgp 64512neighbor 10.0.0.2 remote-as 64512neighbor 10.0.0.3 remote-as 64512neighbor 10.0.0.4 remote-as 64512!

R1:

router bgp 64512neighbor 10.0.0.1 remote-as 64512neighbor 10.0.0.3 remote-as 64512neighbor 10.0.0.4 remote-as 64512

!

R2:


43

44 v1.0

BGP Path Attributes

• Path attributes, some examples are shown below:

ORIGIN

AS_PATH

NEXT_HOP LOCAL_PREF

ATOMIC_AGGREGATE

COMMUNITY

MULTI_EXIT_DISC

ORIGINATOR_ID

CLUSTER_LIST

44

23

5/22/20

45 v1.0

BGP Path Attributes

• Path attributes fall into four separate categories:

Well-knownmandatory

• ORIGIN• AS_PATH• NEXT_HOP

Well-knowndiscretionary

• LOCAL_PREF• ATOMIC_AGGREGA

TE

Optionaltransitive

• AGGREGATOR• COMMUNITY

Optionalnon-transitive

• MULTI_EXIT_DISC• ORIGINATOR_ID• CLUSTER_LIST

45

46 v1.0

Example of BGP Routing Table

• Part of an example BGP routing table on one router


46

24

5/22/20

47 v1.0



AS Path and Origin

47

48 v1.0



Next Hop

48

25

5/22/20

49 v1.0



Local Preference

49

50 v1.0



MED(MULTI_EXIT_

DISC)

50

26

5/22/20

51 v1.0

Insert Prefixes into BGP

Examples in Cisco IOS Functionnetwork 192.168.1.0 mask 255.255.255.0 Add the specific route

192.168.1.0/24 into BGP routing table.

redistribute OSPF redistribute all the routes in OSPF routing table into BGP routing table.


51

52 v1.0

Inserting prefixes into BGP – network command

• Configuration Example

• A matching route must exist in the routing table before the network is announced

• Forces origin to be “IGP”

router bgp 64512bgp router-id 10.0.0.1address-family ipv4 unicastnetwork 10.0.0.0 mask 255.0.0.0

ip route 10.0.0.0 255.0.0.0 Null0

52

27

5/22/20

53 v1.0

Lab Demo

• I will demo some configuration and results of BGP on the virtual lab “Deploying

BGP (Cisco IOS)” ”

• If you also want to try, here is the information:

o Access https://academy.apnic.net/en/virtual-labs/

o Click “Launch” on the “Deploying BGP (Cisco IOS)” lab

• We will add the BGP labs on Junos OS and Mikrotik RouterOS in a month.

53

54 v1.0

Homework

• 1. Please try to complete the “Deploying BGP (Cisco IOS)” lab

o Access https://academy.apnic.net/en/virtual-labs/

o Click “Launch” on the “Deploying BGP (Cisco IOS)” lab

• 2. Complete the quizzes on “https://academy.apnic.net/en/quizz/bgp-basics/”, then

get your certificate

• Any issue? Contact us: [email protected]

54

https://academy.apnic.net/en/virtual-labs/

https://academy.apnic.net/en/virtual-labs/

https://academy.apnic.net/en/quizz/bgp-basics/

http://apnic.net

28

5/22/20

55 v1.0

Acknowledgements

• Figure 1: Adapted from Jeff Doyle and Jennifer DeHaven Carroll (2001). Routing TCP/IP, Volume

II (CCIE Professional Development). USA: Cisco Press, Figure 2-20. The BGP Finite State

Machine

55

56 v1.0

Wish you and your family stay safe and healthy!

Last but not least

56

29

5/22/20

57

57

slides bgp basics...4271 external BGP are commonly abbreviated as IBGP and EBGP. 29 30 v1.0 Internal...

Documents

Transcript of slides bgp basics...4271 external BGP are commonly abbreviated as IBGP and EBGP. 29 30 v1.0 Internal...