1. Week 11
Interdomain routing with BGP
Ethernet

2. Agenda
• Interdomain routing
• Peering links
• BGP basics
• BGP convergence

3. Interdomain routing
l Goals
l Allow to transmit IP packets along the best
path towards their destination through
several transit domains while taking into
account their routing policies of each
domain without knowing their detailed
topology
l From an interdomain viewpoint, best path
often means cheapest path
u Each domain is free to specify inside its
routing policy the domains for which it
agrees to provide a transit service and
the method it uses to select the best path
to reach each destination

4. Routing policies
l A domain specifies its routing policy by
defining on each BGP router two sets of filters
for each peer
l Import filter
u Specifies which routes can be accepted by
the router among all the received routes
from a given peer
l Export filter
u Specifies which routes can be advertised by
the router to a given peer

5. Agenda
• Routing in IP networks
• Interdomain routing
• Peering links
• BGP basics
• BGP convergence

6. Border Gateway Protocol l Path vector protocol
u BGP router advertises its best route to each
destination
AS1 AS2
AS4
2001:db8:1/48
AS5
lprefix:2001:db8:1/48
lASPath: AS1
lprefix: 2001:db8:1/48
ASPath: ::AS2:AS4:AS1
lprefix: 2001:db8:1/48
lASPath: AS4:AS1
lprefix: 2001:db8:1/48
ASPath: AS1
l ... with incremental updates

7. BGP : Principles
operation l Principles
l BGP relies on the
incremental exchange of path vectors
BGP session established
over
TCP connection between
peers
Each peer sends all its
active routes
As long as the BGP session
remains up
Incrementally update BGP routing
tables
AS3
R1
R2
AS4
BGP
session
BGP Msgs

8. BGP basics
(2)
l 2 types of BGP messages
l UPDATE (path vector)
u advertises a route towards one prefix
u Destination address/prefix
u Interdomain path (AS-Path)
u Nexthop
l WITHDRAW
u a previously announced route is not
reachable anymore
u Unreachable destination address/prefix

9. BGP router
BGP Loc-RIB
Peer[N]
All
BGP Msgs
from Peer[N] BGP Msgs
Peer[1]
Import filter
Attribute
manipulation
Peer[N]
Peer[1]
Export filter
Attribute
manipulation
acceptable
routes
BGP Decision
Process
BGP Routing Information Base
Contains all the acceptable routes
learned from all Peers + internal routes
l BGP decision process selects
the best route towards each destination
BGP Msgs
from Peer[1]
to Peer[N]
BGP Msgs
to Peer[1]
Import filter(Peer[i])
Determines which BGM Msgs
are acceptable from Peer[i] Export filter(Peer[i])
Determines which
routes can be sent to Peer[i]
One best
route to each
destination
BGP Adj-RIB-In
BGP Adj-RIB-Out

10. Example
AS20
R2
AS30
AS10
UPDATE
lprefix: 2001:db8:12/48,
lNextHop:R1
lASPath: AS10
UPDATE
lprefix: 2001:db8:12/48,
lNextHop:R2
lASPath: AS20:AS10
R1 R3
2001:db8:12/48
BGP
R4
AS40
BGP
BGP
UPDATE
lprefix: 2001:db8:12/48,
lNextHop:R1
lASPath: AS10
UPDATE
lprefix: 2001:db8:12/48,
lNextHop:R4
lASPath: AS40:AS10
l What happens if link AS10-AS20 goes down ?

11. How to prefer some
routes over others ?
RA RB
R1
Backup: 2Mbps
Primary: 34Mbps
AS1
AS2

12. How to prefer some
routes over others
• Limitations
RA
R1 R2
R3
RB
Cheap
Expensive
AS1
AS2
AS3
AS4
R5 AS5

13. BGP router
BGP RIB
Peer[N]
Peer[1]
Import filter
Attribute
manipulation
Peer[N]
Peer[1]
Export filter
Attribute
manipulation
BGP Msgs
from Peer[N]
BGP Msgs
from Peer[1]
BGP Msgs
to Peer[N]
BGP Msgs
All
acceptable
routes
BGP Decision
Process
One best to Peer[1]
route to each
destination
Import filter
l Selection of acceptable routes
l Addition of local-pref attribute
inside received BGP Msg
lNormal quality route : local-pref=100
lBetter than normal route :local-pref=200
lWorse than normal route :local-pref=50
Simplified BGP Decision Process
l Select routes with highest
local-pref
l If there are several routes,
choose routes with the
shortest ASPath
l If there are still several routes
tie-breaking rule

14. BGP session
• Session establishment
def initialize_BGP_session( RemoteAS, RemoteIP):
# Initialize and start BGP session
# Send BGP OPEN Message to RemoteIP on port 179
# Follow BGP state machine
# advertise local routes and routes learned from
peers*/
for d in BGPLocRIB :
B=build_BGP_Update(d)
S=Apply_Export_Filter(RemoteAS,B)
if (S != None) :
send_Update(S,RemoteAS,RemoteIP)
# entire RIB has been sent
# new Updates will be sent to reflect local or
distant
# changes in routers

15. Simple export filter
def apply_export_filter(RemoteAS, BGPMsg) :
# check if RemoteAS already received route
if RemoteAS is BGPMsg.ASPath :
BGPMsg=None
# Many additional export policies can be
configured :
# Accept or refuse the BGPMsg
# Modify selected attributes inside BGPMsg
return BGPMsg

16. Simple import filter
def apply_import_filter(RemoteAS, BGPMsg):
if MysAS in BGPMsg.ASPath :
BGPMsg=None
# Many additional import policies can be
configured :
# Accept or refuse the BGPMsg
# Modify selected attributes inside BGPMsg
return BGPMsg

17. Processing UPDATE
def Recvd_BGPMsg(Msg, RemoteAS) :
B=apply_import_filer(Msg,RemoteAS)
if (B== None): # Msg not acceptable
return
if IsUPDATE(Msg):
Old_Route=BestRoute(Msg.prefix)
Insert_in_RIB(Msg)
Run_Decision_Process(RIB)
if (BestRoute(Msg.prefix) != Old_Route) :
# best route changed
B=build_BGP_Message(Msg.prefix);
S=apply_export_filter(RemoteAS,B);
if (S!=None) : # announce best route
send_UPDATE(S,RemoteAS,RemoteIP);
else if (Old_Route != None) :
send_WITHDRAW(Msg.prefix,RemoteAS, RemoteIP)

18. Processing
WITHDRAW
else : # Msg is WITHDRAW
Old_Route=BestRoute(Msg.prefix)
Remove_from_RIB(Msg)
Run_Decision_Process(RIB)
if (Best_Route(Msg.prefix) !=Old_Route):
# best route changed
B=build_BGP_Message(Msg.prefix)
S=apply_export_filter(RemoteAS,B)
if (S != None) : # still one best route towards Msg.prefix
send_UPDATE(S,RemoteAS, RemoteIP);
else if(Old_Route != None) : # No best route anymore
send_WITHDRAW(Msg.prefix,RemoteAS,RemoteIP);

19. How to prefer routes
?routes over others (3)
?
RA RB
R1
Backup: 2Mbps
Primary: 34Mbps
AS1
AS2
RPSL-like policy for AS1
aut-num: AS1
import: from AS2 RA at R1 set localpref=100;
from AS2 RB at R1 set localpref=200;
accept ANY
export: to AS2 RA at R1 announce AS1
to AS2 RB at R1 announce AS1
RPSL-like policy for AS2
aut-num: AS2
import: from AS1 R1 at RA set localpref=100;
from AS1 R1 at RB set localpref=200;
accept AS1
export: to AS1 R1 at RA announce ANY
to AS2 R1 at RB announce ANY

20. How to prefer routes ?
routes over others (4) ?
RA
R1 R2
R3
RB
Cheap
Expensive
AS1
AS2
AS3
AS4
R5 AS5
RPSL policy for AS1
aut-num: AS1
import: from AS2 RA at R1 set localpref=100;
from AS4 R2 at R1 set localpref=200;
accept ANY
export: to AS2 RA at R1 announce AS1
to AS4 R2 at R1 announce AS1
u AS1 will prefer to send over cheap link
u But the flow of the packets destined to
AS1 will depend on the routing policy of
the other domains

21. Agenda
• Interdomain routing
• Peering links
• BGP basics
• BGP convergence
• Ethernet

22. Limitations of local-pref
l In theory
u Each domain is free to define its order of
preference for the routes learned from
external peers
2001:db8:1/48
AS1
Preferred paths for AS4
Preferred paths for AS3
AS3 AS4
1. AS3:AS1
2. AS1
1. AS4:AS1
2. AS1
u How to reach 1.0.0.0/8 from AS3 and

23. Limitations of local-pref
l AS1 sends its UPDATE messages ...
2001:db8:1/48
AS1
lP: 2001:db8:1/48
lASPath: AS1
AS3 AS4
UPDATE
Preferred paths for AS3
1. AS4:AS1
2. AS1
Routing table for AS3
2001:db8:1/48 ASPath: AS1 (best)
Preferred paths for AS4
1. AS3:AS1
2. AS1
UPDATE
lP: 2001:db8:1/48
lASPath: AS1
Routing table for AS4
2001:db8:1/48 ASPath: AS1 (best)

24. Limitations of local-pref
l First possibility
l AS3 sends its UPDATE first...
AS1
AS3 AS4
Preferred paths for AS3
1. AS4:AS1
2. AS1
2001:db8:1/48
Routing table for AS3
2001:db8:1/48 ASPath: AS1 (best) UPDATE
lP: 2001:db8:1/48
lASPath: AS3:AS1
Preferred paths for AS4
1. AS3:AS1
2. AS1
Routing table for AS4
2001:db8:1/48 ASPath: AS1
2001:db8:1/48 ASPath:AS3:AS1 (best)
u Stable route assignment

25. Limitations of local-pref
l AS4 sends its UPDATE first...
l2001:db8:1/48
AS1
Preferred paths for AS4
1. AS3:AS1
2. AS1
AS3 AS4
Routing table for AS4
2001:db8:1/48 ASPath: AS1 (best)
Preferred paths for AS3
1. AS4:AS1
2. AS1
UPDATE
lPrefix: 2001:db8:1/48
lASPath: AS4:AS1
Routing table for AS3
2001:db8:1/48 ASPath: AS1
2001:db8:1/48 ASPath: AS4:AS1 (best)
u Another (but different) stable route assignment

26. Limitations of local-pref
l AS3 and AS4 send their UPDATE together...
AS1
AS3 AS4
Preferred paths for AS3
1. AS4:AS1
2. AS1
2001:db8:1/48
UPDATE
lP: 2001:db8:1/48
lASPath: AS3:AS1
Preferred paths for AS4
1. AS3:AS1
2. AS1
UPDATE
lP: 2001:db8:1/48
ASPath: AS4:AS1
u AS3 prefers indirect path -> withdraw
u AS4 prefers indirect path -> withdraw

27. Limitations of local-pref
l AS3 and AS4 send their UPDATE together...
AS1
AS3 AS4
Preferred paths for AS3
1. AS4:AS1
2. AS1
2001:db8:1/48
Preferred paths for AS4
1. AS3:AS1
2. AS1
WITHDRAW
lP: 2001:db8:1/48
WITHDRAW
lP: 2001:db8:1/48
u AS3 : indirect route is not available anymore
u AS3 will reannounce its direct route...
u AS4 : indirect route is not available anymore
u AS4 will reannounce its direct route...

28. More limitations
local pref
l Unfortunately, interdomain routing may not
converge at all in some cases...
AS1
AS0
Preferred paths for AS1
1. AS3:AS0
2. other paths
Preferred paths for AS4
1. AS1:AS0
2. other paths
AS3 AS4
Preferred paths for AS3
1. AS4:AS0
2. other paths
u How to reach a destination inside AS0 in
this case ?

29. local-pref and
economical
l In practicer, elolcaalti-oprnefsish oiftpens combined
with filters to enforce economical
relationships
Prov1 Prov2
$ $
AS1
Peer1
Peer2
Peer3
Peer4
Cust1 Cust2
$ Customer-provider
$
Shared-cost
$
Local-pref values used by AS1
> 1000 for the routes received from a Customer
500 – 999 for the routes learned from a Peer
< 500 for the routes learned from a Provider

30. local-pref
l Which route will be used by AS1 to reach AS5 ?
AS1
$
$
AS4
AS2
AS3
Shared-cost
$
AS5 $ Customer-provider
$
l and how will AS5 reach AS1 ?
$
$
AS8
AS6
AS7
$
$
Internet paths are often asymmetrical

31. Intern•eNt S1FN9et90s
• American backbone
• AUP : no commercial
traffic
• Some regional
networks
• US regions
• national networks in
Europe
• Universities/research
labs
• connected to regional

32. • Dozen transit ISPs
shared-cost
• Uunet, Level3, OTIP,
Internet early 2000s
...
• Tier-2 ISPs
• Regional/ National
ISPs
• FT, BT, DT, ...
• Tier-3 ISPs
• Smaller ISPs,
EntreprisesNetworks,
Content providers
•

33. Today’s Internet
• Hyper Giants
• google, microsoft,
yahoo, amazon, ...
• google peers 70%
ISPs
• Tier-1 ISPs
• Tier-2 ISPs
• Tier-3 ISPs
Craig Labovitz), Scott Iekel-Johnson, Danny McPher•son, JMon Oabenrhyeid ep, Faerneamr Jianhagniasn, at IXPs
Internet Inter-Domain Traffic, SIGCOMM 2010

34. Internet size
Source: http://bgp.potaroo.net

35. BGP routing tables
Source: http://bgp.potaroo.net

36. BGP : IPv4
Source: http://bgp.potaroo.net

37. AS7007 incident

38. RIPE RIS
https://stat.ripe.net/AS2611#tabId=at-a-glance
https://stat.ripe.net/widget/looking-glass#
w.resource=2001:6a8::/32
https://stat.ripe.net/2001:6A8::/32#tabId=routing

39. Youtube and
Pakistan
http://www.ripe.net/internet-coordination/
news/industry-developments/
youtube-hijacking-a-ripe-ncc-
ris-case-study

40. Exercise

41. Agenda
• Interdomain routing
• Ethernet

42. l Most widely used LAN
Ethernet/802.3
l First developed by Digital, Intel and Xerox
l Standardised by IEEE and ISO
• Medium Access Control
• CSMA/CD with exponential backoff
• Bandwidth: 10 Mbps
• Two ways delay
• 51.2 microsec on Ethernet/802.3
• => minimum frame size : 512 bits
• Cabling
• 10Base5 : (thick) coaxial cable

44. Ethernet Addresses
l Each Ethernet adapter has a unique
address
l Ethernet Address format
l 48 bits addresses
u Source Address
24 bits OUI
(adapter manufacturer)
00
u Destination address
24 bits
(identifier of adapter)
u If high order bit is 0, host unicast
address
• If high order bit is 1, host multicast address

45. Ethernet
Frames l DIX Format
• proposed by Digital, Intel and Xerox
Preamble
[8 bytes]
Destination
address
Source address
Type
[2 bytes]
Data
[46-1500 bytes
CRC [32 bits]
Used to mark the beginning of the frame
Allows the receiver to synchronise its
clock to the sender’s clock
Indication of the type of packet contained
inside the frame
Upper layer protocol must ensure that
the payload of the Ethernet frame is
at least 46 bytes and at most 1500 bytes

46. Ethernet
Service
l An Ethernet network provides a
connectionless unreliable service
l Transmission modes
l unicast, multicast, broadcast
l Even if in theory the Ethernet service is
unreliable, a good Ethernet network should
l deliver frames to their destination with a very
hig probability of delivery
l not reorder the transmitted frames
• reordering is obviously impossible on a
bus

47. Ethernet with
structured cabling
• How to perform CSMA/CD in a star-shaped
network ?
Collision domain : set of stations that could be in collision

48. Ethernet hub
l A hub is a relay operating in the physical
layer
Datalink Datalink
Physical
Physical
Host A Hub Host B

49. Larger Ethernet
Hub
l With hubs ?
l Interconnect hubs together
Hub
Issue : maximum 51.2 microsec
delay between any pair of stations
Collision domain : entire network
Hub
Hub

50. Ethernet Switch
• Ethernet switch
• understands MAC addresses and filters
frames based on their addresses
Address Port
A West
B South
C East
Eth : A
Eth : B
Eth : C
Src:A Dst:B

51. Ethernet switch
l A switch is a relay that operates in the
datalink layer
Network Network
Datalink
Physical Phys. Phys.
Datalink
Physical
Host A Switch Host B

52. Frame processing
Arrival of frame F on port P
src=F.Source_Address;
dst=F.Destination_Address;
UpdateTable(src, P); // src heard on port P
if (dst==broadcast) || (dst is multicast)
{
for(Port p!=P) // forward all ports
ForwardFrame(F,p);
}
else
{
if(dst isin AddressPortTable) { ForwardFrame(F,AddressPortTable(}

53. •NHoew ttow deosirgnk n ertweodrksu thnatd suarvnivec linyk
and node failures ?
• Add reAdddurenssdanPotr tswitches
A West
B South
C East
Eth : A
Eth : B
Eth : C
Src:A Dst:C
Address Port
A North
B South
C East
Address Port
A West
B South
C North

54. Network redundancy
(2)
Address Port
Eth : A
Eth : B
Eth : C
Src:A Dst:C
Address Port
Address Port

55. Spanning tree
• Each switch has a unique identifier
• The switch with the lowest id is the root
• Disable all links that do not belong to the
Switch 1
Switch 7
Switch 9
Switch 2
Switch 22
Switch 44
spanning tree

56. Building the spanning
tree l 802.1d protocol
l 802.1d uses Bridge PDUs (BPDUs)
containing
u Root ID : identifier of the current root
switch
u Cost : Cost of the shortest path between
the switch transmitting the BPDU and the
root switch
u Transmitting ID : identifier of the switch
that transmits the BPDU
l The BPDUs are sent by switches over their
attached LANs as multicast frames but
they are never forwarded
• switches that implement 802.1d listen to a

57. Ordering of BPDUs
l BPDUs can be strictly ordered
l BPDU11[R=R1,C=C1, T=T1] is better than
BPDU2 [R=R2,C=C2, T=T2] if
u R1<R2
u R1=R2 and C1<C2
u R1=R2 and C1=C2 and T1<T2
l Example
BPDU1 BPDU2
R1 C1 T1 R2 C2 T2
29 15 35 31 12 32
35 80 39 35 80 40
35 15 80 35 18 38

58. 802.1d port states
l 802.1d port state based on received BPDUs
l Root port
u port on which best 802.1d BPDU received
l Designated port
u a port is designated if the switch’s BPDU
is better than the best BPDU received on
this port
u Switch’s BPDU is
u current root, cost to root, switch identifier
l Blocked port (only receives 802.1d
BPDUs)

59. Port states and
activityReceive
BPDUs
Transmit
BPDUs
Blocked yes no
Root yes no
Designated yes yes
Learn
Addresses
Forward Data
Frames
Inactive no no
Active yes yes