Interdomain Routing and The Border Gateway Protocol (BGP)

1
Interdomain Routing and The Border Gateway
Protocol (BGP)
2
Todays Big Picture
Large ISP
Large ISP
Stub
Small ISP
GPRS
Access Network
Stub
Stub
Large number of diverse networks
3
Internet AS Map caida.org
4
Autonomous System(AS)

• Internet is not a single network
• Collection of networks controlled by different
  administrations
• An autonomous system is a network under a single
  administrative control
• IANA
• An AS owns unique IP prefixes
• Every AS has a unique AS number
• ASes need to inter-network themselves to form a
  single virtual global network
• Need a common protocol for communication

5
Who speaks Inter-AS routing?
AS2
BGP
AS1
border router
internal router

• Two types of routers
• Border router (Edge)
• Internal router (Core)
• Two border routers of different ASes will have a
  BGP session

6
Autonomous Systems (ASes)

• An autonomous system is an autonomous routing
  domain that has been assigned an Autonomous
  System Number (ASN).
• All parts within an AS remain connected.

7
IP Address Allocation and Assignment Internet
Registries
IANA www.iana.org
APNIC www.apnic.org
ARIN www.arin.org
RIPE www.ripe.org
Allocate to National and local
registries and ISPs Addresses assigned
to customers by ISPs
RFC 2050 - Internet Registry IP Allocation
Guidelines RFC 1918 - Address Allocation
for Private Internets RFC 1518 - An
Architecture for IP Address Allocation with CIDR
8
Whois servers (AS, IP)

• http//www.ripe.net/perl/whois
• AS2588
• http//ws.arin.net/cgi-bin/whois.pl
• AS701
• http//www.apnic.net/apnic-bin/whois.pl
• AS4808

9
AS Numbers (ASNs)
ASNs are 16 bit values.
64512 through 65535 are private
Currently over 20,000 in use.

• Genuity 1
• MIT 3
• JANET 786
• UC San Diego 7377
• ATT 7018, 6341, 5074,
• UUNET 701, 702, 284, 12199,
• Sprint 1239, 1240, 6211, 6242,

ASNs represent units of routing policy
10
Partial View of www.cl.cam.ac.uk (128.232.0.20)
Neighborhood
AS 20757 Hanse
AS 5089 NTL Group
AS 3356 Level 3
AS 1239 Sprint
AS 6461 AboveNet
AS 3257 Tiscali
AS 702 UUNET
AS 13127 Versatel
AS 4637 REACH
AS 20965 GEANT
AS 786 ja.net
AS 5459 LINX
AS 1213 HEAnet (Irish academic and
research)
Originates gt 180 prefixes, Including
128.232.0.0/16
AS 4373 Online Computer Library
Center
AS 7 UK Defense Research Agency
11
How Many ASNs are there today?
18,217
Thanks to Geoff Huston. http//bgp.potaroo.net on
October 26, 2004
12
IP network assignment process
13
RIR Allocations - Current
Allocated
14
BGP Routing Table - Current
15
How many prefixes today?
Note numbers actually depends point of view
Thanks to Geoff Huston. http//bgp.potaroo.net on
October 26, 2004
16
The Gang of Four
Link State
Vectoring
RIP
OSPF
IGP
EIGRP
BGP
EGP
17
BGP-4

• BGP Border Gateway Protocol
• Is a Policy-Based routing protocol
• Is the de facto EGP of todays global Internet
• Relatively simple protocol, but configuration is
  complex and the entire world can see, and be
  impacted by, your mistakes.

• 1989 BGP-1 RFC 1105
• Replacement for EGP (1984, RFC 904)
• 1990 BGP-2 RFC 1163
• 1991 BGP-3 RFC 1267
• 1995 BGP-4 RFC 1771
• Support for Classless Interdomain Routing (CIDR)

18
The Border Gateway Protocol (BGP)
BGP
RFC 1771

optional extensions RFC 1997 (communities) RFC
2439 (damping) RFC 2796 (reflection) RFC3065
(confederation)

routing policy configuration languages
(vendor-specific)

Current Best Practices in management of
Interdomain Routing
BGP was not DESIGNED. It EVOLVED.
19
BGP Operations (Simplified)
Establish session on TCP port 179
AS1
BGP session
Exchange all active routes
AS2
While connection is ALIVE exchange route UPDATE
messages
Exchange incremental updates
20
Four Types of BGP Messages

• Open Establish a peering session.
• Keep Alive Handshake at regular intervals.
• Notification Shuts down a peering session.
• Update Announcing new routes or withdrawing
  previously announced routes.

announcement
prefix attributes values
21
BGP Attributes
Value Code
Reference ----- -----------------------------
---- --------- 1 ORIGIN
RFC1771 2 AS_PATH
RFC1771 3 NEXT_HOP
RFC1771 4
MULTI_EXIT_DISC RFC1771 5
LOCAL_PREF RFC1771
6 ATOMIC_AGGREGATE
RFC1771 7 AGGREGATOR
RFC1771 8 COMMUNITY
RFC1997 9 ORIGINATOR_ID
RFC2796 10 CLUSTER_LIST
RFC2796 11 DPA
Chen 12
ADVERTISER RFC1863 13
RCID_PATH / CLUSTER_ID RFC1863
14 MP_REACH_NLRI
RFC2283 15 MP_UNREACH_NLRI
RFC2283 16 EXTENDED
COMMUNITIES Rosen ... 255
reserved for development
Most important attributes
Not all attributes need to be present in every
announcement
From IANA http//www.iana.org/assignments/bgp-par
ameters
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Attributes are Used to Select Best Routes
192.0.2.0/24 pick me!
192.0.2.0/24 pick me!
192.0.2.0/24 pick me!
Given multiple routes to the same prefix, a BGP
speaker must pick at most one best route (Note
it could reject them all!)
192.0.2.0/24 pick me!
23
BGP Route Processing
Open ended programming. Constrain
ed only by vendor configuration language
Apply Policy filter routes tweak attributes
Apply Policy filter routes tweak attributes
Receive BGP Updates
Best Routes
Transmit BGP Updates
Based on Attribute Values
Best Route Selection
Apply Import Policies
Best Route Table
Apply Export Policies
Install forwarding Entries for best Routes.
IP Forwarding Table
24
Route Selection Summary
Highest Local Preference
Enforce relationships
Shortest ASPATH
Lowest MED
traffic engineering
i-BGP lt e-BGP
Lowest IGP cost to BGP egress
Throw up hands and break ties
Lowest router ID
25
BGP Routing Tables
show ip bgp BGP table version is 111849680, local
router ID is 203.62.248.4 Status codes s
suppressed, d damped, h history, valid, gt best,
i - internal Origin codes i - IGP, e - EGP, ? -
incomplete Network Next Hop
Metric LocPrf Weight Path . . . gti192.35.25.0
134.159.0.1 50 0
16779 1 701 703 i gti192.35.29.0
166.49.251.25 50 0 5727
7018 14541 i gti192.35.35.0 134.159.0.1
50 0 16779 1 701 1744
i gti192.35.37.0 134.159.0.1
50 0 16779 1 3561 i gti192.35.39.0
134.159.0.3 50 0 16779 1
701 80 i gti192.35.44.0 166.49.251.25
50 0 5727 7018 1785
i gti192.35.48.0 203.62.248.34
55 0 16779 209 7843 225 225 225 225 225
i gti192.35.49.0 203.62.248.34
55 0 16779 209 7843 225 225 225 225 225
i gti192.35.50.0 203.62.248.34
55 0 16779 3549 714 714 714
i gti192.35.51.0/25 203.62.248.34
55 0 16779 3549 14744 14744 14744 14744
14744 14744 14744 14744 i . . .
Thanks to Geoff Huston. http//www.telstra.net/ops
on July 6, 2001

• Use whois queries to associate an ASN with
  owner (for example, http//www.arin.net/whois/ar
  inwhois.html)
• 7018 ATT Worldnet, 701 Uunet, 3561 Cable
  Wireless,

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AS Graphs Can Be Fun
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Policy Transit vs. Nontransit
A transit AS allows traffic with neither source
nor destination within AS to flow across the
network
AS 701
ATT CBB
AS 701
UUnet
AS144
A nontransit AS allows only traffic originating
from AS or traffic with destination within AS
Bell Labs
IP traffic
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Customers and Providers
provider
customer
Customer pays provider for access to the Internet
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The Peering Relationship
Peers provide transit between their respective
customers Peers do not provide transit between
peers Peers (often) do not exchange
traffic allowed
traffic NOT allowed
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Peering Provides Shortcuts
Peering also allows connectivity between the
customers of Tier 1 providers.
31
Peering Wars
Peer
Dont Peer

• Reduces upstream transit costs
• Can increase end-to-end performance
• May be the only way to connect your customers to
  some part of the Internet (Tier 1)

• You would rather have customers
• Peers are usually your competition
• Peering relationships may require periodic
  renegotiation

Peering struggles are by far the most
contentious issues in the ISP world! Peering
agreements are often confidential.
32
Policy-Based vs. Distance-Based Routing?
Host 1
Cust1
Minimizing hop count can violate commercial
relationships that constrain inter- domain
routing.
ISP1
ISP3
Host 2
ISP2
Cust3
Cust2
33
What is Routing Policy

• Policy refers to arbitrary preference among a
  menu of available routes (based upon routes
  attributes)
• Public description of the relationship between
  external BGP peers
• Can also describe internal BGP peer relationship
• Eg Who are my BGP peers
• What routes are
• Originated by a peer
• Imported from each peer
• Exported to each peer
• Preferred when multiple routes exist
• What to do if no route exists?

34
Routing Policy Example

• AS1 originates prefix d
• AS1 exports d to AS2, AS2 imports
• AS2 exports d to AS3, AS3 imports
• AS3 exports d to AS5, AS5 imports

35
Routing Policy Example (cont)

• AS5 also imports d from AS4
• Which route does it prefer?
• Does it matter?
• Consider case where
• AS3 Commercial Internet
• AS4 Internet2

36
Import and Export Policies

• Inbound filtering controls outbound traffic
• filters route updates received from other peers
• filtering based on IP prefixes, AS_PATH,
  community
• Outbound Filtering controls inbound traffic
• forwarding a route means others may choose to
  reach the prefix through you
• not forwarding a route means others must use
  another router to reach the prefix
• Attribute Manipulation
• Import LOCAL_PREF (manipulate trust)
• Export AS_PATH and MEDs

37
ASPATH Attribute
AS 1129
135.207.0.0/16 AS Path 1755 1239 7018 6341
Global Access
AS 1755
135.207.0.0/16 AS Path 1239 7018 6341
135.207.0.0/16 AS Path 1129 1755 1239 7018 6341
Ebone
AS 12654
RIPE NCC RIS project
135.207.0.0/16 AS Path 7018 6341
AS7018
135.207.0.0/16 AS Path 3549 7018 6341
135.207.0.0/16 AS Path 6341
ATT
AS 3549
AS 6341
135.207.0.0/16 AS Path 7018 6341
Global Crossing
ATT Research
135.207.0.0/16
Prefix Originated
38
Shorter Doesnt Always Mean Shorter
Mr. BGP says that path 4 1 is better
than path 3 2 1
In fairness could you do this right and
still scale? Exporting internal state would
dramatically increase global instability and
amount of routing state
Duh!
AS 4
AS 3
AS 2
AS 1
39
Routing Example 1
40
Routing Example 2
41
Tweak Tweak Tweak (TE)

• For inbound traffic
• Filter outbound routes
• Tweak attributes on outbound routes in the hope
  of influencing your neighbors best route
  selection
• For outbound traffic
• Filter inbound routes
• Tweak attributes on inbound routes to influence
  best route selection

outbound routes
inbound traffic
inbound routes
outbound traffic
In general, an AS has more control over outbound
traffic
42
LOCAL PREFERENCE
Local preference used ONLY in iBGP
AS 4
local pref 80
AS 3
local pref 90
local pref 100
AS 2
AS 1
Higher Local preference values are more preferred
13.13.0.0/16
43
Implementing Backup Links with Local Preference
(Outbound Traffic)
AS 1
primary link
backup link
Set Local Pref 100 for all routes from AS 1
Set Local Pref 50 for all routes from AS 1
AS 65000
Forces outbound traffic to take primary link,
unless link is down.
Well talk about inbound traffic soon
44
Multihomed Backups (Outbound Traffic)
AS 1
AS 3
provider
provider
primary link
backup link
Set Local Pref 100 for all routes from AS 1
Set Local Pref 50 for all routes from AS 3
AS 2
Forces outbound traffic to take primary link,
unless link is down.
45
ASpath prepending
AS 1
AS 3
provider
provider
192.0.2.0/24 ASPATH 2 2 2 2 2 2 2 2 2 2 2 2 2 2
192.0.2.0/24 ASPATH 2
backup
primary
customer
192.0.2.0/24
AS 2
Padding in this way is often used as a form of
load balancing
46
COMMUNITY Attribute to the Rescue!
AS 3 normal customer local pref is 100, peer
local pref is 90
AS 1
AS 3
provider
provider
192.0.2.0/24 ASPATH 2 COMMUNITY 370
192.0.2.0/24 ASPATH 2
backup
primary
Customer import policy at AS 3 If 390 in
COMMUNITY then set local preference to 90 If
380 in COMMUNITY then set local preference
to 80 If 370 in COMMUNITY then set local
preference to 70
customer
192.0.2.0/24
AS 2
47
BGP Summary

• BGP4 is the protocol used on the Internet to
  exchange routing information between providers,
  and to propagate external routing information
  through networks.
• Each autonomous network is called an Autonomous
  System.
• ASs which inject routing information on their own
  behalf have ASNs.

48
BGP Peering

• BGP-speaking routers peer with each other over
  TCP sessions, and exchange routes through the
  peering sessions.
• Providers typically try to peer at multiple
  places. Either by peering with the same AS
  multiple times, or because some ASs are
  multi-homed, a typical network will have many
  candidate paths to a given prefix.

49
The BGP Route

• The BGP route is, conceptually, a promise to
  carry data to a section of IP space. The route
  is a bag of attributes.
• The section of IP space is called the prefix
  attribute of the route.
• As a BGP route travels from AS to AS, the ASN of
  each AS is stamped on it when it leaves that AS.
  Called the AS_PATH attribute, or as-path in
  Cisco-speak.

50
BGP Route Attributes

• In addition to the prefix, the as-path, and the
  next-hop, the BGP route has other attributes,
  affectionately known as knobs and
  twiddles -
• weight, rarely used - sledgehammer
• local-pref, sometimes used - hammer
• origin code, rarely used
• MED (metric) - a gentle nudge