Transcript Presentation3

Department of Computer and IT Engineering
University of Kurdistan
Computer Networks II
Border Gateway protocol (BGP)
By: Dr. Alireza Abdollahpouri
Internet structure: network of networks
local
ISP
Tier 3
ISP
Tier-2 ISP
local
ISP
local
ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
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Hierarchical Routing
 aggregate routers into
regions, “autonomous
systems” (AS)
 routers in same AS run
same routing protocol
 “intra-AS” routing
protocol
 routers in different
AS can run different
intra-AS routing
protocol
gateway routers



special routers in AS
run intra-AS routing
protocol with all other
routers in AS
also responsible for routing
to destinations outside AS
 run inter-AS routing
protocol with other
gateway routers
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Internet’s Area Hierarchy
 What is an Autonomous System (AS)?
 A set of routers under a single technical
administration, using Intra-AS routing protocols
(e.g., RIP, OSPF) and common metrics to route
packets within the AS and using an Inter-AS
routing protocol to route packets to other AS’s
 Each AS assigned unique ID
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Intra-AS and Inter-AS routing
BGP
C.b
B.a
A.a
a
b
C
A.c
d
A
Host1
a
b
c
Intra-AS routing
within AS A
( RIP, OSPF, …)
a
Host2
c
B
b
Intra-AS routing
within AS B
( RIP, OSPF, …)
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AS Categories
 Stub: an AS that has only a single connection
to one other AS - carries only local traffic.
 Multi-homed: an AS that has connections to
more than one AS, but does not carry transit
traffic
 Transit: an AS that has connections to more
than one AS, and carries both transit and local
traffic (under certain policy restrictions)
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AS Categories
AS1
AS3
AS1
AS2
AS1
AS3
AS2
Transit
Stub
AS2
Multi-homed
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Internet inter-AS routing: BGP
 BGP (Border Gateway Protocol): the de facto
standard
 BGP is a Path Vector protocol:
 similar to Distance Vector protocol
 each Border Gateway broadcast to neighbors
(peers) entire path (i.e., sequence of AS’s) to
destination
 BGP routes to networks (ASs), not individual
hosts
 E.g., Gateway X may send its path to dest. Z:

Path (X,Z) = X,Y1,Y2,Y3,…,Z
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Internet inter-AS routing: BGP
Suppose: gateway X send its path to peer gateway W
 W may or may not select path offered by X
 cost, policy (don’t route via competitors AS), loop
prevention reasons.
 If W selects path advertised by X, then:
Path (W,Z) = W, Path (X,Z)
 Note: X can control incoming traffic by controlling its
route advertisements to peers:
 e.g., don’t want to route traffic to Z -> don’t advertise
any routes to Z
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BGP: controlling who routes to you
B
W
X
A
provider
network
customer
network
C
Y



A,B,C are provider networks
X,W,Y are customer (of provider networks)
X is dual-homed: attached to two networks
 X does not want to route from B via X to C
 .. so X will not advertise to B a route to C
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BGP operation
Q: What does a BGP router do?
 Receiving and filtering route advertisements from
directly attached neighbor(s).
 Route selection.
 To route to destination X, which path (of several
advertised) will be taken?
 Sending route advertisements to neighbors.
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Initial routing tables in path vector routing
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Stabilized tables for four autonomous systems
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BGP messages
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BGP messages
 OPEN: opens TCP connection to peer and
authenticates sender
 UPDATE: advertises new path (or withdraws old)
 KEEPALIVE keeps connection alive in absence of
UPDATES; also ACKs OPEN request (send
periodically, every 30 seconds)
 NOTIFICATION: reports errors in previous msg;
also used to close connection
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Policy with BGP
 BGP provides capability for enforcing various
policies
 Policies are not part of BGP: they are provided
to BGP as configuration information
 BGP enforces policies by choosing paths from
multiple alternatives and controlling
advertisement to other AS’s
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Examples of BGP Policies
 A multi-homed AS refuses to act as transit
 Limit path advertisement
 A multi-homed AS can become transit for some
AS’s
 Only advertise paths to some AS’s
 An AS can favor or disfavor certain AS’s for
traffic transit from itself
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I-BGP and E-BGP
External BGP (E-BGP): BGP runs between different ASs
Internal BGP (I-BGP): BGP runs between two peers in the same AS
R1
AS1
E-BGP
R3
R4
AS2
R2
I-BGP
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AS-Path
 Sequence of AS’s a route traverses
 Used for loop detection and to apply policy
AS-3
AS-4
130.10.0.0/16
AS-2
120.10.0.0/16
AS-5
110.10.0.0/16
AS-1
120.10.0.0/16 AS-2 AS-3 AS-4
130.10.0.0/16 AS-2 AS-3
110.10.0.0/16 AS-2 AS-5
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BGP Operations (Simplified)
Establish session on
TCP port 179
AS1
BGP session
Exchange all
active routes
AS2
Exchange incremental
updates
While connection
is ALIVE exchange
route UPDATE messages
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Example: Multiple AS Paths
AS 128.2/16
9
AS
701
AS
7018
128.2/16
9 701
128.2/16
9 7018 1239
AS
73
AS
1239
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Shorter Doesn’t Always Mean Shorter
Path 4 1 is “better”
than path 3 2 1
AS 4
AS 3
AS 2
AS 1
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Questions
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