Feb 12, 2008 CS573: Network Protocols and Standards

1

Border Gateway Protocol (BGP)

Network Protocols and Standards

Winter 2007-2008

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Current Internet Architecture

(1999-2000)

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Allocated AS Numbers

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BGP Routing The de facto standard for inter-AS

routing Path Vector Protocol

Extension of Distance Vector Protocol Each Border Gateway broadcasts to

neighbors (peers) the entire path (i.e., sequence of ASes) to the destination

Example: X stores the following path to destination Z Path (X, Z) = X, Y1, Y2, …, Z Different from RIP distance vector

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BGP Routing Now X may send this “path vector” to

another border router W through BGP A promise that I will take care of traffic for Z

Gateway W may or may not select the offered path, based on: Cost Policy Additional reasons (later!)

If W selects the path advertised by X Path (W, Z) = W, Path (X, Z)

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Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer network:

provider network

BGP Routing Policy

A, B, C are provider networks X, W, Y are customers (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 should not advertise to B that a route to C exists!

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Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer network:

provider network

BGP Routing Policy

A advertises to B the path AW B advertises to X the path BAW

So that X can reach W Should B advertise to C the path BAW?

No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers

B wants to route only to/from its customers!

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Transit versus Non-transit AS

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BGP Operation

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BGP Operation: Messages Peers exchange BGP messages using TCP OPEN

Opens a TCP connection to peer Authenticates the sender

UPDATE Advertises a new path (or withdraws old)

KEEPALIVE Keeps connection alive in the absence of updates Serves as ACK to an OPEN request

NOTIFICATION Reports error in a previous message Closes a connection

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A BGP Update Message TIME : Tue Jun 1 00:08:03 2004 LENGTH : 72 TYPE : Zebra BGP SUBTYPE : Zebra BGP Message SOURCE_AS : 7018 DEST_AS : 6447 INTERFACE : 0 SOURCE_IP : 12.0.1.63 DEST_IP : 128.223.60.102 MESSAGE TYPE : Update/Withdraw WITHDRAW : ANNOUNCE : 64.166.88.0/24 ATTRIBUTES : ATTR_LEN : 29 ORIGIN : 0 ASPATH : 7018 701 19714 NEXT_HOP : 12.0.1.63 MED : N/A LOCAL_PREF : N/A ATOMIC_AGREG : N/A AGGREGATOR : N/A COMMUNITIES : 7018:5000

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BGP Attributes

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BGP Route Selection

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BGP Route Selection

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eBGPiBGP

18.0.0.0/8

Border router/Egress

AB

C D

Autonomous System (AS)

eBGP and iBGP

Route

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Distributing Reachability Information within the AS

Note: Border routers also need iBGP sessions with I internal routers

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Route Reflectors

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Problems with Route Reflectors Problem 1

Routers may not choose best route Because, RR only relects its own best

route Problem 2

RR choose their best route and make full mesh

This may result in forwarding loops See BGPsep paper (Infocom 2006)

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Distributing Reachability Info to other Autonomous Systems

With eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1.

1c can then use iBGP to distribute this new prefix reach info to all routers in AS1

1b can then re-advertise the new reach info to AS2 over the 1b-to-2a eBGP session

When router learns about a new prefix, it creates an entry for the prefix in its forwarding table.

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session

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BGP Route Oscillations

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BGP Route Oscillations

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BGP Route Flap Dampening

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BGP Route Flap Dampening

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Cisco IOS Example