CS540/TE630
Computer Network Architecture
Spring 2009
Tu/Th 10:30am-Noon
Sue Moon
Routing
 What do you remember from undergrad
networking courses?
Questions
BGP
 De-facto standard inter-domain routing
protocol
 Became popular only in 1995


significant increase in # of ISPs
CIDR introduced in 1995
 Path vector algorithm
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Configuration and Policy
 A BGP node decides which routes to share
with its neighbor
 A BGP node can selectively accept and reject
messages
 What to share and what to accept

determined by routing policy
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Four Basic BGP Messages
 Open


Establishes BGP session (TCP port #179)
Sets the hold timer
 Notification


Report unusual conditions
Terminates the TCP session and gives an indication (holder
timer expiry, bad peer AS, malformed attribute list, etc.)
 Update

Inform neighbor of new/old routes that become
active/inactive
 Keepalive

Inform neighbor that connection is still alive
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UPDATE Message
 Advertise/Withdraw prefixes
Withdrawn routes length (2 bytes)
Withdrawn routes (variable length)
Total path attributes length (2 bytes)
Path attributes (variable length)
Reachability information (variable length)
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Attributes
 ORIGIN


Who originated the announcement?
IGP, EGP or Incomplete (often for static routes)
 AS-PATH


list of AS's
useful to detect and prevent loops
 NEXT HOP



For EBGP, IP addr of neighbor that announced
For IBGP, if route originated inside, IP addr of neighbor
For IBGP, if route originated outside, EBGP node that learned of route,
is carried unaltered into IBGP
 Multi-Exit Discriminator (MED)
 Local Preference
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Attribute: Multi-Exit Discriminator (MED)
 When ASes have multiple
interconnecting links
 Lower, more preferred
 Non-transitive
R1
AS1
143.248.0.0/16
MED=2
R3
R2
143.248.0.0/16
MED=10
AS2
R4
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Attribute: Local Pref
143.248.0.0/16
 Indicates preference among
multiples paths for the same
prefix

AS1
higher, more preferred
 Exchanged between IBGP
peers only
 Often used to select a specific
egress point for a particular
destination
AS3
AS2
AS4
Destination
AS Path
Local Pref
143.248.0.0/16
AS3 AS1
300
143.248.0.0/16
AS2 AS1
100
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BGP Decision Process
1.
2.
3.
4.
5.
6.
Highest LOCAL-PREF
Shortest AS-PATH
Lowest ORIGIN (IGP < EGP < Incomplete)
Lowest MED
Min cost path to NEXT HOP using IGP metrics
BGP Router ID to break tie
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Input Policy Engine
 Inbound filtering



filter based on IP prefixes, AS_PATH, community
deny = BGP won't reach that prefix via the peer
accept = traffic to that prefix via the peer
 Attribute manipulation

Sets attributes on accepted routes
• E.g.: Specify LOCAL-PREF to set priorities among
multiple peers
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Output Policy Engine
 Outbound filtering

forward = peers may route traffic via you
 Attribute manipulation

Sets attributes such as AS-PATH and MEDs
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Transit vs. Nontransit
Transit
AS3
AS1
C3
C1
AS2
C2
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Routing Engine
BGP
Input Policy
BGP Table
BGP
Output Policy
IP Routing
Table
Forwarding
Table
OSPF Topology
Shortest Path
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References & Acknowledgements
 Some use of Nina Taft's tutorial slides on BGP
 BGP4 Inter-Domain Routing in the Internet, John W. Stewart,
Addison-Wesley, 1998
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