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How BGP Works
Young Xu, Product Marketing Manager

2© 2017 ThousandEyes Inc. All Rights Reserved.
About ThousandEyes
Network Intelligence platform
that gives you a complete
picture from users to internal
and cloud-based applications
Routing!
User
App
End-to-End Performance Data
App
Performance!
User
Experience!
Network
Topology!
Routing
Topology!
Enterprise, Endpoint and Cloud Agents
Network
Connectivity!
And Route Monitors!
Surface insights from
a global data set
Lightweight, flexible
data collection
Unified view of diverse
performance data
Solve issues across
shared infrastructure
See any network like
it’s your own

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Border Gateway Protocol
BGP-4 RFC4271
Where do I forward traffic to reach an IP
address in an external network?
How can I control the route and composition
of inbound traffic to my network?

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• Autonomous System
– Collection of IP prefixes
– Common routing policy
to other ASes
– Registered by an RIR
(regional Internet registry)
– Denoted by a name and number
• A Private AS can be used when
a single upstream exists
– External routing policy is identical
IP Blocks and Autonomous Systems
AS 200
2.2.2.0/24
2.2.3.0/24
Autonomous
System

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• Inter-Network – Used for routing
between networks (Autonomous
Systems), or within large networks
• Reachability – BGP defines how one AS
can reach another, described as a path
vector (AS Path)
• Policy-Based – BGP makes it possible
for an AS to apply policies (e.g. multi-
homing, failover, commercial terms)
• Decentralized – Each AS makes policy
decisions autonomously, using BGP to
coordinate and share routes
Key BGP Concepts

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• Neighboring
routers, within or
between ASes,
establish a TCP
connection on
port 179.
• BGP messages
include:
– Open
– Update
– Notification
– Keep Alive
BGP the Protocol
AS 100
1.1.1.0/24
AS 300
3.3.3.4/22
AS 400
4.4.0.0/16
AS 200
2.2.2.0/24
Border Router
Origin
Autonomous
System
Internal
Router
BGP peers exchange
routes, within and
among ASes

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• An update message may advertise
routes, withdraw routes, or both
• Any number of routes may be
withdrawn
• Any number of routes may be
advertised
– They must all share the same attributes
– These attributes include AS Path and
Origin
• Therefore, you’ll expect at least one
Update message for each Origin AS
BGP Update Message
BGP Header
Withdrawn Routes
(n Prefix/Length tuples)
Path Attributes
7 well-known attributes:
Origin, AS Path, Next Hop, MED,
Local Pref, Atomic Aggregate, Aggregate
Optional attributes:
Community, Originator
Network Layer Reachability Info
(n Prefix/Length tuples)

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AS 300
3.3.3.4/22
• Describes available
routes using a path
vector
• Each AS will
prepend itself onto
the AS Path
• Associated with an
origin AS and prefix
• Avoids routing loops
by rejecting any AS
Path containing the
local AS
AS Path
AS 100
1.1.1.0/24
AS 400
4.4.0.0/16
AS 200
2.2.2.0/24
Origin
Autonomous
System
300 100
100100
200 100

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The Update Process
Loc-RIB
Routes with policies
applied
Adj-RIB-In
Unprocessed routes
Adj-RIB-Out
Routes to advertiseNewly
learned
routes
Newly
advertised
routes
FIB
Routes with next-hop
and interfaces
IP Routing Table
Routes aggregated
across protocols
Next-hops resolved
Interfaces calculated
Locally learned
routes added
1. Preference calculated (PIB)
2. Route selection
• Ensure resolvability
• Break ties
3. Route
dissemination
• Aggregation
Incorporated with IS-IS, OSPF,
etc. by Administrative Distance

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The routing application builds a Routing Information Base (RIB) to map
learned prefixes and routes
Example of the routing table for AS100 (show ip bgp)
* = valid; > = best
Routing Tables (RIBs)
Network
Next Hop
Metric (MED)
Local Pref
Weight
Path
*> 1.1.1.0/24 10.1.12.2
0
0
i
*>
2.2.2.0/24
10.1.14.4
0
0
200 i
*>
2.2.3.0/24
10.1.14.4
0
0
200 i
*>
3.3.3.4/22 10.1.16.6
0
0
300 i
*>
4.4.0.0/16
10.1.16.6
0
32768
300 400 i
*
4.4.0.0/16
10.1.14.4
0
0
200 400 i

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• Highest weight (de facto standard)
– Set by the local router
• Highest local preference
– Set by the local AS, typically based on commercial relationships
• Shortest AS Path
– The route that traverses the fewest ASes
• Origin type
– Internal-learned (IGP) routes preferred
• Multi-Exit Discriminator (MED)
– A preference set by the origin AS
• Additional tiebreaking and multipath criteria…
Route Selection

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• Match the most specific prefix
– If none available, then the prefix is not reachable
• Forward traffic to the correct interface
– Based on information placed in the FIB, learned from BGP (and
other protocols)
• Thus, a forwarding decision is influenced by:
– Specificity of IP prefix
– Internal routes
– BGP routes, their attributes and the local routing policy
Making Forwarding Decisions

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• Generally, BGP speakers within an AS must communicate with one
another in a full mesh, each updating one another
• But this can be hard to scale in large ASes
• Alternatives to full meshes exist, including:
Coordinating Within an AS
Route reflection (hub-spoke) Confederations (AS subdomains)

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• Communities
– Communicate to neighbors how to
advertise routes they learn from you,
and vice versa
– ISPs publish community definitions on how advertisements will be handled
– Often used for local preference, no-export, prepending, geographic or peering
limitations
• MED (Multi-Exit Discriminator)
– Communicate preferred inbound paths to a neighbor
• Prepending
– Inserting the local AS to the AS Path multiple times to lengthen the path and
reduce its preference by others
Coordinating Between ASes

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Demo

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BGP Route Visualization Shows Preferred Routes
WV Fiber
Switch
Level 3

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Routes Are Reflected in Traffic Paths
WV Fiber
Switch
Level 3

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Watch the webinar:
www.thousandeyes.com/webinars/how-bgp-works