How BGP Works

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How BGP Works
Young Xu, Product Marketing Manager

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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?

•  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

•  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

•  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

•  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withdrawn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Withdrawn Routes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)

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

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

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

•  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

•  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

•  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)

•  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

BGP Route Visualization Shows Preferred Routes
WV Fiber
Switch
Level 3

Routes Are Reﬂected in Traﬃc Paths
WV Fiber
Switch
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How BGP Works

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