BGP is the routing protocol used between autonomous systems (AS) on the Internet to exchange routing and reachability information. It uses path vector routing to exchange routing information between ASes and allows for policies to influence path selection. BGP attributes like AS path, next hop, local preference, and communities are used to apply policies and influence the best path selection. Techniques like route reflection and confederations help scale the internal BGP mesh within a large AS.

1. Introduction to the
Border Gateway Protocol (BGP)
Paul Ferguson
pferguson@cisco.com
Consulting Engineer
Internet Service Providers
cisco Systems, Inc.
BGP
 What
 Why
 Who
 When
 Where
BGP version 4
 RFC 1771
 Intelligent route selection based
on most specific prefix and shortest Autonomous System (AS) path
 Classless Inter Domain Routing (CIDR) support
 Current Internet standard for
inter-domain (AS) exterior routing
Border Gateway Protocol (BGP)
 BGP version 4 is the current
de facto exterior routing protocol
in the Internet
 BGP- 4 is required for CIDR
Introduction
 Used for carrying routing information between AS’s
 Path vector protocol

2.  Incremental
 Runs over TCP (port 179)
 Conveys information about AS
path topology
BGP Between AS’s
PPT Slide
General Operation
 Learns multiple paths via internal
and external BGP speakers
 Picks the best path and installs in
the IP forwarding table
 Policies applied by influencing the
best path selection
When should I use BGP?
 Dual- or multi-homed
 Providing partial or full Internet
routing to a downstream customer
 Anytime the AS path information is required
 Multi-Homed AS
 AS 100
 AS 200
 AS 300
 A
 B
 C

3.  D
BGP is not needed...
 ... if singularly homed
 … if not providing downstream routing
 Use a default route!
 PPT Slide
 Default Route
 Static Route
 204.10.0/23
 Upstream
 Provider
 AS100
BGPAttributes
 What is an attribute?
 AS path
 Next hop
 Local preference
 Multi-exit discriminator (MED)
 Others
What Is an Attribute?
 Describes the characteristics of a particular prefix

4.  Transitive or non-transitive
 13 attributes
 Some are mandatory
BGPAttributes—AS Path
 Sequence of AS’s a route has traversed
 Loop detection
 Apply policy
 AS-Path
 AS 100
 AS 300
 AS 200
 AS 500
 AS 400
 170.10.0.0/16
 180.10.0.0/16
 150.10.0.0/16
 180.10.0.0/16 300 200 100
 170.10.0.0/16 300 200
 150.10.0.0/16 300 400
 180.10.0.0/16 300 200 100
 170.10.0.0/16 300 200
BGPAttributes—Next Hop
 Next hop to reach a network
 Usually the local net is the next hop
in eBGP session
 Third party next hop in iBGP session

5.  eBGP and Next Hop
 160.10.0.0/16
 150.10.0.0/16
 150.10.1.1
 150.10.1.2
 AS 100
 AS 300
 AS 200
 150.10.0.0/16 150.10.1.1
 160.10.0.0/16 150.10.1.1
 A
 B
BGPAttributes—Next Hop (Cont.)
 Next hop of external routes not changed when
announced to iBGP neighbors
 iBGP and Next Hop
 160.10.0.0/16
 150.10.0.0/16
 150.10.1.1
 150.10.1.2
 AS 100
 AS 300
 AS 200
 150.10.0.0/16 150.10.1.1
 160.10.0.0/16 150.10.1.1
 B
 C
 A
BGPAttributes—Next Hop (Cont.)
 IGP should carry route to next hops
 Recursive route look-up

6.  Decouples BGP from actual physical topology
BGPAttributes—Local Preference
 Local to AS
 Used to influence BGP path selection
 Path with highest local preference wins
 Local Preference
 AS 400
 AS 200
 160.10.0.0/16
 AS 100
 AS 300
 A
 B
 C
 D
 E
 160.10.0.0/16 500
 > 160.10.0.0/16 800
 500
 800
Multi-Exit Discriminator (MED)
 Non-transitive
 Used to convey the relative preference of entry points
 Influences best path selection
 Comparable if paths are from same AS
 IGP metric can be conveyed as MED

7.  Multi-Exit Discriminator
 AS 201
 AS 200
 A
 C
 B
 192.68.1.0/24
 192.68.1.0/24 1000
 192.68.1.0/24 2000
Applying Policy with BGP
 Policy based on AS path, community
or the network
 Rejecting/accepting selected routes
 Set attributes to influence path selection
Communities
 BGP attribute
 Used to group destinations
 Represented as an integer
 Each destination could be member
of multiple communities
 Community attribute carried across AS’s
 Useful in applying policies
Community Attribute

8.  RFC1997, RFC1998
 Optional attribute
 Range: 0 to 4,294,901,760
 Method to group destinations
into communities and apply
routing decisions (accept, prefer,
redistribute, etc.) using route-maps
Community attribute (cont.)
 Format -- aa:nn
 `aa’ -- ASN
 `nn’ -- 1 to 4,294,901,760
 BGP Communities
 Community 10:200
 Community 10:300
 Community 10:200
 Community 10:300
Load Balancing
 BGP does not load-balance traffic;
it chooses & installs a “best” route.
 BGP Load Balancing Problem
 “Since BGP picks a „best‟ route based upon most specific prefix and shortest
AS_PATH,
it becomes non-trivial to figure out how to manually direct specific portions of
internal traffic (prefixes) in a distributed fashion across multiple external
gateways.”

9. eBGP Multihop
BGP Multipath
Difficulties in Load Balancing
Internal BGP (iBGP)
 Same routing protocol as BGP,
different application
 iBGP should be used when AS_PATH information must
remain intact between multiple eBGP peers
 All iBGP peers must be fully meshed, logically; an iBGP
peer will not advertise
a route learned by one iBGP peer to another iBGP peer
(readvertisement restriction to prevent looping)
 PPT Slide
 AS 1
 AS 2
 eBGP
 eBGP
 eBGP
 iBGP
 iBGP
 Upstream
 Provider B
 AS200
 Upstream
 Provider A
 AS100
iBGP

10. Scaling the iBGP mesh
 Two methods:
BGP Confederations
BGP Route Reflectors
BGP Confederations
 Method to subdivide a single AS into multiple, internal
sub-AS’s, yet still advertise a single AS to external peers
 Reduces iBGP mesh
 BGP Confederation
 AS10
 AS20
 AS30
 AS300
BGP Route Reflectors (RR)
 Another method to reduce iBGP mesh
 iBGP re-advertisement restrictions
are relaxed
 Single iBGP peer advertises (reflects) routes to
subordinate iBGP peers
 Clients must not peer with RR’s
outside of cluster
 Route Reflectors
 (RR)

11.  AS300
 (RR)
 (RR)
Asymmetric Routing Paradigm
 In the Internet, it is impossible
to control end-to-end routing
and paths traversed
 Packets may not always traverse
same downstream path as they
do when forwarded upstream
Asymmetric Routing
Achieving stability
 Using loopbacks
 BGP/IGP Interaction
 Peer groups
 Route-flap dampening
 BGP soft reconfiguration
Stable iBGP Peering
 Peer with loop-back addresses
 iBGP session is not dependent
on a single interface

12.  Loop-back interface does not go down
 Peering to Loop-Back Address
 AS 100
Relationship with IGP
 BGP carries full Internet routing table
 IGP’s are used to carry next hop and interior network
information
 Routes are never redistributed from BGP into IGP (and
vice versa)
 Recursive route lookup
Peer Groups
 iBGP neighbors receive same update
 Group peers with same outbound policy
 Updates are generated once per group
 Makes configuration easier
 Members could have different
inbound policy
 eBGP neighbors
Route Flap Dampening
 Route flap

13. Going up and down of path
Change in attribute
 Ripples through the entire Internet
 Consumes CPU
 Reduce scope of route flap propagation
Route Flap Dampening (Cont.)
 Fast convergence for normal
route changes
 History predicts future behavior
 Suppress oscillating routes
 Advertise stable suppressed routes
Flap Dampening: Terminology
 Penalty
 Decay
 Half-life time
 Suppress-limit
 Reuse-limit
 Suppress


14.  Route Flap Dampening
 0
 1
 2
 3
 4
 5
 6
 7
 8
 9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 0
 1
 2
 3
 4
 Suppress-Limit
 Reuse-Limit
 Penalty
 Time

15. Flap Dampening: Operation
 Add penalty for each flap
 Exponentially decay penalty
 Penalty above suppress-limit—
do not advertise up route
 Penalty decayed below reuse-limit—advertise route
 History path
Flap Dampening: Operation (Cont.)
 Done only for external path
 Alternate paths still usable
 Suppress-limit, reuse-limit and half-life time give
control
 Less overhead
BGP soft reconfig
 Soft reconfig allows BGP policies to be configured &
activated without clearing the BGP session
 Does not invalidate forwarding cache, hence no short-
term interruptions
 Outbound preferable over inbound reconfig

16. Cisco IOS™ Version Information
 Peer-groups—10.2
 Communities—10.3
 Route-reflectors—10.3
 Confederation—10.3
 Route flap dampening—11.0
 BGP Multipath, Soft Reconfig—11.2
Summary
 Use BGP only when needed!
 IGP and conventional BGP
network problems
 BGP gives flexibility and control
 Route reflectors and confederations helps iBGP mesh
scale
Questions?
Thanks for attending!