Networking guide

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!