1. Introduction to BGP

2. What Is BGP? Border Gateway Protocol BGP-4
The de-facto interdomain routing protocol
BGP enables policy in routing:
Which information gets advertised and how
BGP is a Distance Vector like protocol
Within an AS, Internal Gateway Protocol (IGP or I-BGP)
Vectoring Distributed Routing protocol
Each Router knows little about the network topology
Only best next-hops are chosen by each router for each destination network
Best end2end paths result from composition of all next-hop choices
Does not require any notion of distance
Does not require uniform policies at all routers

3. Internet Structure Original idea Backbone service provider “ Consumer”
ISP
Large corporation “
Consumer ”
ISP
Small “
Consumer ”
ISP “
corporation
Consumer ”
ISP
Small
Small
Small
corporation
corporation
corporation
CS 640

4. Internet Structure Today Backbone service provider Peering point
Large corporation
Small
corporation “
Consumer ”
ISP
CS 640

5. Route Propagation in the Internet
Autonomous System (AS)
corresponds to an administrative domain
examples: University, company, backbone network
assign each AS a 16-bit number
Two-level route propagation hierarchy
interior gateway protocol (each AS selects its own)
exterior gateway protocol (Internet-wide standard)
Routes information is propagated at various levels
hosts know local router
local routers know site routers
site routers know core router
core routers know everything
CS 640

6. Popular Interior Gateway Protocols
RIP: Route Information Protocol
distributed with BSD Unix
distance-vector algorithm
based on hop-count (infinity set to 16)
OSPF: Open Shortest Path First
recent Internet standard
uses link-state algorithm
supports load balancing
supports authentication
CS 640

7. EGP: Exterior Gateway Protocol
Overview
Original standard for Internet routing protocol (c 1983)
designed for tree-structured Internet
Single backbone
concerned with reachability, not optimal routes
Protocol messages
neighbor acquisition: one router requests that another be its peer; peers exchange reachability information
neighbor reachability: one router periodically tests if the another is still reachable; exchange HELLO/ACK messages;
uses a k-out-of-n rule: ¼ to stay up, ¾ to establish
routing updates: peers periodically exchange their routing tables (including route weights) using a basic distance vector method
There can be multiple connections between ASs
CS 640

8. Limits of EGP
At first glance, EGP seems like a distance vector protocol since updates carry lists of destinations and distances – but distances are NOT reliable.
EGP was designed to support tree topologies, not meshes
False routes injected by accident can have really bad consequences (black holes) – there is no easy way for dealing with this problem
Loops can easily occur – all we are doing is forwarding routing tables
EGP was not designed to easily support fragmented IP packets – all data is assumed to fit in MTU.
Solutions to these and other EGP problems were all manual
CS 640

9. BGP-4: Border Gateway Protocol
BGP-1 developed in 1989 to address problems with EGP.
Assumes Internet is an arbitrarily interconnected set of ASs
AS traffic types
Local
starts or ends within an AS
Transit
passes through an AS
AS Types
stub AS: has a single connection to one other AS
carries local traffic only
multihomed AS: has connections to more than one AS
refuses to carry transit traffic
transit AS: has connections to more than one AS
carries both transit and local traffic
CS 640

10. BGP-4 contd. Each AS has: BGP goal: find loop free paths between ASs
one or more border routers
Handles inter-AS traffic
one BGP speaker for an AS that participates in routing
BGP speaker establishes BGP sessions with peers and advertises:
local network names
other reachable networks (transit AS only)
gives path information including path weights (MEDs)
withdrawn routes
BGP goal: find loop free paths between ASs
Optimality is secondary goal
It’s neither a distance-vector nor a link-state protocol
Hard problem
Internet’s size (~12K active ASs) means large tables in BGP routers
Autonomous domains mean different path metrics
Need for flexibility
CS 640

11. BGP Example Speaker for AS2 advertises reachability to P and Q
network , , , and , can be reached directly from AS2
Speaker for backbone advertises
networks , , , and can be reached along the path (AS1, AS2).
Speaker can cancel previously advertised paths
Backbone network
(AS 1)
Regional provider A
(AS 2)
Regional provider B
(AS 3)
Customer P
(AS 4)
Customer Q
(AS 5)
Customer R
(AS 6)
Customer S
(AS 7)
128.96
CS 640

12. Some BGP details Path vectors are most important innovation in BGP
Enables loop prevention in complex topologies
If AS sees itself in the path, it will not use that path
Routes can be aggregated
Based on CIDR (classless) addressing
Routes can be filtered
Runs over TCP
Most of the same messages as EGP
Open, Update, Notify, Keepalive
BGP session have only recently been made secure
CS 640

13. BGP in practice 10-20 “tier 1” ASs which are the Internet backbone
Clearly convergence is an issue – why?
Black holes are always a potential problem
There are lots of BGP updates every day!
BGP is really the heart of the Internet
BGP is a means by which network operators control congestion in the Internet.
BGP is really a big problem!
CS 640

14. How A BGP graph Looks Like
AS 2
AS 5
Each AS has designated BGP routers
BGP routers of an AS communicate internally with another protocol (IGP)
AS 4
BGP-speaking routers peer with each other over TCP sessions, and exchange routes through the peering sessions.
Providers typically try to peer at multiple places. Either by peering with the same AS multiple times,
Or because soma Ass are multi-homed, a typical network will have many candidate paths to a given prefix.
--The BGP Route is conceptually a “promise” to carry data to a section of IP space. The route is a “bag” of
Attributes.
--The section of IP space is called the “prefix” attribute of the route.
--As a BGP route travels from AS to AS, the ASN of each AS is stamped on it when it leaves that AS. Called
The AS_PATH attribute, or “as-path” in Cisco-speak
It provides a mechanism for loop detection. Policies may be applied based on AS path.
AS 3
AS 1

15. What is different with BGP?
BGP goal: enable business relationships
Opts for: flexibility, scalability
Performance optimization is secondary

16. Some Basic Numbers ~20,000 Autonomous Systems approx.
Corporate Networks
ISP Internal Networks
National Service Providers
Identified by ASN a 16 bit value
Assigned by IANA
Superlinear growth
From Griffin RFC 1930:”… the administration of an AS appears to other Ases to have a single coherent interior
Routing plan and presents a consistent picture of what networks are reachable through it.

17. Size of the Routing Table at the core of the Internet
Source:

18. Routing is Based on Prefixes
A BGP Routing table has prefixes for entries
For a IP address of a packet, find longest match
Example: packet IP
/16 match for 16 bits
/24 is a longer match
No matches:
/24

19. Prefix Matching in More Detail
For a IP address of a packet, find longest match
Example: Compare
packet IP
With /16
IP :
Mask :
AND :
Prefix :
Equal? Yes

20. Advertising Routing Information
Each AS advertises what it can reach from each BGP router
Policies I: filter what you advertise
Policies II: filter from what you hear advertised
Build up a BGP routing table
Remember which prefix you hear from which link

21. What Does a Routing Table Look Like?
Prefix
Origin AS
Path
/16
123
/24 15
Origin AS “owns” the address

22. Basic AS relationships
Customer – Provider
Customer pays Provider for service
The Customer is always right
Peer to Peer: mutual cooperation
Ex. MCI and AT&T
Sibling-Sibling
Ex. AT&T research and AT&T wireless

23. The Internet as a Directed Graph
Every edge is bidirectional
Business relationships are represented
Provider
Customer
Peer

24. The Initial Idea Data flows between customers-providers
Top level providers are peers
They exchange information to ensure connectivity
What can possibly go wrong?

25. And then came the rain… Thousands of ASs Complicated relationships
Multiple providers for one AS!!
Multihoming
Traffic engineering
I want to use multiple paths and load balance

26. AS Relationships
Provider
Customer
100
200
Peer
Peer 10 11 12 13 1 3 4 2
Customer – Provider: customer pays and is always right
Peer to Peer: Exchange traffic only between their customers
Sibling-Sibling: Exchange traffic at will

27. The Rules of BGP Routing
Transit traffic: traffic that does not go to my customers (or their customers)
A provider carries any traffic to or from a customer
Peers exchange traffic only if between their customers

28. How BGP Policy Restricts Routing
Provider
Customer
100
Peer
200
Peer 10 11 12 13 1 3 4 2
Routing rules:
Provider accept everything
Peer only if it is for its customers
Path Properties:
Up then down
No up-down-up, at most 1 peer-peer steps

29. Implementing BGP Rules
What do you do with an advertisement:
Through customer link
Advertise to all (customers, peers, providers)
Through provider link
Advertise to customer only (and possibly siblings)
Through peer link
Through sibling link
Advertise to all

30. How Policies Affect Routing
A Provider will get rid of traffic as soon as possible,
But a Provider will carry the traffic for its customer
Did anyone say traffic is asymmetric?
Customer 1
ISP1
ISP2
Customer 2