Interdomain Traffic Engineering with
BGP
By
Behzad Akbari
Spring 2011
These slides are based on the slides of Tim. G. Griffin (AT&T) and Shivkumar (RPI)
1
Real World: Multiple Links Between Domains
Multiple links
Middle of path
4
3
5
2
7
1
6
Web server
Client
2
BGP Decision Process

Highest local preference

Lowest AS path length

Lowest MED (with same next hop AS)

I-BGP < E-BGP

Lowest IGP cost to next hop

Lowest router ID of BGP speaker
3
BGP Route Selection Process






Prefer largest LOCAL-PREF
If same LOCAL-PREF prefer the shortest AS-PATH.
If AS-PATHs are the same prefer the lowest MED.
If MED is same, prefer min-cost NEXT-HOP
If routes learned from EBGP or IBGP, prefer paths learnt
from EBGP
Final tie-break: Prefer the route with I-BGP ID (IP address)
4
Route Selection Summary
Highest Local Preference
Enforce relationships
Shortest ASPATH
Lowest MED
i-BGP < e-BGP
traffic engineering
Lowest IGP cost to BGP
egress
Lowest router ID
5
Hot-Potato Routing
multiple egress points
dest
New York
San Francisco
ISP network
10
9
Dallas
Hot-potato routing = route to closest egress point
when
there from
is more
than to peers
-All traffic
customer
one
-Allroute
traffictotodestination
customer prefixes
with multiple connections
6
Hot-Potato Routing Change
dest
New York
San Francisco
- failure
- planned maintenance 11
- traffic engineering
ISP network
9
11
Consequences:
Transient forwarding instability
Traffic shift
Interdomain routing changes
10
Dallas Routes to thousands
of destinations switch
egress points!!!
7
Tuning BGP to control the outgoing traffic



Principle
 To control its outgoing traffic, a domain must tune the BGP
decision process on its own routers
How to tune the BGP decision process ?
Filter some routes learned from some peers

local-pref


usual method of enforcing economical relationships
MED


usually, MED value is set when sending a route
but some routers allow to insert a MED in a received route


allows to prefer routes over others with same AS Path length
IGP cost to nexthop


setting of IGP cost for intradomain traffic engineering
several routes in forwarding table instead of one
8
Load-Balancing Knobs in BGP


LOCAL-PREF: outbound traffic, local preference (box-level
knob)
MED: Inbound-traffic, typically from the same ISP (link-level
knob)
AS1
Local Preference
AS2
MED
9
Local Preference Attribute


Local to AS
 It is never advertised to an eBGP
peer.
AS 3847
Used to influence BGP path selection
F
G
E
C
208.1.1.0/24


Default 100
 Highest local-pref preferred
For example, you can express the
policy “prefer private connect” by
making the “local_pref” be 150 and
leaving all other peers at 100.
D
80
208.1.1.0/24
100
Preferred by all
AS3847 routers
A
B
208.1.1.0/24
AS 6201
10
Controlling incoming traffic by outbound BGP routes

Outbound BGP routes make traffic come in




It’s a lot harder to control inbound traffic as other ASs’
policies complicate your life!
If you are a stub AS with a single connection
 Not much you need to do except to filter out routes not in your AS
If you are a multi-homed stub AS,
 Want to control through which link/provider that traffic to certain
destinations in your AS may take, to load balance or for back-up
If you are an ISP, you want to minimize transit cost,



carry transit traffic from customers only !
use “hot-potato” routing to hand off traffic to
peers/providers as soon as possible
to load balance, or to ensure reliability with back-up
routes
11
Why Inbound Traffic is Hard to Manage

Other ASes decide how to send to you



Destination-based routing
Other ASes decide which path to take
Based on their own policies
2
4
1
p
3
AS 2 doesn’t know how AS 1 will send traffic toward p 12
Tuning BGP to control the incoming traffic

Principle


To control its incoming traffic, a domain must tune the BGP
advertisements sent by its own routers
How to tune the BGP advertisements ?

Do not announce some routes to from some peers


MED


advertise some prefixes only to some peers
insert MED=IGP cost, usually requires bilateral agreement
AS-Path

artificially increase the length of AS-Path
13
AS Prepending

Artificial increasing AS path length



Prepend your own AS in the path
E.g., turn “3 4 5” into “3 3 3 4 5”
Hope to make the path less attractive
1
“3 4 5”
3
“3 3 3 4 5”
14
ASPATH Padding: Shed inbound traffic
AS 1
provider
192.0.2.0/24
ASPATH = 2 2 2
192.0.2.0/24
ASPATH = 2
primary
backup
customer
AS 2
192.0.2.0/24
Padding will (usually)
force inbound
traffic from AS 1
to take primary link
15
Padding May Not Shut Off All Traffic
AS 1
AS 3
provider
provider
192.0.2.0/24
ASPATH = 2
192.0.2.0/24
ASPATH = 2 2 2 2 2 2 2 2 2 2 2 2 2 2
primary
backup
customer
AS 2
192.0.2.0/24
AS 3 will send traffic on
“backup” link because it
prefers customer routes and
local preference is
considered before ASPATH
length!
Padding in this way is often
used as a form of load
balancing
16
Multiple Exit Discriminator (MED)

Tell your neighbor what you want




MED attribute to indicate receiver preference
Decision process picks route with smallest MED
Can use MED for “cold potato” routing
But, have to get your neighbor to accept MEDs
1
“3 4 5” with MED=1
3
“3 4 5” with MED=2
17
Hot Potato Routing: Closest Egress Point
192.44.78.0/24
egress 2
egress 1
15
56
IGP distances
This Router has two BGP routes to 192.44.78.0/24.
Hot potato: get traffic off of your network as
Soon as possible. Go for egress 1!
18
Getting Burned by the Hot Potato
2865
High bandwidth
Provider
backbone
17
SFF
Low bandwidth
customer
backbone
Heavy
Content
Web Farm
NYC
15
56
San Diego
tiny http request
huge http reply
19
Cold Potato Routing with MEDs
(Multi-Exit Discriminator Attribute)
Prefer lower
MED values
2865
17
Heavy
Content
Web Farm
192.44.78.0/24
MED = 56
192.44.78.0/24
MED = 15
15
56
192.44.78.0/24
This means that MEDs must be considered BEFORE
IGP distance!
Note1 : some providers will not listen to MEDs
Note2 : MEDs need not be tied to IGP distance
20
MEDs Can Export Internal Instability
2865
17
FLAP
FLAP
192.44.78.0/24
MED = 56 OR 10
192.44.78.0/24
MED = 15
10
15
Heavy
Content
Web Farm
FLAP
FLAP
56
FLAP
FLAP
192.44.78.0/24
21