1. Interdomain Traffic Engineering with BGPBy
Behzad Akbari
Fall 2008
These slides are based on the slides of Tim. G. Griffin (AT&T) and Shivkumar (RPI)

2. Real World: Multiple Links Between Domains
Middle of path 4 3 5 2 7 6 1
Web server
Client

3. BGP Decision Process Lowest IGP cost to next hop
Highest local preference
Lowest AS path length
Lowest ORIGIN type
Lowest MED (with same next hop AS)
I-BGP < E-BGP
Lowest IGP cost to next hop
Lowest router ID of BGP speaker

4. BGP Route Selection Process
If NEXTHOP is inaccessible do not consider the route.
Prefer largest LOCAL-PREF
If same LOCAL-PREF prefer the shortest AS-PATH.
If all paths are external prefer the lowest ORIGIN code (IGP<EGP<INCOMPLETE).
If ORIGIN codes 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)

5. Route Selection Summary
Highest Local Preference
Enforce relationships
Shortest ASPATH
Lowest MED
traffic engineering
i-BGP < e-BGP
Lowest IGP cost
to BGP egress
Lowest router ID

6. Hot-Potato Routing dest multiple egress points New York San Francisco9 10
ISP network
this and the next slide explain the problem. explain egress point
link weights determine both intradomain path and selection of egress points
Dallas
All traffic from customer to peers
All traffic to customer prefixes
with multiple connections
Hot-potato routing = route to closest egress point when there is more than
one route to destination

7. Hot-Potato Routing Change
dest 11
New York
San Francisco 9 10
ISP network
- failure
- planned maintenance
- traffic engineering 11
a typical BGP table has 150K routes.
non-linearity in the system: small changes inside the network may cause many external routes to shift egress points.
anecdote: performance impact for some application
Routes to thousands
of destinations switch
egress points!!!
Dallas
Consequences:
Transient forwarding instability
Traffic shift
Interdomain routing changes

8. 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

9. 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)
Local Preference
AS1
AS2
MED

10. Local Preference Attribute
Local to AS
It is never advertised to an eBGP peer.
Used to influence BGP path selection
AS 3847 F E G C D /
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. /
Preferred by all
AS3847 routers A B
/24
AS 6201

11. 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

12. 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 p 1 4 3
AS 2 doesn’t know how AS 1 will send traffic toward p

13. 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
advertise some prefixes only to some peers
MED
insert MED=IGP cost, usually requires bilateral agreement
AS-Path
artificially increase the length of AS-Path

14. AS Prepending Artificial increasing AS path length “3 4 5” “3 3 3 4 5”
Prepend your own AS in the path
E.g., turn “3 4 5” into “ ”
Hope to make the path less attractive
“3 4 5” 1 3
“ ”

15. ASPATH Padding: Shed inbound traffic
provider
/24
ASPATH =
/24
ASPATH = 2
Padding will (usually)
force inbound
traffic from AS 1
to take primary link
primary
backup
customer
/24
AS 2

16. Padding May Not Shut Off All Traffic
AS 1
AS 3
provider
provider
/24
ASPATH = 2
/24
ASPATH =
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
primary
backup
customer
/24
AS 2

17. 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
“3 4 5” with MED=1 1 3
“3 4 5” with MED=2

18. Hot Potato Routing: Closest Egress Point
/24
egress 2
egress 1
IGP distances 56 15
This Router has two BGP routes to /24.
Hot potato: get traffic off of your network as
Soon as possible. Go for egress 1!

19. Getting Burned by the Hot Potato
Heavy
Content
Web Farm
2865
High bandwidth
Provider
backbone 17
SFF
NYC
Low bandwidth
customer
backbone 56 15
San Diego
tiny http request
huge http reply

20. Cold Potato Routing with MEDs (Multi-Exit Discriminator Attribute)
Heavy
Content
Web Farm
Prefer lower
MED values
2865 17
/24
MED = 56
/24
MED = 15 56 15
/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

21. MEDs Can Export Internal Instability
Heavy
Content
Web Farm
2865 17
FLAP
FLAP
/24
MED = 56 OR 10
/24
MED = 15 10
FLAP
FLAP 56 15
FLAP
/24