1. BGP supplement
Abhigyan Sharma

2. Autonomous systems
AS is a network under a single administrative domain …
… glued to other ASes by the Border Gateway Protocol
But, “Not all ASes are created equal”
Tier 1 (global),
Tier 2 (regional)
Tier 3 (maybe stub ASes)  may not need to run BGP, if they configure a default gateway router

3. Key routing protocols External BGP (eBGP) Internal BGP (iBGP)
Exchange routes with other ASes to obtain reachability information
Internal BGP (iBGP)
Disseminate routes learned via eBGP among routers in AS
Interior Gateway Protocol
Optimize path metrics using a variety of techniques (shortest path, distance vector)

4. BGP protocol IP Prefix: Attributes
OPEN  initiate BGP session over a TCP conneciton with another BGP speaker and exchange all routes with one another
UPDATE 
Announcements : advertise new routes
Withdrawal: remove previously advertised routes
KEEP-ALIVE  because TCP does not have keep alives
Format of messages:
IP Prefix: Attributes

5. Exporting and importing routes
Exporting: which routes should I advertise to another eBGP speaker
Importing: if I receive multiple routes to the same prefix, which one should I install locally
Complex decisions, but one key factor is typical AS policies
Customer > peer > provider

6. LOCAL_PREF attribute How are policies implemented in BGP?
LOCAL_PREF attribute: sets a weight (e.g., 100) for routes originating from a router in neighbouring AS.
LOCAL_PREF is propagated among routers within AS but not outside!
RTC# router bgp 256 neighbor remote−as 100 neighbor remote−as 256 bgp default local−preference 150

7. Disseminating routes within an AS: iBGP
Why?
Multiple eBGP speakers learn routes from different ASes
Must propagate all routes to all routers inside AS (including non-eBGP speakers)
Goals:
Loop-free forwarding: obvious!
Complete visibility: all routers must reach the decision they would have reached if they received all eBGP advertisements

8. iBGP Why do we need a new protocol?
IGP protocols do not provide incremental updates
IGP protocols do not implement all of BGP’s attributes
How to sync eBGP speakers to 100s of routers in a tier-1 AS?
Full-mesh: e(e − 1)/2 + ei BGP sessions (quadratic scaling problem)
Route reflectors:
Configure a group of routers as a client of BGP route reflector

9. Route reflector operations

10. NEXT_HOP attribute
IP address of next hop router along the path to the destination
Mandatory attribute
When crossing AS boundary, NEXT_HOP is updated to the IP of border router

11. ASPATH attribute Complete list of ASes along the path
Achieves loop avoidance
Gives a hint about length of route
LOCAL_PREF > ASPATH 1 2 3 4
/16
/16
AS 1
/16
AS2, AS 1
/16
AS 3, AS 2, AS 1

12. MED (Multi-exit discriminator)
If two ASes linked at multiple location, then MED value may accompany path advertisements
lower MED value  better path
Provides an AS an option to control location of incoming traffic
Provides neighboring AS guidance in choosing among multiple next hop routers
May be ignored!
MED 200
AS 1
AS 2
MED 100

13. Route importing based on multiple criteria
LOCAL_PREF
ASPATH
MED
eBGP > iBGP
IGP path
Router ID
Decreasing order of importance