CS540/TE630 Computer Network Architecture Spring 2009 Tu/Th 10:30am-Noon Sue Moon

Routing  What do you remember from undergrad networking courses?

Questions

4 BGP  De-facto standard inter-domain routing protocol  Became popular only in 1995  significant increase in # of ISPs  CIDR introduced in 1995  Path vector algorithm

5 Configuration and Policy  A BGP node decides which routes to share with its neighbor  A BGP node can selectively accept and reject messages  What to share and what to accept  determined by routing policy

6 Four Basic BGP Messages  Open  Establishes BGP session (TCP port #179)  Sets the hold timer  Notification  Report unusual conditions  Terminates the TCP session and gives an indication (holder timer expiry, bad peer AS, malformed attribute list, etc.)  Update  Inform neighbor of new/old routes that become active/inactive  Keepalive  Inform neighbor that connection is still alive

7 UPDATE Message  Advertise/Withdraw prefixes Withdrawn routes length (2 bytes) Withdrawn routes (variable length) Total path attributes length (2 bytes) Path attributes (variable length) Reachability information (variable length)

8 Attributes  ORIGIN  Who originated the announcement?  IGP, EGP or Incomplete (often for static routes)  AS-PATH  list of AS's  useful to detect and prevent loops  NEXT HOP  For EBGP, IP addr of neighbor that announced  For IBGP, if route originated inside, IP addr of neighbor  For IBGP, if route originated outside, EBGP node that learned of route, is carried unaltered into IBGP  Multi-Exit Discriminator (MED)  Local Preference

9 Attribute: Multi-Exit Discriminator (MED)  When ASes have multiple interconnecting links  Lower, more preferred  Non-transitive AS1 AS2 R1 R3R4 R /16 MED= /16 MED=10

10 Attribute: Local Pref  Indicates preference among multiples paths for the same prefix  higher, more preferred  Exchanged between IBGP peers only  Often used to select a specific egress point for a particular destination AS1 AS4 AS2 AS /16 DestinationAS PathLocal Pref /16AS3 AS /16AS2 AS1100

11 BGP Decision Process 1.Highest LOCAL-PREF 2.Shortest AS-PATH 3.Lowest ORIGIN (IGP < EGP < Incomplete) 4.Lowest MED 5.Min cost path to NEXT HOP using IGP metrics 6.BGP Router ID to break tie

12 Input Policy Engine  Inbound filtering  filter based on IP prefixes, AS_PATH, community  deny = BGP won't reach that prefix via the peer  accept = traffic to that prefix via the peer  Attribute manipulation  Sets attributes on accepted routes E.g.: Specify LOCAL-PREF to set priorities among multiple peers

13 Output Policy Engine  Outbound filtering  forward = peers may route traffic via you  Attribute manipulation  Sets attributes such as AS-PATH and MEDs

14 Transit vs. Nontransit AS3 AS2 AS1 C1 C3 C2 Transit

15 Routing Engine BGP Input Policy BGP Table IP Routing Table OSPF Topology Shortest Path Forwarding Table BGP Output Policy

16 References & Acknowledgements  Some use of Nina Taft's tutorial slides on BGP  BGP4 Inter-Domain Routing in the Internet, John W. Stewart, Addison-Wesley, 1998