1. BGP4 - Border Gateway Protocol

2. Autonomous Systems Routers under a single administrative control are grouped into autonomous systems Identified by a 16 bit Autonomous System Number (ASN) Normally BGP chooses the route with shortest AS PATH as “best”

3. Types of Autonomous Systems Single-homed Multi-homed non-transit Multi-homed transit

4. ASN Numbers 16-bit Integer, 1 thru 65535 Public ASNs 1 thru 64511 –Assigned by ARIN requires multihoming Private ASNs 64512 thru 65535

5. Typical ASN Connections

6. Sample ASNs 2553FSU209Qwest 3447Sprint-TLH701UUNET 6912Hayes3996FIRN 8103DMS22909Comcast 2379Sprint-Orld3506CSIT 1239Sprint-BB10490GIT-I2

7. FSU ASN Connections

8. sh ip bgp neighbor HCS-TLH-2#sh ip bgp sum BGP router identifier 204.194.39.157, local AS number 6912 BGP table version is 37851125, main routing table version 37851125 BGP activity 2780683/27562509 prefixes, 23934532/23593311 paths Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 157.130.64.213 4 701 9884946 287646 37851089 0 0 07:09:12 113693 157.130.68.173 4 701 9704492 294562 37851067 0 0 4d09h 113702 157.130.81.185 4 701 10456245 293187 37851089 0 0 4d09h 113702 204.90.26.101 4 8103 292617 9817792 37851089 0 0 3d00h 82 204.194.39.133 4 6912 295291 292759 37851120 0 0 3w1d 37 204.194.39.158 4 2553 292764 12413184 37851089 0 0 4d09h 4 HCS-TLH-2#

9. sh ip bgp nei 204.194.39.158 routes HCS-TLH-2#sh ip bgp neighbor 204.194.39.158 routes BGP table version is 37851272, local router ID is 204.194.39.157 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 128.186.0.0 204.194.39.158 0 0 2553 2553 i *> 144.174.0.0 204.194.39.158 0 2553 2553 3506 i *> 146.201.0.0 204.194.39.158 0 0 2553 2553 i *> 192.80.53.0 204.194.39.158 86 0 2553 2553 i Total number of prefixes 4 HCS-TLH-2#

10. Route Servers To figure out Internet backbone problems, there are several public route servers Many can be found at www.traceroute.org On the next slide, we look at one route server’s entries for the FSU ASN

11. sh ip bgp 128.186.0.0 route-views.oregon-ix.net>sh ip bgp 128.186.0.0 BGP routing table entry for 128.186.0.0/16, version 1501796 Paths: (51 available, best #6) 6539 3561 1239 2379 3447 2553 216.18.63.137 from 216.18.63.137 (216.18.63.137) Origin IGP, localpref 100, valid, external 5056 701 6912 2553 2553 167.142.3.6 from 167.142.3.6 (167.142.225.101) Origin IGP, localpref 100, valid, external 267 2914 701 6912 2553 2553 204.42.253.253 from 204.42.253.253 (204.42.253.253) Origin IGP, localpref 100, valid, external Community: 267:2914 2914:420 2914:2000 2914:3000 11537 10490 2553 198.32.8.252 from 198.32.8.252 (198.32.8.252) Origin IGP, localpref 100, valid, external, best Community: 11537:950

12. IBGP BGP routers within the same AS speak IBGP with each other BGP routers that speak to each other are called “peers” or “neighbors” IBGP speakers within an AS need not be directly connected to each other, but must be fully meshed or use a route reflector

13. EBGP BGP routers in different ASNs speak EBGP to each other Normally are directly connected Sometimes use EBGP Multihop

14. EBGP - IBGP

15. Public Tools Route Servers, Traceroute Servers, Looking Glass servers at www.traceroute.org Digex Looking-Glass nitrous.digex.net

16. Protocol BGP peers speak using tcp/179 Initially send full routing table, followed by periodic updates Updates are incremental BGP table version keeps track of the state Routing entries include subnet and support CIDR Table entries also have associated attributes

17. BGP attributes Origin ASPATH Weight Local Preference Multi-Exit Descriminator (MED) Communities Origin Type Next-Hop

18. BGP Attributes 2 Both mandatory and optional attributes Route Maps used to manipulate attributes Eg: Can use route maps to prepend the ASPATH on routing annoucements

19. Route Maps Manipulate routes and their attributes See Doyle vol 1, chapter 14, p805 Each has a line number and is processed sequentially (similar to BASIC) Match on IP address, next hop, metric, etc. Can set various attributes

20. BGP Net Advertisements Normally, use a NETWORK statement for networks to be advertised BGP will only advertise a network to its BGP peers if there is a local route for the network. Local routes are often added to the NULL 0 device for this purpose Can disable synchronization but not recommended.

21. BGP Advertisments 2 router bgp 2553 network 128.186.0.0 mask 255.255.0.0 ip route 128.186.0.0 255.255.0.0 null 0

22. Weight Attribute Cisco specific Default 32768 Highest weight preferred when multiple paths to a destination exist Local relevance (not propogated)

23. Local Preference Attribute Only applies when multiple routes to a destination all with the same weight Default is 100 Attribute exchanged within AS Highest local preference is preferred Used to control exit point when your AS has mulitple EBGP connections

24. Metric / MED Attribute MED=Multi Exit Descriminator Gives external AS hints as to your preferred entry to your network Not always honored, ASPATH prepend may be better Low value is preferred

25. Community Attribute Used for grouping destinations Allows applying routing decisions to according to communities. Must explicitly send communities Also used for other enhancements like MPLS/VPNs

26. ASPATHs List of ASNs representing the path to a desination Use ASPATH filters with regular expressions to match “ip as-path 5 permit ^2553” (Matches routes originating from FSU AS) Applied with filter-list or route-map

27. ASPATH Regular Expressions.Single Character Wildcard ^Beginning of String $End of String _Match beginning,end,comma,brace, or space *Zero or more occurances +One or more occurances

28. Regular Expressions Examples _2553_Paths that traverse FSU ^2553$Origin FSU ^2553.*Path from FSU ^$Null path from this AS Experiment with: sh ip bgp regexp REGULAR-EXP

29. Internet Backbone Architecture Backbone ISPs provide service Backbone ISPs peer at the Internet NAPs Large backbone ISPs also privately peer Peering arrangements are largely determined by finances

30. References Internet Routing Architectures 2/e - Halabi RFC1771 BGP4 Case Studies - Halabi