1. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-1 Connecting an Enterprise Network to an ISP Network Considering the Advantages of Using BGP

2. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-2 BGP Terminology  Autonomous system (AS): a collection of networks under a single administrative domain  Interdomain routing: routing between the customer and the ISP  Internal routing: uses IGP protocol (RIP, OSPF, EIGRP, and so on) to exchange routing information inside the AS  External routing: uses EGP protocol (BGP) to exchange routes between autonomous systems  Two BGP implementations: –Internal BGP (IBGP): when BGP is used inside an AS –External BGP (EBGP): when BGP is used between autonomous systems

3. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-3 Multihoming Options Multihoming options with respect to connections  Using a single connection to an individual ISP  Using multiple connections to an individual ISP Multihoming options with respect to routing  Default routes from all providers  Default routes and partial Internet routing from the providers  Full Internet routing from the providers

4. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-4 Default Routes from Providers  Customer A receives the default route from each ISP.

5. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-5 Default Routes from Providers (cont.)  One of the ISPs is used for sending traffic out of the customer network.  Can result in the suboptimal routing of packets.

6. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-6 Default Routes and Partial Table from Providers  Customer A receives the default route from each ISP.  Customer A receives a partial routing table from each ISP.

7. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-7 Default Routes and Partial Table from Providers (cont.)  The partial table is used to forward traffic to the correct ISP.  If the destination is unknown, then a default route to one of the ISPs is used.

8. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-8 Full Internet Routing from Providers  Customer A receives a full routing table from each ISP.  Requires that enough memory and CPU resources are available.

9. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-9 Autonomous System  Autonomous system (AS) – a collection of networks under a single technical administration. –16-bit numbers (as of January 2009 32-bit numbers are available) –Ranging from 1 to 65535 –Private AS: 64512–65535  Internet Assigned Numbers Authority (IANA) allocates AS numbers.  IGPs operate within an AS.  BGP is used between autonomous systems.

10. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-10 BGP Routing Between Autonomous Systems  BGP is used to provide an interdomain routing system.  BGP guarantees the exchange of loop-free routing information.  BGP works differently than IGPs. –BGP is a policy-based routing protocol. –Control traffic flow using multiple BGP path attributes.

11. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-11 Path Vector Functionality  BGP announces: –Paths –Networks that are reachable at the end of the path  The path is described by using attributes.  The administrator can define data flow through autonomous systems.

12. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-12 BGP Routing Policies  BGP can support any policy conforming to the hop-by-hop (AS-by-AS) routing paradigm.

13. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-13 Features of BGP BGP is a path vector protocol with the following properties:  Reliable updates: BGP runs on top of TCP (port 179)  Incremental, triggered updates only  Periodic keepalive messages to verify TCP connectivity  Rich metrics (called path vectors or attributes)  Designed to scale to huge internetworks (for example, the Internet) It has enhancements over distance vector protocols.

14. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-14 When to Use BGP  BGP should be used if one of the following is true: –An AS is a transit AS –An AS is multihomed –Inter-AS routing policy must be manipulated  BGP should not be used if one of the following is true: –Single-homed AS –Insufficient memory and processor resources to handle BGP routing –Insufficient understanding of route filtering and BGP path selection process

15. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-15 BGP Databases  BGP neighbor table –List of BGP neighbors  BGP table –List of all networks learned from each BGP neighbor –Multiple paths to same destination network can be present –Each path is associated with BGP attributes  IP routing table (forwarding database) –List of best paths to destination networks used to forward traffic

16. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-16 BGP Message Types BGP defines the following message types:  Open, which includes hold time and BGP router ID  Keepalive  Update –Information for one path only (could be to multiple networks) –Includes path attributes and networks  Notification –When an error is detected –BGP connection closed after message is sent

17. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-17 Summary  BGP is typically used for interdomain routing.  Three common ways to perform multihoming with BGP are as follows: –Each ISP passes only a default route. –Each ISP passes only a default route and specific provider- owned routes. –Each ISP passes all routes.  BGP is the external routing protocol used between autonomous systems. Forwarding is based on policies and not on best path.  BGP routers exchange network reachability information called path vectors, made up of path attributes.  A router running BGP keeps its own tables to store BGP information that it receives from and sends to other routers, including a neighbor table, a BGP table, and an IP routing table.

18. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—6-18