Planning the
Enterprise-to-ISP
Connection
Connecting an Enterprise Network to an ISP Network
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-1
Session Origin Initiation
Enterprise session initiation requirement:
 One-way: Connectivity from an enterprise network toward the Internet is
the only connectivity required.
 Two-way: Connectivity from the Internet to an enterprise network is also
required.
Solutions:
 One-way: private IP address space with address translation
 Two-way: public IP address space (in combination with private)
and proper routing
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-2
Enterprise Network-to-ISP Connectivity
Requirements
 Public IP address space (subpool or whole /24 subnet)
 Link type and bandwidth availability
 Routing options
 Connection redundancy
 Independency in regard to an ISP:
– Public IP address space
– AS number
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-3
Reachability
 Circuit emulation
 Static routes
 MPLS VPNs
 BGP
Static routes and BGP are typically selected for Internet
connectivity.
Previously covered
Not covered yet
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-4
Using Circuit Emulation
 Used to provide different Layer 2 connectivity to customers via the
common Layer 3 infrastructure of a service point
– Ethernet, Frame Relay, PPP, HDLC, ATM, Layer 2 connectivity
– No routing with the service point from the customer perspective
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-5
Using Static Routes
 The customer uses the default route toward the ISP.
 The service provider uses static route(s) for customer public networks.
 No automatic adjustment to any changes in the network.
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-6
Using MPLS VPN
 Used to connect multiple customer locations via a service provider’s common
Layer 3 infrastructure
– A special VPN can be used to provide Internet connectivity
– Routing used can be static or dynamic depending on the SP
– The customer routers are connected to the service provider PE routers
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-7
Using BGP
 The customer deploys BGP to announce its public networks
 The ISP announces a default route, a subset of Internet routes,
or a complete Internet routing table
 Typically used for inter-AS routing
CPE—Customer to Provider Edge router
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-8
Enterprise Network-to-ISP Connection Options
 Single-homed
 Dual-homed
 Multihomed
 Dual-multihomed
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-9
Single-homed ISP Connectivity
Link failure results in broken connectivity.
Option 1: Routing with static routes
 A default route from an enterprise network
 A static route(s) from an ISP for customer networks
Option 2: Routing with BGP
 The customer announces its public network(s)
 The ISP announces the default route to the customer
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-10
Dual-homed ISP Connectivity
Characteristics:
 Connected with two links to the same ISP
 Can use a single router or two edge routers
 Can use static routes or BGP
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-11
Multihomed ISP Connectivity
Characteristics:
 Connected to two or more different ISPs
 Can use a single router or multiple edge routers
 Dynamic routing with BGP
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-12
Dual-multihomed ISP Connectivity
Characteristics:
 Connected to two or more different ISPs with two links per ISP
 Typically uses multiple edge routers (one per ISP)
 Dynamic routing with BGP
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-13
Summary
 Connecting an enterprise network to an ISP requires, at a
minimum, a public IP address pool, a proper link to the ISP,
consideration of redundancy requirements, and the proper
routing protocol.
 To exchange routing updates with an ISP, the customer can use
different options. Static routes and BGP are the options that are
most commonly used.
 The way in which the customer connects to an ISP depends on
the redundancy requirements, where a single-homed connectivity
has no redundancy, and the dual-multihomed connectivity has the
most redundancy built in.
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-14
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—6-15