© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—5-1 Customer-to-Provider Connectivity with BGP Connecting a Multihomed Customer to Multiple Service Providers

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—5-2 Outline Overview Configuring BGP for Multihomed Customers Multihomed Customer Address Space Selection Multihomed Customer AS Number Selection AS Number Translation Primary/Backup Link Selection BGP Incoming Link Selection Load Sharing with Multiple Providers Summary

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—5-3 Configuring BGP for Multihomed Customers

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—5-4 Configuring BGP for Multihomed Customers (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—5-5 Multihomed Customer Address Space Selection Provider-independent address space If the customer owns the address space, there should be no limitations regarding announcing it to both service providers. Provider-assigned address space If the customer uses ISP-assigned small address blocks, then there is no purpose in using BGP to provide redundant connectivity. NAT is easier to implement and solves the problem of reverse path.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—5-6 Multihomed Customer AS Number Selection Registered, public AS number (recommended): Preferred option, but difficult to get Does not require ISPs to assign a private AS number Consistent routing information in the Internet Private AS number (discouraged): Easier to get (even easier with AS translation) –One private AS number: The customer has to be able to use the same private AS number with multiple providers. –Multiple private AS numbers: The customer gets a private AS number assigned by each provider and uses one of them internally; the others have to be translated. Causes inconsistent routing information

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—5-7 AS Number Translation On one EBGP adjacency, the real AS number is used. On the other EBGP adjacency, the AS number is translated to the one assigned by the second ISP.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—5-8 AS Number Translation (Cont.) neighbor ip-address local-as private-as router(config-router)# Optionally, the customer can get two different private AS numbers assigned by the service providers. Internally, the customer can use an ISP-assigned AS number or even any other private AS number. Externally, the customer is seen as one private AS number to ISP 1 and as a different AS to ISP 2. Note: When you are using this option, the AS path of the customer network contains two AS numbers. The ISP has to adapt the incoming AS-path filters.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—5-9 Primary/Backup Link Selection Outgoing link selection: You can use the same solution as with multihomed customers connected to one service provider. Incoming link selection: You cannot use the MED because it can be sent only to the neighboring AS and no farther. You must use other means such as BGP communities or AS- path prepending to achieve incoming link selection.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—5-10 BGP Incoming Link Selection BGP communities: –Customer sets the appropriate BGP community attribute on updates sent to the backup ISP –Requires the ISP to translate the BGP community attribute to a local preference attribute that is lower than the default value of 100 –May not work in all situations AS-path prepending: –Multiple copies of customer AS number prepended to the AS path to lengthen the AS path sent over the backup link –Customer not dependent on the provider configuration –Always works

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—5-11 BGP Incoming Link Selection Using BGP Communities

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—5-12 BGP Incoming Link Selection Using BGP Communities (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—5-13 BGP Incoming Link Selection Using AS-Path Prepending

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—5-14 Load Sharing with Multiple Providers Load sharing for outgoing traffic: You can use the same solution as with multihomed customers connected to one service provider. Load sharing for incoming traffic: The only load-sharing option that you can use in this setup is to separate address space into two or more smaller address blocks. Some traffic analysis is needed to fine-tune address space separation according to link bandwidths. You should use AS-path prepending to ensure symmetric routing as well as backup for noncontiguous address blocks.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—5-15 Summary Customers that require the maximum redundancy in their network design should implement a configuration that is multihomed to multiple service providers. A customer that is multihomed to multiple BGP service providers must advertise its address space to both ISPs and take care not to transmit any routing information between the two ISPs. The internal addresses of the customer must be advertised to both ISPs. Depending on the addressing scheme that is used by the customer, NAT may be required. Customers that are connected to only one ISP do not require a public AS number, while customers connected to multiple ISPs must use an AS number that all ISPs agree to.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—5-16 Summary (Cont.) You can use AS number translation to prepend a different AS number to the AS path, which allows the customer to use a single private AS number in the network. Outgoing route selection in primary/backup connectivity is achieved using local preference. Incoming route selection should be implemented using either BGP communities to tag customer routes or AS-path prepending. Load-sharing configurations for outgoing traffic are the same as those used in the scenario in which the customer is multihomed to a single provider. You can perform load sharing of incoming traffic when you are multihomed to multiple providers only if separate address spaces are advertised to each provider. You can also use AS-path prepending of this configuration for fine-tuning.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—5-17