Options and features available to Frontier BGP Customers
Introduction to BGP
BGP performs interdomain routing in IP networks. BGP is an exterior gateway protocol (EGP), which means that it performs routing between multiple autonomous systems or domains and exchanges routing and reachability information with other BGP systems. Autonomous Systems are identified by what is called an AS number or Autonomous System Number.
BGP is necessary when multiple connections exist between a single autonomous system and one or more Internet Service Providers. This configuration is known as multihoming and is useful for those customers requiring higher availability and redundancy than a single Internet connection may provide. The BGP routing protocol is very configurable. Some options and features provided and supported by Frontier are outlined below.
Public AS versus Private AS
When multi homed to more than one service provide (known as traditional multihoming), a public AS number is required. These public AS numbers must be acquired from a globally recognized registry, ARIN for North America, RIPE for Europe, and APNIC for Asia. Costs incurred for obtaining these AS numbers differ from one registry to another.
When multihomed to a single service provider, a public AS number is not required. This is because the AS number must only be unique to that service provider. For this reason, private AS numbers may be used. Private AS number are those reserved numbers in the range 64512 though 65535. These have been allotted by IANA (Internet Assigned Numbers Authority) in the same manner that Private IP addresses have been allotted.
Characteristics of both Public AS and Private AS are outlined below.
- Requires registered public AS
- Requires announcement of netblocks with /24 or shorter prefix length
- Used for multihoming to more than one provider
- Provides ISP redundancy
- Private AS obtained from service provider
- No prefix length requirement associated with netblock announcement
- Used for multihoming to a single provider
- Not required to purchase public AS
Frontier offers both Public and Private AS BGP peering depending on customer needs.
Frontier Route Advertisements
Frontier offers several choices regarding the types of prefixes that are advertised to the customer. Each type is explained below.
Default Route Only
Frontier sends only a default route to the customer (0.0.0.0/0). This is different than a static default that the customer may set themselves in that this default route learned via BGP inherits all of the dynamic characteristics that BGP provides. Choosing this option is useful when low memory use is a priority on the customer router. Note that choosing only a default route may lead suboptimal routing and packet delivery.
Frontier sends to the customer Frontier (AS5650) routes and Frontier EBGP customer routes. At present they number approximately 750. This option is useful when memory constraints on the customer router are important however the customer still wants the ability to make semi-informed routing decisions. Obtaining partial routes without a default is usually not useful due to the fact that it may limit reachability to the entire Internet. For this reason, obtaining only partial routes is not very common and is normally recommended the customer receive partial plus default routes.
Partial plus Default Routes
Frontier sends to the customer Frontier (AS5650) routes and Frontier EBGP customer routes as well as a default route. This option is useful when memory constraints on the customer router are important however the customer still wants the ability to make semi-informed routing decisions. Obtaining the default route in addition to the partial route ensures reachabilty to other networks not specifically advertised with partial routes.
Frontier sends the customer full Internet routes which make up explicit network announcements to every destination on the Internet. At present full Internet routes number approximately 105,000. Obtaining full Internet routes is useful for customers where memory utilization is not an issue on their router as full Internet routes may require 128 MB of memory or more. Obtaining full Internet routes allows for fully intelligent routing decisions and optimal packet delivery.
Full plus Default Routes
Frontier sends the customer full Internet routes as well as a default route. Obtaining full Internet routes is useful for customers where memory utilization is not an issue on their router. In addition they receive a default route which allows routing of packets where an explicit route does not exist. Some customers prefer this due to the fact that it may offer a safety net of sorts. This may not always work however because it may simply result in moving the lack of reachability from the customer's network to Frontier's network. For this reason, choosing Full routes plus a default is not very common.
Routing Policy Control
Frontier routing policy can be manipulated by the customer in a number of different ways. Some affect routing within Frontier's network only, and some affect routing Internet-wide.
Frontier listens to certain BGP communities sent by the customer which allow for manipulation of local-preference on Frontier's side of the BGP session. Local preference is a well known discretionary attribute which affects path selection within Frontier's network only. For additional information on BGP communities including a listing of specific BGP communities Frontier listens for, please see the Customer BGP Routing Policy.
Frontier allows customers to perform autonomous system path prepending which effectively lengthens the as-path seen by BGP speaking routers for netblocks which the customer originates. The as-path is a well known mandatory attribute. Since as-path length plays a large roll in determining path selection, customers may choose to manipulate this in a way that may make one path less favorable than another. Changes to the as-path are seen Internet-wide.
Customers may decide to adjust local preference internal to their own autonomous system. Doing this will affect outbound path selection inside the customer's network. Due to the fact that local preference is so high on the BGP decision making process, this is frequently used to set up a primary and backup Internet connection scenario.