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BGP and BGP4+ Introduction
Software Reference Supplement for SwitchBlade® x8112, x908, x900 and x610 Series Switches
C613-50032-01 REV D AlliedWare Plus
TM
Operating System - Software Version 5.4.3-2.6 1.5
Internal and External BGP Concepts
BGP defines two classes of neighbors (peers): internal BGP (iBGP) and external BGP (eBGP).
These terms use the perspective of a single router, with the terms referring to whether a
BGP neighbor is in the same ASN (iBGP) or a different ASN (eBGP).
A BGP router behaves differently in several ways depending on whether the peer
(neighbor) is an iBGP or eBGP peer. The differences include different rules about what
must be true before the two routers can become neighbors, different rules about which
routes the BGP best path algorithm chooses as best, and even some different rules about
how the routers update the BGP AS_Path PA.
When advertising to an eBGP peer, a BGP router updates the AS_Path PA, but it does not
do so when advertising to an iBGP peer.
Public and Private ASNs
IANA administers the assignment of ASNs as it does with IP address prefixes. ASNs must be
assigned as unique values since if ASNs are duplicated, the BGP loop prevention process
may prevent parts of the Internet learning about a route.
IANA controls the ASN numbering space. Using the same process as for IPv4 addresses,
ASNs are assigned to different organizations. The previous 16-bit BGP ASN has a decimal
range of 0 through 65,535. The 32-bit BGP ASN has a decimal range of 1 through
4,294,967,295.
Like the public IPv4 address space, the public BGP ASN space has similar issues. To help
overcome this issue, the ASN assignment process requires that each AS justify whether it
truly needs a publicly unique ASN or whether it can just as easily use a private ASN. RFC
5398 reserves a small range of ASNs for use in documentation so that documentation
avoids the use of ASNs assigned to organizations.
Private ASNs allow routers inside an AS to participate with BGP, using the same ASN as
other organizations. An AS can use a private AS where the AS connects to only one other
ASN. Loops cannot occur at that point in the BGP topology, so unique ASNs in that
network do not exist. Loops cannot occur with BGP advertising the best path for a prefix.
Outbound Routing Toward the Internet
The main reason to use BGP between an Enterprise and an ISP is to influence the choice of
best path (or best route). However, since the majority of the end-to-end routes exist inside
the Internet, it is hard to determine which exit point from the Enterprise is a better route.
Enterprises typically have two major classes of options for outbound routing toward the
Internet: default routing and BGP. Using default routes is reasonable. This section
discusses default routes toward the Internet, and outbound routes toward the Internet.