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• Aggregating Routes
• Configuring BGP Confederations
• Enabling Route Flap Dampening
• Changing BGP Timers
• Enabling BGP Neighbor Soft-Reconfiguration
• Enabling or disabling BGP neighbors
• Route Map Continue
• Enabling MBGP Configurations
• BGP Regular Expression Optimization
• Debugging BGP
• Sample Configurations
Autonomous Systems (AS)
BGP autonomous systems (ASs) are a collection of nodes under common administration with common network routing policies.
Each AS has a number, which an internet authority already assigns. You do not assign the BGP number.
AS numbers (ASNs) are important because the ASN uniquely identifies each network on the internet. The Internet Assigned
Numbers Authority (IANA) has reserved AS numbers 64512 through 65534 to be used for private purposes. IANA reserves ASNs
0 and 65535 and must not be used in a live environment.
You can group autonomous systems into three categories (multihomed, stub, and transit), defined by their connections and
operation.
● multihomed AS — is one that maintains connections to more than one other AS. This group allows the AS to remain
connected to the Internet in the event of a complete failure of one of their connections. However, this type of AS does not
allow traffic from one AS to pass through on its way to another AS. A simple example of this group is seen in the following
illustration.
● stub AS — is one that is connected to only one other AS.
● transit AS — is one that provides connections through itself to separate networks. For example, in the following illustration,
Router 1 can use Router 2 (the transit AS) to connect to Router 4. Internet service providers (ISPs) are always transit ASs,
because they provide connections from one network to another. The ISP is considered to be “selling transit service” to the
customer network, so thus the term Transit AS.
When BGP operates inside an AS (AS1 or AS2, as seen in the following illustration), it is referred to as Internal BGP (IBGP
Internal Border Gateway Protocol). When BGP operates between ASs (AS1 and AS2), it is called External BGP (EBGP External
Border Gateway Protocol). IBGP provides routers inside the AS with the knowledge to reach routers external to the AS. EBGP
routers exchange information with other EBGP routers as well as IBGP routers to maintain connectivity and accessibility.
Figure 16. Internal BGP
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Border Gateway Protocol IPv4 (BGPv4)