Service Manual

• Filtering on an AS-Path Attribute
• Filtering BGP Routes Using AS-PATH Information
• Redistributing Routes
• Enabling Additional Paths
• Configuring IP Community Lists
• Filtering Routes with Community Lists
• Manipulating the COMMUNITY Attribute
• Changing MED Attributes
• Changing the LOCAL_PREFERENCE Attribute
• Configuring the local System or a Different System to be the Next Hop for BGP-Learned Routes
• Changing the WEIGHT Attribute
• Enabling Multipath
• Filtering BGP Routes Using Route Maps
• Filtering BGP Routes Using AS-PATH Information
• Filtering BGP Routes
• Configuring BGP Route Reflectors
• Aggregating Routes
• Configuring BGP Confederations
• Enabling Route Flap Dampening
• Changing BGP Timers
• Enabling BGP Neighbor Soft-Reconfiguration
• 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.
Border Gateway Protocol IPv4 (BGPv4) 206