Administrator Guide

• Manipulating the COMMUNITY Attribute
• Changing MED Attributes
• Changing the LOCAL_PREFERENCE Attribute
• Conguring the local System or a Dierent 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
• Conguring BGP Route Reectors
• Aggregating Routes
• Conguring BGP Confederations
• Enabling Route Flap Dampening
• Changing BGP Timers
• Enabling BGP Neighbor Soft-Reconguration
• Enabling or disabling BGP neighbors
• Route Map Continue
• Enabling MBGP Congurations
• BGP Regular Expression Optimization
• Debugging BGP
• Sample Congurations
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 identies 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), dened 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 trac 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.
164
Border Gateway Protocol IPv4 (BGPv4)