
Customizing BGP Services
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An autonomous system can include one or more BGP speakers that provide
external route information for the networks within the AS. An AS containing a
single BGP speaker with a single external BGP connection is a stub AS. The BGP
speaker is providing external route information for the networks contained within
its AS only.
Bay Networks supports two versions of the Border Gateway Protocol: BGP-3 and
BGP-4.
• BGP-3 assumes that each advertised network is a natural class network (A, B,
or C) based on its high-order bits. BGP-3 cannot advertise subnets or
supernets.
• BGP-4 has no concept of address classes. Each network listed in the Network
Layer Reachability Information (NLRI) portion of an Update message
contains a prefix length field, which describes the length of the mask
associated with the network. This allows for both supernet and subnet
advertisement. The supernet advertisement is what makes classless
interdomain routing (CIDR) deployment possible.
Interior BGP in Intra-AS Routing
Bay Networks implements Interior BGP (IBGP) intra-AS routing. Under IBGP,
each router in the AS runs an IGP for internal routing updates and also maintains
an IBGP connection to each BGP border router. The IBGP information is used in
conjunction with the IGP route to the authoring BGP border router to determine
the next hop to use for external networks.
No BGP information is carried by the IGP. Each router uses IBGP exclusively to
determine reachability to external networks. When an IBGP update for a network
is received, it can be passed on to IP for inclusion in the forwarding tables only if
a viable IGP route to the correct border gateway is available.
IBGP in a Transit AS
An AS with more than one BGP speaker can use IBGP to provide a transit service
for networks outside the AS. An AS that provides such a service for BGP speakers
is known as a transit AS (see Figure 6-2).
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