•
If the C bit received in the BFD packets was 1, BGP immediately flushes all routes,
determining that the forwarding plane on the BGP peer has gone down.
•
If the C bit received in the BFD packets was 0, BGP marks all routes as stale but does
not flush them because the forwarding plane on the BGP peer might be working and
only the control plane has gone down.
Managing a Large-Scale AS
BGP requires that IBGP peers be fully meshed, creating significant routing overhead as
the number of peers increases. The number of IBGP sessions increases rapidly with the
number of routers:
For example, in an AS with 9 BGP peers, the peers can conduct 36 sessions:
BGP provides the following two alternative configuration strategies to reduce the number
of fully meshed peers:
•
Configure confederations.
•
Configure route reflectors.
Both of these strategies are complex and can create their own problems. Neither strategy
is typically used unless the mesh of IBGP peers approaches 100 sessions per peer.
Configuring a Confederation
IBGP requires that BGP speakers within an AS be fully meshed. You can reduce the IBGP
mesh inside an AS by subdividing the AS into a confederation of sub-ASs. Each sub-AS
must be fully meshed internally, but the sub-ASs do not have to be fully meshed with
each other. Confederations are most useful when the number of IBGP speakers within
an AS increases to the point that each router has about 100 peering sessions.
Figure 41 on page 142 shows a simpler topology. AS 29 consists of 10 fully meshed IBGP
peers (for clarity, only the BGP sessions are shown). Border router Salem has an EBGP
session with a neighbor in AS 325. Border router Boston has an EBGP session with a
neighbor in AS 413.
141Copyright © 2010, Juniper Networks, Inc.
Chapter 1: Configuring BGP Routing
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