Configuring Backdoor Routes; Figure 39: Administrative Distance And Synchronization - Juniper BGP - CONFIGURATION GUIDE V 11.1.X Configuration Manual

Chapter 1: Configuring BGP Routing
In this example, both router Albany and router Boston have synchronization turned
on. When synchronization is on, BGP propagates a received route to EBGP peers,
even if the IP forwarding table contains a non-BGP route with a better administrative
distance than the BGP route. This example demonstrates why synchronization is
needed.

Figure 39: Administrative Distance and Synchronization

Router Boston does not advertise the route externally to router Philly. At first, this is
because router Boston has not yet heard about the prefix from router NY, and
therefore the IGP route does not appear in router Boston's IP routing table.
BGP routes are not propagated until a route to the prefix by means of any IGP appears
in the IP routing table. In other words, routers connected by means of an IGP must
have a route to the prefix before a BGP speaker can advertise the route it learned
from a peer.
When the RIP route appears on router Boston, the router has both an IBGP route and
a RIP route to the same prefix. Even though the RIP route has a better administrative
distance, the IBGP route is propagated to router Philly because synchronization is
turned on.

Configuring Backdoor Routes

In certain network topologies, a BGP speaker might learn routes to the same prefix
from an external BGP peer and by means of an IGP protocol. Consider the network
structure shown in Figure 40 on page 140.
A company has established an OSPF link between routers NY and Boston. This private
link between the two routers is known as a backdoor link. Router NY learns two routes
to prefix 172.19.0.0/16; one by means of OSPF from router Boston, and one by
means of EBGP from router LA through router SanDiego. As was shown in Table 21
on page 136, EBGP routes have an administrative distance of 20 and are preferred
over IGP routes, which have much higher administrative distances. In this example,
the longer path by means of EBGP is preferred over the OSPF backdoor path with
its distance of 110.
139
Interactions Between BGP and IGPs