Figure 39: Administrative Distance And Synchronization - Juniper JUNOSE 11.2.X BGP AND MPLS Configuration Manual

JunosE 11.2.x BGP and MPLS Configuration Guide
Example 2
136
Suppose router KC originates 172.17.24.0/21 and advertises the route to router Chicago
by means of EBGP. Both router KC and router Chicago are directly connected to the
network represented by 172.17.24.0/21. If you issue the show ip route command on router
Chicago, the BGP route does not appear. Instead, only the connected route is displayed.
Both routes are in the IP routing table, but the show ip route command displays only the
best route. (Use the show ip route all command to display all best routes; in this case
the BGP route and the connected route.) Connected routes have a default distance of
0. Routes learned by means of EBGP have a default value of 20. The connected route is
a better route than the EBGP route and appears in the command display.
In practice, if two BGP peers are connected to the same network, both peers should
originate the route.
Consider the network structure shown in Figure 39 on page 136. Router Chicago originates
prefix 192.168.11.0/24 and advertises it by means of EBGP to router Albany. Router Albany
advertises the route to router Boston by means of IBGP.
Router Albany also redistributes the route into the interior gateway protocol RIP, which
informs router NY of the route. Router NY propagates the route to router Boston by means
of RIP, from which it is injected into BGP.
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
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