Juniper IGP - CONFIGURATION GUIDE V11.1.X, Configuration Manual

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The global routing table is a database maintained by IP on the SRP module. It contains
at most one route per protocol to each prefix in the table. Each of these routes is the
best route known by a given protocol to get to that prefix. The IP routing table does
not, for example, have two OSPF routes to 10.5.11.0/24; it will have only one (if any)
OSPF route to that prefix. It might also have a BGP route to the prefix, and a RIP
route to the prefix, but no more than one route to a prefix per protocol.
IP compares the administrative distances for the routes to each prefix and selects
the overall best route regardless of protocol. The best route to 10.5.11.0/24 might
be via IS-IS. The best route to 192.168.0.0/16 might be via EBGP, and so on. These
selected overall best routes to each prefix are used to create the forwarding table.
The forwarding table is pushed to each line module. The line modules use their local
instance of the forwarding table to forward the packets that they receive. When the
global IP routing table is updated, so are the forwarding tables on the line modules.
Figure 10 on page 26 illustrates a very simple network composed of three networks
and two routers. The hosts that are attached to each network are not shown, because
each router makes its forwarding decisions based on the network number and not
on the address of each individual host. The router uses ARP to find the physical
address that corresponds to the Internet address for any host or router on networks
directly connected to it.
Figure 10: Routers in a Small Network
Table 3 on page 27 and Table 4 on page 27 represent information from the routing
tables for routers NY and LA. Each routing table contains one entry for each route
for each protocol or route type. Each routing table entry includes the following
information:
■
The destination IP network address.
■
The IP address of the next-hop router.
■
The type of network, such as static, directly connected, or the particular protocol.
■
An administrative distance that is used to select the least-cost route among
multiple routes to the same destination network. The least-cost (best) route is
placed in the forwarding table. The administrative distance is not included in
the forwarding table.
■
A metric that is used by protocols to which the route is redistributed to select
the least-cost route among multiple routes to the same destination network. The
metric is not used to determine the best route to be placed in the forwarding
table. The metric is also not listed in the forwarding table.
26 ■
IP Routing
JUNOSe 11.0.x IP, IPv6, and IGP Configuration Guide