Routing Table Features in the Junos OS
The Junos OS maintains two databases for routing information:
- Routing table—Contains all the routing information learned by all routing protocols. (Some vendors refer to this kind of table as a routing information base [RIB].)
- Forwarding table—Contains the routes actually used to forward packets. (Some vendors refer to this kind of table as a forwarding information base [FIB].)
By default, the Junos OS maintains three routing tables: one for IP version 4 (IPv4) unicast routes, a second for multicast routes, and a third for MPLS. You can configure additional routing tables.
The Junos OS maintains separate routing tables for IPv4 and IP version 6 (IPv6) routes.
The Junos OS installs all active routes from the routing table into the forwarding table. The active routes are routes that are used to forward packets to their destinations. The Junos operating system kernel maintains a master copy of the forwarding table. It copies the forwarding table to the Packet Forwarding Engine, which is the component responsible for forwarding packets.
The Junos routing protocol process generally determines the active route by selecting the route with the lowest preference value. The Junos OS provides support for alternate and tiebreaker preferences, and some of the routing protocols, including BGP and MPLS, use these additional preferences.
You can add martian addresses and static, aggregate, and generated routes to the Junos routing tables, configuring the routes with one or more of the properties shown in Table 1.
Table 1: Routing Table Route Properties
Description | Static | Aggregate | Generated |
|---|---|---|---|
Destination address | X | X | X |
Default route to the destination | X | X | X |
IP address or interface of the next hop to the destination | X | – | – |
Label-switched path (LSP) as next hop | X | – | – |
Drop the packets, install a reject route for this destination, and send Internet Control Message Protocol (ICMP) unreachable messages | X | X | X |
Drop the packets, install a reject route for this destination, but do not send ICMP unreachable messages | X | X | X |
Cause packets to be received by the local router | X | – | – |
Associate a metric value with the route | X | X | X |
Type of route | X | X | X |
Preference values | X | X | X |
Additional preference values | X | X | X |
Independent preference (qualified-next-hop statement) | X | – | – |
BGP community information to associate with the route | X | X | X |
Autonomous system (AS) path information to associate with the route | X | X | X |
OSPF tag strings to associate with the route | X | X | X |
Do not install active static routes into the forwarding table | X | – | – |
Install the route into the forwarding table | X | – | – |
Permanently retain a static route in the forwarding table | X | – | – |
Include only the longest common leading sequences from the contributing AS paths | – | X | – |
Include all AS numbers for a specific route | – | X | – |
Retain an inactive route in the routing and forwarding tables | X | X | X |
Remove an inactive route from the routing and forwarding tables | X | X | X |
Active policy to associate with the route | – | X | X |
Specify that a route is ineligible for readvertisement | X | – | – |
Specify route to a prefix that is not a directly connected next hop | X | – | – |
