Understanding Routing Policies
- date_range31-Aug-17
For closed to routing platform vendors, the catamenia of routes occurs betwixt diverse protocols. If, for example, y'all desire to configure redistribution from RIP to OSPF, the RIP procedure tells the OSPF procedure that it has routes that mightiness last included for redistribution. In Junos OS, in that location is non much straight interaction betwixt the routing protocols. Instead, in that location are fundamental gathering points where all protocols install their routing information. These are the principal unicast routing tables inet.0 together with inet6.0.
From these tables, the routing protocols calculate the best road to each goal together with house these routes inwards a forwarding table. These routes are together with then used to forwards routing protocol traffic toward a destination, together with they tin give notice last advertised to neighbors.
Importing together with Exporting Routes
Two terms—import and export—explain how routes motility betwixt the routing protocols together with the routing table.
When the Routing Engine places the routes of a routing protocol into the routing table, it is importing routes into the routing table.
When the Routing Engine uses active routes from the routing tabular array to ship a protocol advertisement, it is exportingroutes from the routing table.
NOTEThe procedure of moving routes betwixt a routing protocol together with the routing tabular array is described always from the signal of sentiment of the routing table. That is, routes are imported into a routing tabular array from a routing protocol together with they are exported from a routing tabular array to a routing protocol. Remember this distinction when working alongside routing policies.
As shown in Figure 1, y'all purpose import routing policies to command which routes are placed inwards the routing table, together with export routing policies to command which routes are advertised from the routing tabular array to neighbors.
Figure 1: Importing together with Exporting Routes
In general, the routing protocols house all their routes inwards the routing tabular array together with advertise a express laid of routes from the routing table. The full general rules for treatment the routing information betwixt the routing protocols together with the routing tabular array are known equally the routing policy framework.
The routing policy framework is composed of default rules for each routing protocol that create upwards one's hear which routes the protocol places inwards the routing tabular array together with advertises from the routing table. The default rules for each routing protocol are known as default routing policies.
You tin give notice create routing policies to preempt the default policies, which are ever present. A routing policy allows y'all to modify the routing policy framework to adapt your needs. You tin give notice create together with implement your ain routing policies to produce the following:
Control which routes a routing protocol places inwards the routing table.
Control which active routes a routing protocol advertises from the routing table. An active route is a road that is chosen from all routes inwards the routing tabular array to accomplish a destination.
Manipulate the road characteristics equally a routing protocol places the road inwards the routing tabular array or advertises the road from the routing table.
You tin give notice manipulate the road characteristics to command which road is selected equally the active road to accomplish a destination. The active road is placed inwards the forwarding tabular array together with is used to forwards traffic toward the route's destination. In general, the active road is likewise advertised to a router's neighbors.
Active together with Inactive Routes
When multiple routes for a goal be inwards the routing table, the protocol selects an active road together with that road is placed inwards the appropriate routing table. For equal-cost routes, the Junos OS places multiple adjacent hops inwards the appropriate routing table.
When a protocol is exporting routes from the routing table, it exports active routes only. This applies to actions specified past times both default together with user-defined export policies.
When evaluating routes for export, the Routing Engine uses solely active routes from the routing table. For example, if a routing tabular array contains multiple routes to the same goal together with 1 road has a preferable metric, solely that road is evaluated. In other words, an export policy does non evaluate all routes; it evaluates solely those routes that a routing protocol is allowed to advertise to a neighbor.
By default, BGP advertises active routes. However, y'all tin give notice configure BGP to advertise inactive routes, which become to the same goal equally other routes but convey less preferable metrics.
Explicitly Configured Routes
An explicitly configured route is a road that y'all convey configured. Direct routes are non explicitly configured. They are created equally a consequence of IP addresses beingness configured on an interface. Explicitly configured routes include aggregate, generated, local, together with static routes. (An aggregate route is a road that distills groups of routes alongside mutual addresses into 1 route. A generated route is a road used when the routing tabular array has no information virtually how to accomplish a detail destination. A local route is an IP address assigned to a router interface. A static route is an unchanging road to a destination.)
The policy framework software treats straight together with explicitly configured routes equally if they are learned through routing protocols; therefore, they tin give notice last imported into the routing table. Routes cannot last exported from the routing tabular array to the pseudoprotocol, because this protocol is non a existent routing protocol. However, aggregate, direct, generated, together with static routes tin give notice last exported from the routing tabular array to routing protocols, whereas local routes cannot.
Dynamic Database
In Junos OS Release 9.5 together with later, y'all tin give notice configure routing policies together with for certain routing policy objects inwards a dynamic database that is non bailiwick to the same verification required past times the touchstone configuration database. As a result, y'all tin give notice speedily commit these routing policies together with policy objects, which tin give notice last referenced together with applied inwards the touchstone configuration equally needed. BGP is the solely protocol to which y'all tin give notice apply routing policies that reference policies configured inwards the dynamic database. After a routing policy based on the dynamic database is configured together with committed inwards the touchstone configuration, y'all tin give notice speedily brand changes to existing routing policies past times modifying policy objects inwards the dynamic database. Because Junos OS does non validate configuration changes to the dynamic database, when y'all purpose this feature, y'all should attempt out together with verify all configuration changes earlier committing them.
