Routing Policy

23rd February 2019

Overview

Junos routing policy allows an administrator to alter the default behaviour of a routing protocol. All routing protocols try to determine the best path to a destination based on the protocols definition of the best "metric". The Junos routing policy framework allows an administrator to influence the best path based on their business needs.

Usage Examples

  • Alter the best path to a destination
  • Modify or ignore routes that are advertised to you
  • Modify or ignore routes that are advertised to your neighbours
  • Redistribute routes between protocols

Policy Application

There are two places routing policy can be applied.

  • Inbound - Import policy
  • Outbound - Export policy

The use of a routing policy alters a routers view of the network topology. If you apply a policy inbound it will alter the local routers perception of the network topology. Applying a policy outbound will alter a neighbours perception of the network topology.

BGP policies can be applied at three levels. The below list is order from the most to least preferred.

  • Neighbor
  • Group
  • Global

Less preferred policies are ignored if a more preferred match is also configured.

Policy Processing

  • Routing policy is only evaluated against active routes in the routing table.
  • Policies are evaluated in a daisy-chain order known as a policy-chain.
  • Evaluation is halted once a policy match is found and the policy contains a terminating action.
  • If no match is found evaluation will continue to the next policy.
  • If no match is found in any policy the default policy will be evaluated.

Terminating Action

Terminating actions define how a route match is utilised. The options are accept and reject. Their usage is as follows.

  • Accept - Route will be placed in the routing table and advertised to neighbours
  • Reject - Route is ignored and will not be placed in the routing table

Action Modifiers

Prior to a route being accepted or rejected by a terminating action the routes attributes can be modified. Below are some common usage examples of action modifiers.

  • local-preference - Set the BGP local-preference attribute
  • metric - Set the metric for a route
  • next-hop - Set the next-hop interface for a route
  • origin - Set the BGP origin attribute
  • preference - Set the routes preference (AD)

Flow Control

There are two flow control statements that can be used within routing policy.

  • next term - Moves to the next term in the current policy
  • next policy - Moves to the next policy in the policy chain

Policy Configuration

There are two methods to define a routing policy. term based and non-term based.

Non-term based policies can only have one set of match/action statements.

cmd 

policy-options {
    policy-statement POLICY-NAME {
        from {
            MATCH-CONDITIONS;
        }
        then {
            ACTIONS;
        }
    }
}

Term Based

Term based policies allow for more than one set of match/action statements using a term block.

cmd 

policy-options {
    policy-statement POLICY-NAME {
        term TERM-NAME {
            from {
                MATCH-CONDITIONS;
            }
            then {
                ACTIONS;
            }
        }
        term TERM-NAME {
            from {
                MATCH-CONDITIONS;
            }
            then {
                ACTIONS;
            }
        }
    }
}

Note

If a term does no contain a terminating action the next-term is evaluated.

Route Filters

Route filters are used to apply routing policy to a specific route (or set of routes). There are a number of methods to match a route(s) which are outlined below.

exact

  • The exact prefix
  • 192.168.0.0/16

longer

  • Prefixes more specific than the defined prefix
  • 192.168.0.0/16 longer

orlonger

  • Defined prefix plus more specific prefixes
  • 192.168.0.0/16 orlonger

upto

  • Defined prefix and prefixes up to a defined prefix length
  • 192.168.0.0/16 upto /18

prefix-length-range

  • Defined prefix and routes between a defined prefix length start and end
  • 192.168.0.0/16 prefix-length-range /17-/18

through

  • Defined prefixes and any prefixes in between
  • 192.168.0.0/16 through 192.168.0.0/19

Radix Tree

A radix tree is used to match prefixes based on the route filter conditions. The below diagram[1] is a sample radix tree.

radix-tree-1.png

The below diagram[2] illustrates how route filters affect the selection of prefixes in the radix tree.

radix-tree-2.png

Default Routing Policy

The default manner in which Junos handles routing policy is outlined in the table below.

Protocol Import Export
BGP Accept all received IPv4/6 routes and place them in the inet.0/6 table. Re-advertise all IPv4/6 routes to all BGP neighbors apart from iBGP peers unless local router is a route reflector.
OSPF/v3 Accept all OSPF received IPv4/6 routes and place them in the inet.0/6 table. Reject everything. OSPF uses flooding to advertise local and learned routes.
IS-IS Accept all IS-IS received IPv4/6 routes and place them in the inet.0/6 table. Reject everything. IS-IS uses flooding to advertise local and learned routes.
RIP/ng Accept all RIP/ng received IPv4/6 routes and place them in the inet.0/6 table. Reject everything. An export policy is required to export RIP/ng routes.

References

1 JNCIA: Juniper Networks Certified Internet Associate Study Guide - Chapter 4: Routing Policy - Page 164
2 JNCIA: Juniper Networks Certified Internet Associate Study Guide - Chapter 4: Routing Policy - Page 167

Bibliography

Soricelli, J. Hammond, J. Pildush, G. Van Meter, T. and Warble, T. (2003) JNCIA: Juniper Networks Certified Internet Associate Study Guide. Juniper / Sybex
Author Unknown (2012) JNCIA-Junos Study Guide-Part 2. Juniper Networks