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Example: Using Routing Policy to Set a Preference Value for BGP Routes
This example shows how to use routing policy to set the preference for routes learned from BGP. Routing information can be learned from multiple sources. To break ties among equally specific routes learned from multiple sources, each source has a preference value. Routes that are learned through explicit administrative action, such as static routes, are preferred over routes learned from a routing protocol, such as BGP or OSPF. This concept is called administrative distance by some vendors.
Requirements
No special configuration beyond device initialization is required before you configure this example.
Overview
Routing information can be learned from multiple sources, such as through static configuration, BGP, or an interior gateway protocol (IGP). When Junos OS determines a route’s preference to become the active route, it selects the route with the lowest preference as the active route and installs this route into the forwarding table. By default, the routing software assigns a preference of 170 to routes that originated from BGP. Of all the routing protocols, BGP has the highest default preference value, which means that routes learned by BGP are the least likely to become the active route.
Some vendors have a preference (distance) of 20 for external BGP (EBGP) and a distance of 200 for internal BGP (IGBP). Junos OS uses the same value (170) for both EBGP and IBGP. However, this difference between vendors has no operational impact because Junos OS always prefers EBGP routes over IBGP routes.
Another area in which vendors differ is in regard to IGP distance compared to BGP distance. For example, some vendors assign a distance of 110 to OSPF routes. This is higher than the EBGP distance of 20, and results in the selection of an EBGP route over an equivalent OSPF route. In the same scenario, Junos OS chooses the OSPF route, because of the default preference 10 for an internal OSPF route and 150 for an external OSPF route, which are both lower than the 170 preference assigned to all BGP routes.
This example shows a routing policy that matches routes from specific next hops and sets a preference. If a route does not match the first term, it is evaluated by the second term.
Topology
In the sample network, Device R1 and Device R3 have EBGP sessions with Device R2.
On Device R2, an import policy takes the following actions:
For routes received through BGP from next-hop 10.0.0.1 (Device R1), set the route preference to 10.
For routes received through BGP from next-hop 10.1.0.2 (Device R3), set the route preference to 15.
Figure 1 shows the sample network.
CLI Quick Configuration shows the configuration for all of the devices in Figure 1.
The section #configuration178__bgp-preference-policy-st describes the steps on Device R2.
Configuration
CLI Quick Configuration
To quickly configure
this example, copy the following commands, paste them into a text
file, remove any line breaks, change any details necessary to match
your network configuration, and then copy and paste the commands into
the CLI at the [edit] hierarchy level.
Device R1
set interfaces fe-1/2/0 unit 0 family inet address 10.0.0.1/30 set interfaces lo0 unit 0 family inet address 192.168.0.1/32 set protocols bgp group ext type external set protocols bgp group ext export send-direct set protocols bgp group ext peer-as 200 set protocols bgp group ext neighbor 10.0.0.2 set policy-options policy-statement send-direct term 1 from protocol direct set policy-options policy-statement send-direct term 1 then accept set routing-options autonomous-system 100
Device R2
set interfaces fe-1/2/0 unit 0 family inet address 10.0.0.2/30 set interfaces fe-1/2/1 unit 0 family inet address 10.1.0.1/30 set interfaces lo0 unit 0 family inet address 192.168.0.2/32 set protocols bgp group ext type external set protocols bgp group ext import set-preference set protocols bgp group ext export send-direct set protocols bgp group ext neighbor 10.0.0.1 peer-as 100 set protocols bgp group ext neighbor 10.1.0.2 peer-as 300 set policy-options policy-statement send-direct term 1 from protocol direct set policy-options policy-statement send-direct term 1 then accept set policy-options policy-statement set-preference term term1 from protocol bgp set policy-options policy-statement set-preference term term1 from next-hop 10.0.0.1 set policy-options policy-statement set-preference term term1 then preference 10 set policy-options policy-statement set-preference term term2 from protocol bgp set policy-options policy-statement set-preference term term2 from next-hop 10.1.0.2 set policy-options policy-statement set-preference term term2 then preference 15 set routing-options autonomous-system 200
Device R3
set interfaces fe-1/2/1 unit 0 family inet address 10.1.0.2/30 set interfaces lo0 unit 0 family inet address 192.168.0.3/32 set protocols bgp group ext type external set protocols bgp group ext export send-direct set protocols bgp group ext peer-as 200 set protocols bgp group ext neighbor 10.1.0.1 set policy-options policy-statement send-direct term 1 from protocol direct set policy-options policy-statement send-direct term 1 then accept set routing-options autonomous-system 300
Procedure
Step-by-Step Procedure
The following example requires that you navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the Junos OS CLI User Guide.
To configure Device R2:
Configure the device interfaces.
[edit interfaces] user@R2# set fe-1/2/0 unit 0 family inet address 10.0.0.2/30 user@R2# set fe-1/2/1 unit 0 family inet address 10.1.0.1/30 user@R2# set lo0 unit 0 family inet address 192.168.0.2/32
Configure the local autonomous system.
[edit routing-options] user@R2# set autonomous-system 200
Configure the routing policy that sends direct routes.
[edit policy-options policy-statement send-direct term 1] user@R2# set from protocol direct user@R2# set then accept
Configure the routing policy that changes the preference of received routes.
[edit policy-options policy-statement set-preference] user@R2# set term term1 from protocol bgp user@R2# set term term1 from next-hop 10.0.0.1 user@R2# set term term1 then preference 10 user@R2# set term term2 from protocol bgp user@R2# set term term2 from next-hop 10.1.0.2 user@R2# set term term2 then preference 15
Configure the external peering with Device R2.
[edit protocols bgp group ext] user@R2# set type external user@R2# set export send-direct user@R2# set neighbor 10.0.0.1 peer-as 100 user@R2# set neighbor 10.1.0.2 peer-as 300
Apply the
set-preferencepolicy as an import policy.This affects Device R2’s routing table and has no impact on Device R1 and Device R3.
[edit protocols bgp group ext] user@R2# set import set-preference
Results
From configuration mode, confirm your configuration
by entering the show interfaces, show protocols, show policy-options, and show routing-options commands. If the output does not display the intended configuration,
repeat the instructions in this example to correct the configuration.
user@R2# show interfaces
fe-1/2/0 {
unit 0 {
family inet {
address 10.0.0.2/30;
}
}
}
fe-1/2/1 {
unit 0 {
family inet {
address 10.1.0.1/30;
}
}
}
lo0 {
unit 0{
family inet {
address 192.168.0.2/32;
}
}
}
user@R2# show protocols
bgp {
group ext {
type external;
import set-preference;
export send-direct;
neighbor 10.0.0.1 {
peer-as 100;
}
neighbor 10.1.0.2 {
peer-as 300;
}
}
}
user@R2# show policy-options
policy-statement send-direct {
term 1 {
from protocol direct;
then accept;
}
}
policy-statement set-preference {
term term1 {
from {
protocol bgp;
next-hop 10.0.0.1;
}
then {
preference 10;
}
}
term term2 {
from {
protocol bgp;
next-hop 10.1.0.2;
}
then {
preference 15;
}
}
}
user@R2# show routing-options autonomous-system 200;
If you are done configuring the device, enter commit from configuration mode.
Verification
Confirm that the configuration is working properly.
Verifying the Preference
Purpose
Make sure that the routing tables on Device R1 and Device R2 reflect the fact that Device R1 is using the configured EBGP preference of 8, and Device R2 is using the default EBGP preference of 170.
Action
From operational mode, enter the show route protocols
bgp command.
user@R2> show route protocols bgp
inet.0: 7 destinations, 9 routes (7 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
10.0.0.0/30 [BGP/10] 04:42:23, localpref 100
AS path: 100 I, validation-state: unverified
> to 10.0.0.1 via fe-1/2/0.0
10.1.0.0/30 [BGP/15] 04:42:23, localpref 100
AS path: 300 I, validation-state: unverified
> to 10.1.0.2 via fe-1/2/1.0
192.168.0.1/32 *[BGP/10] 04:42:23, localpref 100
AS path: 100 I, validation-state: unverified
> to 10.0.0.1 via fe-1/2/0.0
192.168.0.3/32 *[BGP/15] 04:42:23, localpref 100
AS path: 300 I, validation-state: unverified
> to 10.1.0.2 via fe-1/2/1.0
Meaning
The output shows that on Device R2, the preference values have been changed to 15 for routes learned from Device R3, and the preference values have been changed to 10 for routes learned from Device R1.