Distance

Scenario to verify BGP distance (administrative distance) configuration.

Administrative distance is how a router decides which route to install when the same destination is reachable through multiple routing protocols. Each protocol has a default distance value, and the route with the lowest distance wins. The default values are: eBGP external routes use 20, iBGP internal routes use 200, locally originated routes use 200, and static routes use 1. This is why static routes typically win over BGP when both exist for the same destination.

There are two commands to modify BGP distances. The first is distance global, which modifies the distance for external routes learned from eBGP peers, internal routes learned from iBGP peers, and local routes originated locally. Each type can be configured independently and uses its default value if not explicitly set. The second command is distance prefix, which sets the distance for all routes received from neighbors whose IP address matches a given prefix. This is useful when you want different distances for different BGP neighbors without changing the global settings.

Test BGP Distance Global

Description

The distance global command modifies the administrative distance for all BGP routes based on their type. It requires configuring all three values together: external, internal, and local. This test first verifies the default behavior where eBGP routes have distance 20 and static routes have distance 1, causing static to win. Then it applies distance global external 5 to lower the eBGP distance below the static route distance of 10, making BGP the preferred path.

Scenario

Note

First we verify default distances: eBGP uses 20, static uses 1, so static wins.

Step 1: Set the following configuration in DUT0 :

set interfaces ethernet eth0 address 10.10.0.100/24
set protocols bgp 100 neighbor peer remote-address 10.10.0.200
set protocols bgp 100 neighbor peer remote-as 200
set protocols bgp 100 neighbor peer route-map import PERMIT
set protocols route-map PERMIT rule 1 action permit
set protocols static route 1.1.1.0/24 blackhole
set system login user admin authentication encrypted-password '$6$GSjsCj8gHLv$/VcqU6FLi6CT2Oxn0MJQ2C2tqnRDrYKNF8HIYWJp68nvXvPdFccDsT04.WtigUONbKYrgKg8d6rEs8PjljMkH0'

Step 2: Set the following configuration in DUT1 :

set interfaces dummy dum0 address 1.1.1.1/24
set interfaces ethernet eth0 address 10.10.0.200/24
set protocols bgp 200 neighbor peer remote-address 10.10.0.100
set protocols bgp 200 neighbor peer remote-as 100
set protocols bgp 200 neighbor peer route-map export PERMIT
set protocols bgp 200 redistribute connected route-map PERMIT
set protocols route-map PERMIT rule 1 action permit
set system login user admin authentication encrypted-password '$6$GSjsCj8gHLv$/VcqU6FLi6CT2Oxn0MJQ2C2tqnRDrYKNF8HIYWJp68nvXvPdFccDsT04.WtigUONbKYrgKg8d6rEs8PjljMkH0'

Attention

Verify eBGP session establishes between DUT0 and DUT1.

Step 3: Run command protocols bgp show ip summary at DUT0 and check if output matches the following regular expressions:

10.10.0.200.*Established

Attention

Verify that DUT0 learns route 1.1.1.0/24 via BGP.

Step 4: Run command protocols bgp show ip at DUT0 and check if output matches the following regular expressions:

1.1.1.0/24

Attention

Verify static route is selected with default distance [1/0] over eBGP [20/0].

Step 5: Run command protocols ip show route at DUT0 and check if output matches the following regular expressions:

B +1.1.1.0/24 \[20/0\].*via 10.10.0.200[\s\S]+S>\* 1.1.1.0/24 \[1/0\].*blackhole

Note

Now we apply distance global external=5. The static route is reconfigured with distance 10, so BGP (5) should win.

Step 6: Set the following configuration in DUT0 :

set interfaces ethernet eth0 address 10.10.0.100/24
set protocols bgp 100 neighbor peer remote-address 10.10.0.200
set protocols bgp 100 neighbor peer remote-as 200
set protocols bgp 100 neighbor peer route-map import PERMIT
set protocols bgp 100 parameters distance global external 5
set protocols route-map PERMIT rule 1 action permit
set protocols static route 1.1.1.0/24 blackhole distance 10
set system login user admin authentication encrypted-password '$6$GSjsCj8gHLv$/VcqU6FLi6CT2Oxn0MJQ2C2tqnRDrYKNF8HIYWJp68nvXvPdFccDsT04.WtigUONbKYrgKg8d6rEs8PjljMkH0'

Attention

Verify eBGP session remains established after configuration change.

Step 7: Run command protocols bgp show ip summary at DUT0 and check if output matches the following regular expressions:

10.10.0.200.*Established

Attention

Verify BGP route is now selected with distance [5/0] over static [10/0].

Step 8: Run command protocols ip show route at DUT0 and check if output matches the following regular expressions:

B>\* 1.1.1.0/24 \[5/0\].*via 10.10.0.200[\s\S]+S +1.1.1.0/24 \[10/0\].*blackhole

Test BGP Distance Prefix

Description

The distance prefix command provides fine-grained control by setting distance based on the neighbor’s IP address rather than changing global values. The prefix parameter must match the neighbor’s IP, not the routes being advertised. In this scenario, DUT0 assigns distance 15 to all routes received from neighbor 10.10.0.200/32. A static blackhole route with distance 10 is also configured. Since 10 is lower than 15, the static route wins, demonstrating that prefix-based distance is being applied to the BGP routes from that specific neighbor.

Scenario

Step 1: Set the following configuration in DUT0 :

set interfaces ethernet eth0 address 10.10.0.100/24
set protocols bgp 100 neighbor peer remote-address 10.10.0.200
set protocols bgp 100 neighbor peer remote-as 200
set protocols bgp 100 neighbor peer route-map import PERMIT
set protocols bgp 100 parameters distance prefix 10.10.0.200/32 distance 15
set protocols route-map PERMIT rule 1 action permit
set protocols static route 1.1.1.0/24 blackhole distance 10
set system login user admin authentication encrypted-password '$6$GSjsCj8gHLv$/VcqU6FLi6CT2Oxn0MJQ2C2tqnRDrYKNF8HIYWJp68nvXvPdFccDsT04.WtigUONbKYrgKg8d6rEs8PjljMkH0'

Step 2: Set the following configuration in DUT1 :

set interfaces dummy dum0 address 1.1.1.1/24
set interfaces ethernet eth0 address 10.10.0.200/24
set protocols bgp 200 neighbor peer remote-address 10.10.0.100
set protocols bgp 200 neighbor peer remote-as 100
set protocols bgp 200 neighbor peer route-map export PERMIT
set protocols bgp 200 redistribute connected route-map PERMIT
set protocols route-map PERMIT rule 1 action permit
set system login user admin authentication encrypted-password '$6$GSjsCj8gHLv$/VcqU6FLi6CT2Oxn0MJQ2C2tqnRDrYKNF8HIYWJp68nvXvPdFccDsT04.WtigUONbKYrgKg8d6rEs8PjljMkH0'

Attention

Verify eBGP session establishes between DUT0 and DUT1.

Step 3: Run command protocols bgp show ip summary at DUT0 and check if output matches the following regular expressions:

10.10.0.200.*Established

Attention

Verify that DUT0 learns route 1.1.1.0/24 via BGP.

Step 4: Run command protocols bgp show ip at DUT0 and check if output matches the following regular expressions:

1.1.1.0/24

Attention

Verify static route [10/0] wins over BGP with prefix distance [15/0].

Step 5: Run command protocols ip show route at DUT0 and check if output matches the following regular expressions:

B +1.1.1.0/24 \[15/0\].*via 10.10.0.200[\s\S]+S>\* 1.1.1.0/24 \[10/0\].*blackhole