Before the router even starts the route selection process it needs to make sure that the route is valid, checks the Martian routes, AS loops and next-hop reachability.
Best Path Selection
| Attribute | Preference | Default Behavior |
|---|---|---|
| 1. Route Preference | Lowest | Default value is 170 |
| 2. Local Preference | Highest | Default learned value is 100 |
| 3. AS-Path | Shortest | Usualy the first time route selection occurs |
| 4. Origin | Lowest | IGP (I) > EGP (E), EGP > incomplete (?) |
| 5. Multi Exit Discriminator (MED) | Lowest | Compared when routes are learned from the same AS |
| 6. Route Type | External | Internal vs External, External prefered |
| 7. IGP Cost | Lowest | lowest cost towards the BGP next-hop |
| 8. Internal vs External | Lowest | Lowest R-ID, Oldest active route |
| 10. Cluster List | Shortest | When route reflection is used |
| 11. Peer IP Address | Lowest | Multiple peerings between the same router |
For JUNOS the route selection results and criteria can be seen in the output of:
- show route details
- show route extensive
Lowest Route Preference
The Juniper default BGP preference is 170 below you can see a route selection which is based on a non-default route preference of 160. Junos OS is kind enough to tell us why the second route has lost in the route selection.
The Inactive Reason is Route Preference
jcluser@vMX4# run show route 192.168.250.0/24 exact detail
inet.0: 14 destinations, 15 routes (14 active,0 holddown,0 hidden)
192.168.250.0/24 (2 entries, 1 announced)
*BGP Preference: 160/-101
Next hop type: Router, Next hop index: 595
Address: 0xc4b5f5c
Next-hop reference count: 2
Source: 10.100.34.1
Next hop: 10.100.34.1 via ge-0/0/1.0,selected
Session Id: 0x141
State: <Active Ext>
Local AS: 64533 Peer AS: 64522
Age: 7
Validation State: unverified
Task: BGP_64522.10.100.34.1
Announcement bits(2):0-KRT 4-BGP_RT_Background
AS path: 64522 I
Accepted
Localpref: 100
Router ID: 10.100.100.3
Thread: junos-main
BGP Preference: 170/-101
Next hop type: Router,Next hop index: 596
Address: 0xc4b5ef4
Next-hop reference count: 1
Source: 10.100.24.1
Next hop: 10.100.24.1 via ge-0/0/0.0,selected
Session Id: 0x140
State: <Ext>
Inactive reason: Route Preference
Local AS: 64533 Peer AS: 64522
Age: 9:18
Validation State: unverified
Task: BGP_64522.10.100.24.1
AS path: 64522 I
Accepted
Localpref: 100
Router ID: 10.100.100.2
Thread: junos-main
Highest Local Preference
The default BGP Local Preference is 100 the route below has been selected for its higher non-default value of 200.
The Inactive Reason is Local Preference
jcluser@vMX4# run show route 192.168.250.0/24 exact detail
inet.0: 14 destinations, 15 routes (14 active, 0 holddown, 0 hidden)
192.168.250.0/24 (2 entries, 1 announced)
*BGP Preference: 170/-201
Next hop type: Router, Next hop index: 595
Address: 0xc4b5f5c
Next-hop reference count: 2
Source: 10.100.34.1
Next hop: 10.100.34.1 via ge-0/0/1.0, selected
Session Id: 0x141
State: <Active Ext>
Local AS: 64533 Peer AS: 64522
Age: 24
Validation State: unverified
Task: BGP_64522.10.100.34.1
Announcement bits (2): 0-KRT 4-BGP_RT_Background
AS path: 64522 I
Accepted
Localpref: 200
Router ID: 10.100.100.3
Thread: junos-main
BGP Preference: 170/-101
Next hop type: Router, Next hop index: 596
Address: 0xc4b5ef4
Next-hop reference count: 1
Source: 10.100.24.1
Next hop: 10.100.24.1 via ge-0/0/0.0, selected
Session Id: 0x140
State: <Ext>
Inactive reason: Local Preference
Local AS: 64533 Peer AS: 64522
Age: 51:52
Validation State: unverified
Task: BGP_64522.10.100.24.1
AS path: 64522 I
Accepted
Localpref: 100
Router ID: 10.100.100.2
Thread: junos-main
Shortest AS Path
The route below has been selected because of its shortest AS Path.
The selected route has 1 AS vs 5 AS.
The Inactive Reason is AS Path
jcluser@vMX4# run show route 192.168.250.0/24 exact detail
inet.0: 14 destinations, 15 routes (14 active, 0 holddown, 0 hidden)
192.168.250.0/24 (2 entries, 1 announced)
*BGP Preference: 170/-101
Next hop type: Router, Next hop index: 596
Address: 0xc4b5ef4
Next-hop reference count: 2
Source: 10.100.24.1
Next hop: 10.100.24.1 via ge-0/0/0.0, selected
Session Id: 0x140
State: <Active Ext>
Local AS: 64533 Peer AS: 64522
Age: 1:23:04
Validation State: unverified
Task: BGP_64522.10.100.24.1
Announcement bits (2): 0-KRT 4-BGP_RT_Background
AS path: 64522 I
Accepted
Localpref: 100
Router ID: 10.100.100.2
Thread: junos-main
BGP Preference: 170/-101
Next hop type: Router, Next hop index: 595
Address: 0xc4b5f5c
Next-hop reference count: 1
Source: 10.100.34.1
Next hop: 10.100.34.1 via ge-0/0/1.0, selected
Session Id: 0x141
State: <Ext>
Inactive reason: AS path
Local AS: 64533 Peer AS: 64522
Age: 15
Validation State: unverified
Task: BGP_64522.10.100.34.1
AS path: 64522 64522 64522 64522 64522 I
Accepted
Localpref: 100
Router ID: 10.100.100.3
Thread: junos-main
Lowest Origin
The active route is chosen based on its lower origin setting of I (IGP), compared to the second route which has an origin of ? (Incomplete).
The inactive reason is Origin
jcluser@vMX4# run show route 192.168.250.0/24 exact detail
inet.0: 14 destinations, 15 routes (14 active, 0 holddown, 0 hidden)
192.168.250.0/24 (2 entries, 1 announced)
*BGP Preference: 170/-101
Next hop type: Router, Next hop index: 596
Address: 0xc4b5ef4
Next-hop reference count: 2
Source: 10.100.24.1
Next hop: 10.100.24.1 via ge-0/0/0.0, selected
Session Id: 0x140
State: <Active Ext>
Local AS: 64533 Peer AS: 64522
Age: 1:45:36
Validation State: unverified
Task: BGP_64522.10.100.24.1
Announcement bits (2): 0-KRT 4-BGP_RT_Background
AS path: 64522 I
Accepted
Localpref: 100
Router ID: 10.100.100.2
Thread: junos-main
BGP Preference: 170/-101
Next hop type: Router, Next hop index: 595
Address: 0xc4b5f5c
Next-hop reference count: 1
Source: 10.100.34.1
Next hop: 10.100.34.1 via ge-0/0/1.0, selected
Session Id: 0x141
State: <Ext Changed>
Inactive reason: Origin
Local AS: 64533 Peer AS: 64522
Age: 10
Validation State: unverified
Task: BGP_64522.10.100.34.1
AS path: 64522 ?
Accepted
Localpref: 100
Router ID: 10.100.100.3
Thread: junos-main
Lowest MED
The active route has a MED value of 20 shown as metric in the output.
The second route has a MED of 150.
Inactive reason Not Best in its group - Route Metric or MED comparison
jjcluser@vMX4# run show route 192.168.250.0/24 exact detail
inet.0: 14 destinations, 15 routes (14 active, 0 holddown, 0 hidden)
192.168.250.0/24 (2 entries, 1 announced)
*BGP Preference: 170/-101
Next hop type: Router, Next hop index: 595
Address: 0xc4b5f5c
Next-hop reference count: 2
Source: 10.100.34.1
Next hop: 10.100.34.1 via ge-0/0/1.0, selected
Session Id: 0x141
State: <Active Ext>
Local AS: 64533 Peer AS: 64522
Age: 36:03 Metric: 20
Validation State: unverified
Task: BGP_64522.10.100.34.1
Announcement bits (2): 0-KRT 4-BGP_RT_Background
AS path: 64522 I
Accepted
Localpref: 100
Router ID: 10.100.100.3
Thread: junos-main
BGP Preference: 170/-101
Next hop type: Router, Next hop index: 596
Address: 0xc4b5ef4
Next-hop reference count: 1
Source: 10.100.24.1
Next hop: 10.100.24.1 via ge-0/0/0.0, selected
Session Id: 0x140
State: <NotBest Ext>
Inactive reason: Not Best in its group -
Route Metric or MED comparison
Local AS: 64533 Peer AS: 64522
Age: 13 Metric: 150
Validation State: unverified
Task: BGP_64522.10.100.24.1
AS path: 64522 I
Accepted
Localpref: 100
Router ID: 10.100.100.2
Thread: junos-main
Route Type
The directly adverised external route is preferred over the external route learned via an internal neighbor.
This can be seen from the Local AS and Peer AS line.
The Local AS and Peer AS line gives information on if the route is learned from an
internal or external neighbor.
The order of preference is:
- Local redistributed routes into BGP (Interior)
- External route directly learned from External BGP peers (Exterior)
- External route learned through an Internal BGP peer (Exterior via Interior)
Inactive reason Interior > Exterior > Exterior via Interior
jcluser@vMX6# run show route 192.168.250.0/24 exact detail
inet.0: 11 destinations, 12 routes (11 active, 0 holddown, 0 hidden)
192.168.250.0/24 (2 entries, 1 announced)
*BGP Preference: 170/-101
Next hop type: Router, Next hop index: 587
Address: 0xc4b5dbc
Next-hop reference count: 2
Source: 10.100.46.1
Next hop: 10.100.46.1 via ge-0/0/3.0, selected
Session Id: 0x141
State: <Active Ext>
Local AS: 64544 Peer AS: 64533
Age: 50
Validation State: unverified
Task: BGP_64533.10.100.46.1
Announcement bits (3):0-KRT 4-BGP_RT_Background
5-Resolve tree 1
AS path: 64533 I
Accepted
Localpref: 100
Router ID: 10.100.100.4
Thread: junos-main
BGP Preference: 170/-101
Next hop type: Indirect, Next hop index: 0
Address: 0xc4b5d54
Next-hop reference count: 1
Source: 10.100.100.5
Next hop type: Router, Next hop index: 349
Next hop: 100.123.0.1 via fxp0.0, selected
Session Id: 0x0
Protocol next hop: 10.100.25.1
Indirect next hop: 0xc61de84 338 INH Session ID: 0x0
State: <Int Ext>
Inactive reason: Interior >
Exterior >
Exterior via Interior
Local AS: 64544 Peer AS: 64544
Age: 6:58 Metric2: 0
Validation State: unverified
Task: BGP_64544.10.100.100.5
AS path: 64522 I
Accepted
Localpref: 100
Router ID: 10.100.100.5
Thread: junos-main
Lowest IGP cost
The active route is chosen based on the lowest IGP cost to the BGP next-hop, shown as protocol next-hop in the output.
The IGP cost is shown as metric2 in the output.
The active route has a metric2 of 5 vs metric2 of 10 for the inactive.
Inactive reason IGP metric
jcluser@vMX4# run show route 192.168.252.0/24 exact detail
inet.0: 16 destinations, 18 routes (16 active, 0 holddown, 0 hidden)
192.168.252.0/24 (2 entries, 1 announced)
*BGP Preference: 170/-101
Next hop type: Router, Next hop index: 595
Address: 0xc4b5f5c
Next-hop reference count: 3
Source: 10.100.34.1
Next hop: 10.100.34.1 via ge-0/0/1.0, selected
Next hop: 10.100.24.1 via ge-0/0/1.0
Session Id: 0x141
State: <Active Ext>
Local AS: 64533 Peer AS: 64522
Age: 16:59 Metric2: 5
Validation State: unverified
Task: BGP_64522.10.100.34.1
Announcement bits (2): 0-KRT 4-BGP_RT_Background
AS path: 64522 64544 I
Accepted
Localpref: 100
Router ID: 10.100.100.3
Thread: junos-main
BGP Preference: 170/-101
Address: 0xc4b5fc4
Next-hop reference count: 2
Source: 10.100.24.1
Next hop type: Router, Next hop index: 0
Next hop: 10.100.34.1 via ge-0/0/1.0, selected
Next hop: 10.100.24.1 via ge-0/0/1.0
Session Id: 0x0
State: <NotBest Ext Changed>
Inactive reason: Not Best in its group - IGP metric
Local AS: 64533 Peer AS: 64522
Age: 16:59 Metric2: 10
Validation State: unverified
Task: BGP_64522.10.100.24.1
AS path: 64522 64544 I
Accepted
Localpref: 100
Router ID: 10.100.100.2
Thread: junos-main
Lowest R-ID or Oldest Active
Internal sessions Router-ID
The inactive route has a router-id 10.100.100.2 which is higher than the 10.100.100.1.
Inactive reason Not Best in its group - Router ID
jcluser@vMX4# run show route 192.168.252.0/24 exact detail
inet.0: 16 destinations, 18 routes (16 active, 0 holddown, 0 hidden)
192.168.252.0/24 (2 entries, 1 announced)
*BGP Preference: 170/-101
Next hop type: Router, Next hop index: 595
Address: 0xc4b5f5c
Next-hop reference count: 2
Source: 10.100.34.1
Next hop: 10.100.34.1 via ge-0/0/1.0, selected
Session Id: 0x141
State: <Active Ext>
Local AS: 64533 Peer AS: 64522
Age: 1:00
Validation State: unverified
Task: BGP_64522.10.100.34.1
Announcement bits (2): 0-KRT 4-BGP_RT_Background
AS path: 64522 64544 I
Accepted
Localpref: 100
Router ID: 10.100.100.1
Thread: junos-main
BGP Preference: 170/-101
Next hop type: Router, Next hop index: 0
Address: 0xc4b5fc4
Next-hop reference count: 1
Source: 10.100.24.1
Next hop: 10.100.24.1 via ge-0/0/0.0, selected
Session Id: 0x0
State: <NotBest Ext Changed>
Inactive reason: Not Best in its group -
Router ID
Local AS: 64533 Peer AS: 64522
Age: 1:00
Validation State: unverified
Task: BGP_64522.10.100.24.1
AS path: 64522 64544 I
Accepted
Localpref: 100
Router ID: 10.100.100.2
Thread: junos-main
External sessions Router-ID
To use the Router-ID as a tie-breaker the command path-selection external-router-id needs to be coinfigured. The inactive route behaves as the internal and has a router-id 10.100.100.2 which is higher than the 10.100.100.1.
Inactive reason Not Best in its group - Router ID
jcluser@vMX4# run show route 192.168.252.0/24 exact detail
inet.0: 16 destinations, 18 routes (16 active, 0 holddown, 0 hidden)
192.168.252.0/24 (2 entries, 1 announced)
*BGP Preference: 170/-101
Next hop type: Router, Next hop index: 595
Address: 0xc4b5f5c
Next-hop reference count: 2
Source: 10.100.34.1
Next hop: 10.100.34.1 via ge-0/0/1.0, selected
Session Id: 0x141
State: <Active Ext>
Local AS: 64533 Peer AS: 64522
Age: 1:00 Metric2: 5
Validation State: unverified
Task: BGP_64522.10.100.34.1
Announcement bits (2): 0-KRT 4-BGP_RT_Background
AS path: 64522 64544 I
Accepted
Localpref: 100
Router ID: 10.100.100.1
Thread: junos-main
BGP Preference: 170/-101
Next hop type: Router, Next hop index: 0
Address: 0xc4b5fc4
Next-hop reference count: 1
Source: 10.100.24.1
Next hop: 10.100.24.1 via ge-0/0/0.0, selected
Session Id: 0x0
State: <NotBest Ext Changed>
Inactive reason: Not Best in its group -
Router ID
Local AS: 64533 Peer AS: 64522
Age: 1:00
Validation State: unverified
Task: BGP_64522.10.100.24.1
AS path: 64522 64544 I
Accepted
Localpref: 100
Router ID: 10.100.100.2
Thread: junos-main
External sessions
For Satbility reasons the oldest route is prefered
The inactive reason Not Best in its group - Active preferred
jcluser@vMX4# run show route 192.168.252.0/24 exact detail
inet.0: 16 destinations, 18 routes (16 active, 0 holddown, 0 hidden)
192.168.252.0/24 (2 entries, 1 announced)
*BGP Preference: 170/-101
Next hop type: Router, Next hop index: 595
Address: 0xc4b5f5c
Next-hop reference count: 2
Source: 10.100.34.1
Next hop: 10.100.34.1 via ge-0/0/1.0, selected
Session Id: 0x141
State: <Active Ext>
Local AS: 64533 Peer AS: 64522
Age: 1:00
Validation State: unverified
Task: BGP_64522.10.100.34.1
Announcement bits (2): 0-KRT 4-BGP_RT_Background
AS path: 64522 64544 I
Accepted
Localpref: 100
Router ID: 10.100.100.3
Thread: junos-main
BGP Preference: 170/-101
Next hop type: Router, Next hop index: 0
Address: 0xc4b5fc4
Next-hop reference count: 1
Source: 10.100.24.1
Next hop: 10.100.24.1 via ge-0/0/0.0, selected
Session Id: 0x0
State: <NotBest Ext Changed>
Inactive reason: Not Best in its group -
Active preferred
Local AS: 64533 Peer AS: 64522
Age: 1:00
Validation State: unverified
Task: BGP_64522.10.100.24.1
AS path: 64522 64544 I
Accepted
Localpref: 100
Router ID: 10.100.100.2
Thread: junos-main
Cluster List
The shortest cluster list will be preferred when we have multiple route reflectors.
Inactive reason Not Best in its group - Cluster list length
Output TBD
Lowest Peer IP
This is the last tie-breaker we can come this when for load-balancing purposed two routers peer two or more time with eachother. The route has been selected based on the lowest peer ip address, shown as Source in the output.
Inactive reason is Not Best in its group - Update source
jcluser@vMX2# run show route 192.168.251.0/24 exact detail
inet.0: 26 destinations, 27 routes (26 active, 0 holddown, 0 hidden)
192.168.251.0/24 (2 entries, 1 announced)
*BGP Preference: 170/-101
Next hop type: Indirect, Next hop index: 0
Address: 0xc4b6234
Next-hop reference count: 3
Source: 10.100.150.1
Next hop type: Router, Next hop index: 607
Next hop: 10.100.12.1 via ge-0/0/0.0, selected
Session Id: 0x143
Protocol next hop: 10.100.100.1
Indirect next hop: 0xc633184 1048574
INH Session ID: 0x144
State: <Active Int Ext>
Local AS: 64500 Peer AS: 64500
Age: 1:37 Metric2: 10
Validation State: unverified
ORR Generation-ID: 0
Task: BGP_64500.10.100.150.1
Announcement bits (3): 0-KRT 4-BGP_RT_Background
5-Resolve tree 4
AS path: 64544 I (Originator)
Cluster list: 1.1.1.1
Originator ID: 10.100.100.1
Accepted
Localpref: 100
Router ID: 10.100.150.1
Thread: junos-main
BGP Preference: 170/-101
Next hop type: Indirect, Next hop index: 0
Address: 0xc4b6234
Next-hop reference count: 3
Source: 10.100.150.2
Next hop type: Router, Next hop index: 607
Next hop: 10.100.12.1 via ge-0/0/0.0, selected
Session Id: 0x143
Protocol next hop: 10.100.100.1
Indirect next hop: 0xc633184 1048574
INH Session ID: 0x144
State: <NotBest Int Ext>
Inactive reason: Not Best in its group -
Update source
Local AS: 64500 Peer AS: 64500
Age: 1:37 Metric2: 10
Validation State: unverified
ORR Generation-ID: 0
Task: BGP_64500.10.100.150.2
AS path: 64544 I (Originator)
Cluster list: 2.2.2.2
Originator ID: 10.100.100.1
Accepted
Localpref: 100
Router ID: 10.100.150.2
Thread: junos-main