4 mins read
BGP – basic multipath
1. Intro
BGP typically selects only one best path for each prefix and installs that route in the forwarding table. When BGP multipath is enabled, the device selects multiple equal-cost BGP paths to reach a given destination, and all these paths are installed in the forwarding table. BGP advertises only the active path to its neighbors, unless add-path is in use.
A path is considered a BGP equal-cost path (and is used for forwarding) if the BGP path selection process performs a tie-break after comparing the IGP cost to the next-hop. By default, all paths with the same neighboring AS, learned by a multipath-enabled BGP neighbor are considered in the multipath selection process.
2. BGP Multipath configuration
The Initial config:
### R1 initial config:
set protocols bgp group AS-200 peer-as 200
set protocols bgp group AS-200 neighbor 17.8.0.2
set protocols bgp group AS-300 peer-as 300
set protocols bgp group AS-300 neighbor 17.8.1.2
### Only one next hop selected:
root@R1# run show route protocol bgp
inet.0: 25 destinations, 26 routes (25 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
192.168.123.0/24 *[BGP/170] 00:00:12, localpref 100
AS path: 200 I, validation-state: unverified
> to 17.8.0.2 via ge-0/0/1.0
[BGP/170] 00:28:55, localpref 100
AS path: 300 I, validation-state: unverified
> to 17.8.1.2 via ge-0/0/2.0
root@R1# run show route protocol bgp detail
inet.0: 25 destinations, 26 routes (25 active, 0 holddown, 0 hidden)
192.168.123.0/24 (2 entries, 1 announced)
*BGP Preference: 170/-101
Next hop type: Router, Next hop index: 598
Address: 0x7bbeab4
Next-hop reference count: 2, key opaque handle: 0x0, non-key opaque handle: 0x0
Kernel Table Id: 0
Source: 17.8.0.2
Next hop: 17.8.0.2 via ge-0/0/1.0, selected
Session Id: 356
State: <Active Ext>
Local AS: 100 Peer AS: 200
Age: 1:03
Validation State: unverified
Task: BGP_200.17.8.0.2
Announcement bits (2): 0-KRT 2-BGP_RT_Background
AS path: 200 I
Accepted
Localpref: 100
Router ID: 17.0.0.2
Thread: junos-main
BGP Preference: 170/-101
Next hop type: Router, Next hop index: 597
Address: 0x7bbeb24
Next-hop reference count: 1, key opaque handle: 0x0, non-key opaque handle: 0x0
Kernel Table Id: 0
Source: 17.8.1.2
Next hop: 17.8.1.2 via ge-0/0/2.0, selected
Session Id: 355
State: <Ext>
Inactive reason: Active preferred
Local AS: 100 Peer AS: 300
Age: 29:46
Validation State: unverified
Task: BGP_300.17.8.1.2
AS path: 300 I
Accepted
Localpref: 100
Router ID: 17.0.0.3
Thread: junos-main Let’s load the multipath config. We must note that R1 receives the 2 paths from different AS numbers, by default Junos will not do any multipath in this case. But, there is solution, we can use the multipath multiple-as option to force Junos applying multipath on different ASs.
### Both neighbors must be multipath enabled !!
root@R1# show | compare
[edit protocols bgp group AS-200]
+ multipath {
+ multiple-as;
+ }
[edit protocols bgp group AS-300]
+ multipath {
+ multiple-as;
+ }
[edit]
root@R1# commit
commit complete
### At this both next hops are being used:
root@R1# run show route protocol bgp
inet.0: 25 destinations, 26 routes (25 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
192.168.123.0/24 *[BGP/170] 00:02:52, localpref 100
AS path: 200 I, validation-state: unverified
> to 17.8.0.2 via ge-0/0/1.0
to 17.8.1.2 via ge-0/0/2.0
[BGP/170] 00:34:26, localpref 100
AS path: 300 I, validation-state: unverified
> to 17.8.1.2 via ge-0/0/2.0
root@R1# run show route protocol bgp detail
inet.0: 25 destinations, 26 routes (25 active, 0 holddown, 0 hidden)
192.168.123.0/24 (2 entries, 1 announced)
*BGP Preference: 170/-101
Next hop type: Router, Next hop index: 0
Address: 0x7e80a3c
Next-hop reference count: 2, key opaque handle: 0x0, non-key opaque handle: 0x0
Kernel Table Id: 0
Source: 17.8.0.2
Next hop: 17.8.0.2 via ge-0/0/1.0, selected
Session Id: 0
Next hop: 17.8.1.2 via ge-0/0/2.0
Session Id: 0
State: <Active Ext>
Local AS: 100 Peer AS: 200
Age: 3:55
Validation State: unverified
Task: BGP_200.17.8.0.2
Announcement bits (3): 0-KRT 2-BGP_RT_Background 3-BGP_Multi_Path
AS path: 200 I
Accepted Multipath
Localpref: 100
Router ID: 17.0.0.2
Thread: junos-main
BGP Preference: 170/-101
Next hop type: Router, Next hop index: 597
Address: 0x7bbeb24
Next-hop reference count: 2, key opaque handle: 0x0, non-key opaque handle: 0x0
Kernel Table Id: 0
Source: 17.8.1.2
Next hop: 17.8.1.2 via ge-0/0/2.0, selected
Session Id: 355
State: <Ext>
Inactive reason: Active preferred
Local AS: 100 Peer AS: 300
Age: 35:29
Validation State: unverified
Task: BGP_300.17.8.1.2
AS path: 300 I
Accepted MultipathContrib
Localpref: 100
Router ID: 17.0.0.3
Thread: junos-main
### But, there is only one next hop in the forwarding table !!!
root@R1# run show route forwarding-table destination 192.168.123.0/24
Routing table: default.inet
Internet:
Destination Type RtRef Next hop Type Index NhRef Netif
192.168.123.0/24 user 0 17.8.0.2 ucst 598 4 ge-0/0/1.0
### Enable per-packet (really per-flow) load balancing for routes installed to kernel:
root@R1# show | compare
[edit policy-options]
+ policy-statement lb {
+ then {
+ load-balance per-packet;
+ }
+ }
[edit routing-options]
+ forwarding-table {
+ export lb;
+ }
[edit]
root@R1# commit
commit complete
### Now we have both next hops installed:
root@R1# run show route forwarding-table destination 192.168.123.0/24
Routing table: default.inet
Internet:
Destination Type RtRef Next hop Type Index NhRef Netif
192.168.123.0/24 user 0 ulst 1048575 2
17.8.0.2 ucst 598 4 ge-0/0/1.0
17.8.1.2 ucst 597 4 ge-0/0/2.0External Resources: