Additional Path Selection (Best2/Best3)
Selection of additional paths can be done at AF or at NEIGHBOR level. In case of selection configured at both levels, Neighbor level selection takes preference over Global Level selection.
Selection of Best 2 Additional Paths at Global Level
R2
#configure terminal | Enter the Configure mode. |
(config)#router bgp 100 | Enter BGP router mode |
(config-router)#address-family ipv4 unicast | Configure the address family ipv4 under router BGP |
(config-router-af)#bgp additional-paths send | Configure R2 to send additional paths to the iBGP neighbor R3 |
(config-router-af)#bgp additional-paths select best 2 | Configure R2 to select best 2 out of all available paths to all iBGP neighbors |
(config-router-af)#exit-address-family | Exit Address Family mode and return to Router mode. |
(config-router)#exit | Exit the router BGP mode and return to the configure mode |
(config)#commit | Apply commit |
(config)#exit | Exit the configure mode |
Selection of Best 2 Additional Paths at Neighbor Level
R3
#configure terminal | Enter the Configure mode. |
(config)#router bgp 100 | Enter BGP router mode |
(config-router)#address-family ipv4 unicast | Configure the address family ipv4 under router BGP |
(config-router-af)#neighbor 10.1.1.2 additional-paths send | Configure R2 to send additional paths to the iBGP neighbor R3 |
(config-router-af)#neighbor 10.1.1.2 advertise additional-paths best 2 | Configure R2 to advertise best 2 out of all available paths to R3 |
(config-router-af)#exit-address-family | Exit Address Family mode and return to Router mode. |
(config-router)#exit | Exit the router BGP mode and return to the configure mode |
(config)#commit | Apply commit |
(config)#exit | Exit the configure mode |
Validation
• Verify that R2 sends only the best 2 paths out of 4 paths available for the route 100.1.1.0/24 to R3.
• Verify on R2 that Transmit path ids(tx path_id) are allocated only for the best 2 paths to 100.1.1.0/24.
• Verify on R2 that paths not selected have a tx path_id of ‘-1’
• Verify that Receive path ids(rx path_id) on R3 match the tx path_ids on R2
R2
#show ip bgp 100.1.1.0
BGP routing table entry for 100.1.1.0/24
Paths: (4 available, best #1, table Default-IP-Routing-Table)
Advertised to non peer-group peers:
12.1.1.2 13.1.1.2 14.1.1.2
200
11.1.1.2 from 11.1.1.2 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external, best
rx path_id: -1 tx path_id: 0
Advertised to non peer-group peers:
10.1.1.2
Last update: Fri Jan 6 06:02:02 2017
200
12.1.1.2 from 12.1.1.2 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external
rx path_id: -1 tx path_id: 1
Advertised to non peer-group peers:
10.1.1.2
Last update: Fri Jan 6 06:02:10 2017
200
13.1.1.2 from 13.1.1.2 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external
rx path_id: -1 tx path_id: -1
Not advertised to any peer
Last update: Fri Jan 6 06:01:48 2017
200
14.1.1.2 from 14.1.1.2 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external
rx path_id: -1 tx path_id: -1
Not advertised to any peer
Last update: Fri Jan 6 06:01:51 2017
R3
#show ip bgp
BGP table version is 168, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, l - labeled
S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*>i 100.1.1.0/24 11.1.1.2 0 100 0 200 i
* i 12.1.1.2 0 100 0 200 i
*> 200.1.1.0 21.1.1.2 0 100 0 300 i
* 22.1.1.2 0 100 0 300 i
* 23.1.1.2 0 100 0 300 i
* 24.1.1.2 0 100 0 300 i
Total number of prefixes 2
#show ip bgp 100.1.1.0
BGP routing table entry for 100.1.1.0/24
Paths: (2 available, best #1, table Default-IP-Routing-Table)
Advertised to non peer-group peers:
21.1.1.2 22.1.1.2 23.1.1.2 24.1.1.2
200
11.1.1.2 (metric 20) from 10.1.1.1 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, internal, best
rx path_id: 0 tx path_id: 0
Not advertised to any peer
Last update: Fri Jan 6 07:26:06 2017
200
12.1.1.2 (metric 20) from 10.1.1.1 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, internal
rx path_id: 1 tx path_id: -1
Not advertised to any peer
Last update: Fri Jan 6 07:27:02 2017
Selection of Best 3 Additional Paths at AF Level
R2
#configure terminal | Enter the Configure mode. |
(config)#router bgp 100 | Enter BGP router mode |
(config-router)#address-family ipv4 unicast | Configure the address family ipv4 under router BGP |
(config-router)#bgp additional-paths send | Configure R2 to send additional paths to the iBGP neighbor R3 |
(config-router)#bgp additional-paths select best 3 | Configure R2 to select best 3 out of all available paths to all iBGP neighbors |
(config-router-af)#exit-address-family | Exit Address Family mode and return to Router mode. |
(config-router)#exit | Exit the router BGP mode and return to the configure mode |
(config)#commit | Apply commit |
(config)#exit | Exit the configure mode |
Selection of Best 3 Additional Paths at Neighbor Level
R2
#configure terminal | Enter the Configure mode. |
(config)#router bgp 100 | Enter BGP router mode |
(config-router)#address-family ipv4 unicast | Configure the address family ipv4 under router BGP |
(config-router)#neighbor 10.1.1.2 additional-paths send | Configure R2 to send additional paths to the iBGP neighbor R3 |
(config-router)#neighbor 10.1.1.2 advertise additional-paths best 3 | Configure R2 to advertise best 3 out of all available paths to R3 |
(config-router-af)#exit-address-family | Exit Address Family mode and return to Router mode. |
(config-router)#exit | Exit the router BGP mode and return to the configure mode |
(config)#commit | Apply commit |
(config)#exit | Exit the configure mode |
Validation
• Verify that R2 sends only the best 3 paths out of 4 paths available for the route 100.1.1.0/24 to R3.
• Verify on R2 that Transmit path ids(tx path_id) are allocated only for the best 3 paths to 100.1.1.0/24.
• Verify that paths not selected have a tx path_id of ‘-1’
• Verify that Receive path ids(rx path_id) on R3 match the tx path_ids on R2
R2
#show ip bgp 100.1.1.0
BGP routing table entry for 100.1.1.0/24
Paths: (4 available, best #1, table Default-IP-Routing-Table)
Advertised to non peer-group peers:
12.1.1.2 13.1.1.2 14.1.1.2
200
11.1.1.2 from 11.1.1.2 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external, best
rx path_id: -1 tx path_id: 0
Advertised to non peer-group peers:
10.1.1.2
Last update: Fri Jan 6 06:02:02 2017
200
12.1.1.2 from 12.1.1.2 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external
rx path_id: -1 tx path_id: 1
Advertised to non peer-group peers:
10.1.1.2
Last update: Fri Jan 6 06:02:10 2017
200
13.1.1.2 from 13.1.1.2 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external
rx path_id: -1 tx path_id: 2
Advertised to non peer-group peers:
10.1.1.2
Last update: Fri Jan 6 06:01:48 2017
200
14.1.1.2 from 14.1.1.2 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external
rx path_id: -1 tx path_id: -1
Not advertised to any peer
Last update: Fri Jan 6 06:01:51 2017
R3
#show ip bgp
BGP table version is 170, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, l - labeled
S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*>i 100.1.1.0/24 11.1.1.2 0 100 0 200 i
* i 13.1.1.2 0 100 0 200 i
* i 12.1.1.2 0 100 0 200 i
*> 200.1.1.0 21.1.1.2 0 100 0 300 i
* 22.1.1.2 0 100 0 300 i
* 23.1.1.2 0 100 0 300 i
* 24.1.1.2 0 100 0 300 i
Total number of prefixes 2
#show ip bgp 100.1.1.0
BGP routing table entry for 100.1.1.0/24
Paths: (3 available, best #1, table Default-IP-Routing-Table)
Advertised to non peer-group peers:
21.1.1.2 22.1.1.2 23.1.1.2 24.1.1.2
200
11.1.1.2 (metric 20) from 10.1.1.1 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, internal, best
rx path_id: 0 tx path_id: 0
Not advertised to any peer
Last update: Fri Jan 6 07:29:38 2017
200
13.1.1.2 (metric 20) from 10.1.1.1 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, internal
rx path_id: 2 tx path_id: -1
Not advertised to any peer
Last update: Fri Jan 6 07:30:05 2017
200
12.1.1.2 (metric 20) from 10.1.1.1 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, internal
rx path_id: 1 tx path_id: -1
Not advertised to any peer
Last update: Fri Jan 6 07:30:05 2017
Verify that R2 sends all 4 paths to 100.1.1.0/24 to R3 with the BGP local preference path attribute set to 140
Verify that R2 sends only the best path to 100.1.1.0/24 to R3 with the BGP local preference path attribute set to 110