BGP add path
By Aravind
When bgp best path selection occurs, it only chooses one path. If there is a need to advertise multiple paths for the same prefix, then BGP introduces a new capability called add-path which will advertise all the paths to a particular prefix. The capability exchange occurs in the BGP OPEN message and the path count indicates how many add paths can be sent. Additionally you can signal how many one would want to receive as well.
Topology considered
<- 1.1.1.1
┌─────────┐
│ │
┌────ibgp─────────┤ PE2 ├───────ebgp──────┐
1.1.1.1:PE1 │ └─────────┘ │
1.1.1.1:PE2 │ │
│ ┌───┴──────┐
┌───────┐ ┌────┴───┐ │ │
│ ├───ibgp───┤ RR 1 │ │ CE │
│ PE1 │ │ │ │ │
└───────┘ └──┬─┬───┘ └───┬──────┘
│ │ │ 1.1.1.1
│ │ │
│ │ ┌──────────┐ │
│ └───ibgp───────────┤ │ │
│ │ PE3 ├──────ebgp─────┘
ibgp └──────────┘
listener
│ <- 1.1.1.1
┌─┴──────────┐
│ monitor │
└────────────┘
Building topology
use crpd-topology-builder, found here and use the add-path.yml topology file
python topo_builder/topo_builder.py -a create -t topologies/add_path.yml
Config router
Follow the below for all containers
python topo_builder/topo_builder.py -a config -t topologies/add_path.yml -cfg backup_pe1.txt -c pe1
...
Without Addpath
- 1.1.1.1 advertised from CE towards PE2 and PE3
- PE2 and PE3 advertise 1.1.1.1 to RR
- RR advertises only best path to PE1
- PE1 sees only one route to 1.1.1.1
RR sees 2 paths
root@rr# run show route 1.1.1.1
inet.0: 17 destinations, 18 routes (17 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
1.1.1.1/32 *[BGP/170] 00:00:18, localpref 100, from 100.1.1.2
AS path: 10000 I, validation-state: unverified
> to 192.168.2.1 via rr_pe2
[BGP/170] 00:00:18, localpref 100, from 100.1.1.3
AS path: 10000 I, validation-state: unverified
> to 192.168.3.1 via rr_pe3
RR advertises only one best path and PE1 sees only 1 path
root@rr# run show route advertising-protocol bgp 100.1.1.1
inet.0: 17 destinations, 18 routes (17 active, 0 holddown, 0 hidden)
Prefix Nexthop MED Lclpref AS path
* 1.1.1.1/32 100.1.1.2 100 10000 I
root@pe1# run show route 1.1.1.1
inet.0: 14 destinations, 14 routes (14 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
1.1.1.1/32 *[BGP/170] 00:00:58, localpref 100, from 100.100.100.100
AS path: 10000 I, validation-state: unverified
> to 192.168.1.2 via pe1_rr
With Addpath
- 1.1.1.1 advertised from CE towards PE2 and PE3
- PE2 and PE3 advertise 1.1.1.1 to RR
- RR enabled with add-path and path-count 2 with send and receive options
- RR advertises 2 paths to PE1 for 1.1.1.1
- PE1 configured with add-path receive
- PE1 sees 2 paths for 1.1.1.1
Configuration
RR add-path send and receive
root@rr# show protocols bgp group RR
type internal;
local-address 100.100.100.100;
family inet {
unicast {
add-path {
receive;
send {
path-count 2;
}
}
}
}
cluster 100.100.100.100;
neighbor 100.1.1.1;
neighbor 100.1.1.2;
neighbor 100.1.1.3;
neighbor 101.101.101.101;
PE1 add-path receive
root@pe1# show protocols bgp
group RR {
type internal;
family inet {
unicast {
add-path {
receive;
}
}
}
neighbor 100.100.100.100 {
local-address 100.1.1.1;
}
}
Outputs
root@rr# run show route 1.1.1.1
inet.0: 17 destinations, 18 routes (17 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
1.1.1.1/32 *[BGP/170] 00:11:25, localpref 100, from 100.1.1.2
AS path: 10000 I, validation-state: unverified
> to 192.168.2.1 via rr_pe2
[BGP/170] 00:10:27, localpref 100, from 100.1.1.3
AS path: 10000 I, validation-state: unverified
> to 192.168.3.1 via rr_pe3
root@rr# run show route advertising-protocol bgp 100.1.1.1
inet.0: 17 destinations, 18 routes (17 active, 0 holddown, 0 hidden)
Prefix Nexthop MED Lclpref AS path
* 1.1.1.1/32 100.1.1.2 100 10000 I
100.1.1.3 100 10000 I
root@rr# run show route advertising-protocol bgp 100.1.1.1 extensive
inet.0: 17 destinations, 18 routes (17 active, 0 holddown, 0 hidden)
* 1.1.1.1/32 (2 entries, 2 announced)
BGP group RR type Internal
Nexthop: 100.1.1.2
Localpref: 100
AS path: [65001] 10000 I
Cluster ID: 100.100.100.100
Originator ID: 100.1.1.2
Addpath Path ID: 1
BGP group RR type Internal
Nexthop: 100.1.1.3
Localpref: 100
AS path: [65001] 10000 I
Cluster ID: 100.100.100.100
Originator ID: 100.1.1.3
Addpath Path ID: 2
As seen from PE1
root@pe1# run show route receive-protocol bgp 100.100.100.100 extensive
inet.0: 14 destinations, 15 routes (14 active, 0 holddown, 0 hidden)
* 1.1.1.1/32 (2 entries, 1 announced)
Accepted
Nexthop: 100.1.1.2
Localpref: 100
AS path: 10000 I (Originator)
Cluster list: 100.100.100.100
Originator ID: 100.1.1.2
Addpath Path ID: 1
Accepted
Nexthop: 100.1.1.3
Localpref: 100
AS path: 10000 I (Originator)
Cluster list: 100.100.100.100
Originator ID: 100.1.1.3
Addpath Path ID: 2
root@pe1# run show route 1.1.1.1 extensive
inet.0: 14 destinations, 15 routes (14 active, 0 holddown, 0 hidden)
1.1.1.1/32 (2 entries, 1 announced)
TSI:
KRT-MFS Advt. to KRT-Netlink
KRT-Netlink in-kernel 1.1.1.1/32 -> {indirect(-)}
*BGP Preference: 170/-101
Next hop type: Indirect, Next hop index: 0
Address: 0x5c31e5d1379c
Next-hop reference count: 2
Kernel Table Id: 0
Source: 100.100.100.100
Next hop type: Router, Next hop index: 0
Next hop: 192.168.1.2 via pe1_rr, selected
Session Id: 0
Protocol next hop: 100.1.1.2
Indirect next hop: 0x5c31e2d79a88 - INH Session ID: 0
State: <Active Int Ext>
Local AS: 65001 Peer AS: 65001
Age: 13:02 Metric2: 2
Validation State: unverified
Task: BGP_65001.100.100.100.100
Announcement bits (3): 1-KRT MFS 2-KRT 6-Resolve tree 1
AS path: 10000 I (Originator)
Cluster list: 100.100.100.100
Originator ID: 100.1.1.2
Accepted
Localpref: 100
Router ID: 100.100.100.100
Addpath Path ID: 1
Thread: junos-main
Indirect next hops: 1
Protocol next hop: 100.1.1.2 Metric: 2 ResolvState: Resolved
Indirect next hop: 0x5c31e2d79a88 - INH Session ID: 0
Indirect path forwarding next hops: 1
Next hop type: Router
Next hop: 192.168.1.2 via pe1_rr
Session Id: 0
100.1.1.2/32 Originating RIB: inet.0
Metric: 2 Node path count: 1
Forwarding nexthops: 1
Next hop type: Router
Next hop: 192.168.1.2 via pe1_rr
Session Id: 0
BGP Preference: 170/-101
Next hop type: Indirect, Next hop index: 0
Address: 0x5c31e5d15c5c
Next-hop reference count: 1
Kernel Table Id: 0
Source: 100.100.100.100
Next hop type: Router, Next hop index: 0
Next hop: 192.168.1.2 via pe1_rr, selected
Session Id: 0
Protocol next hop: 100.1.1.3
Indirect next hop: 0x5c31e2d7bb08 - INH Session ID: 0
State: <NotBest Int Ext Changed>
Inactive reason: Not Best in its group - Router ID
Local AS: 65001 Peer AS: 65001
Age: 12:04 Metric2: 2
Validation State: unverified
Task: BGP_65001.100.100.100.100
AS path: 10000 I (Originator)
Cluster list: 100.100.100.100
Originator ID: 100.1.1.3
Accepted
Localpref: 100
Router ID: 100.100.100.100
Addpath Path ID: 2
Thread: junos-main
Indirect next hops: 1
Protocol next hop: 100.1.1.3 Metric: 2 ResolvState: Resolved
Indirect next hop: 0x5c31e2d7bb08 - INH Session ID: 0
Indirect path forwarding next hops: 1
Next hop type: Router
Next hop: 192.168.1.2 via pe1_rr
Session Id: 0
100.1.1.3/32 Originating RIB: inet.0
Metric: 2 Node path count: 1
Forwarding nexthops: 1
Next hop type: Router
Next hop: 192.168.1.2 via pe1_rr
Session Id: 0
References
[crpd junos