import path limit <number>

[sachin30720041]

Hello,

I am trying to understand the usage of the 'import path limit' command.

     PE1

     /.   \

CE       P----PE3

   \.     /

     PE2

PE1 vrf:

vrf definition vrf1
rd 2:2
!
address-family ipv4
route-target export 2:2
route-target import 7:7
exit-address-family

PE2 VRF:

vrf definition vrf1
rd 2:2
!
address-family ipv4
route-target export 3:3
route-target import 7:7
exit-address-family

PE3 VRF:

vrf definition vrf1
rd 2:2
!
address-family ipv4
route-target export 7:7
route-target import 2:2
route-target import 3:3
exit-address-family

PE3 BGP:

router bgp 64513
...
address-family ipv4 vrf vrf1
import path selection bestpath
import path limit 4
neighbor 100.70.80.8 remote-as 64514
neighbor 100.70.80.8 activate
maximum-paths eibgp 4
exit-address-family

PE3

Router#show ip route vrf vrf1

1.0.0.0/32 is subnetted, 1 subnets
B 1.1.1.1 [200/0] via 3.3.3.3, 00:00:28
                [200/0] via 2.2.2.2, 00:00:28
11.0.0.0/32 is subnetted, 1 subnets
B 11.11.11.11 [200/0] via 3.3.3.3, 00:00:28
                       [200/0] via 2.2.2.2, 00:00:28
111.0.0.0/32 is subnetted, 1 subnets
B 111.111.111.111 [200/0] via 3.3.3.3, 00:00:28
                               [200/0] via 2.2.2.2, 00:00:28
112.0.0.0/32 is subnetted, 1 subnets
B 112.112.112.112 [200/0] via 3.3.3.3, 00:00:28
                               [200/0] via 2.2.2.2, 00:00:28
 113.0.0.0/32 is subnetted, 1 subnets
B 113.113.113.113 [200/0] via 3.3.3.3, 00:00:28
                               [200/0] via 2.2.2.2, 00:00:28

I have configured 'import path limit 4'. But I see 5 routes in vrf1 of PE3. According to my understanding 'import path limit' is used to limit the number of imported routes in a given VRF. So there should be 4 imported routes in vrf1.

 And what is the purpose of 'import path selection bestpath'. Even if I use 'import path selection multipath'. the number of routes in the vrf stays the same. Where one should use 'import path selection bestpath' and where one should use 'mport path selection multipath' command?

Thanks,

Sachin

[Harold Ritter]

Hi @sachin30720041 ,

The "import path limit" is to limit the number of paths per route, not the number of routes. In your case, you have 2 paths for each route, so under the "import path limit 4" that you configured.

To limit the number of routes in the VRF, you can use the following command:

vrf definition vrf1

maximum routes <number of routes> <threshold>

Regards, 

[sachin30720041]

Hello Harold,

Thanks for responding.

When I configure 'maximum-paths ibgp 3' and 'import path limit 2' then I see 3 next hops in the vrf's routing table.

router bgp 64512

address-family ipv4 vrf vrf1
import path limit 2
maximum-paths ibgp 3

show ip route vrf vrf1
B 1.1.1.1 [200/0] via 9.9.9.9, 00:00:04
               [200/0] via 3.3.3.3, 00:00:04
                [200/0] via 2.2.2.2, 00:00:04

Maximum-paths command seems to change the number of next-hops. 'import path limit 2' does not have any impact on the number of paths.

Thanks,

Sachin

[Harold Ritter]

Hi @sachin30720041 ,

Two things here.

1. "import path limit" is part of a feature called "BGP event based VPN import", which allows dynamic import of prefixes from the VPNv4/VPNv6 structures to the the VRF table. Before this feature, prefixes were imported in the VRF table based on the BGP scanner process. "import path limit" is used to limit the number of paths and is particularly useful in SP networks with lots of routes and potentially lots more paths.

2. "maximum-paths ibgp" overrides the limit set by "import paths limit"

Please refer to the following document for more information about the "BGP event based VPN import".

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/15-mt/irg-15-mt-book/irg-event-vpn-import.pdf

Regards,