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  • Writer's pictureMukesh Chanderia

Transit Routing in ACI

Updated: Dec 28, 2023


  1. Default-export route profile with a prefix-list in the L3Out.



In this scenario, the network router needs to communicate to the subnet 172.16.1.0/30 which is behind the Partner Router through the Cisco ACI fabric.


The Cisco ACI fabric needs to advertise (export) 172.16.1.0/30 that is learned from L3Out Partner to L3Out Legacy.


The above objective can be achieved by two ways.


Option 1 - "default-export" Route Profile


Go to Tenant(POC1) --> Networking --> L3Outs --> L3Out (POC1-L3OUT) --> Route map for import and export route control --> Right Click --> Create  Route map for import and export route control --> select "default-export"



select "Match Routing Policy Only"



Now on Contexts --> click on "+"








Please note that the “default-export” route profile takes effect on the L3Out without associating it to L3Out EPG.


When using the "default-export" route profile the additional parameters to be set on the advertised route can be configured in the same "default-export" route profile.


There are several subnets that are received from the partner router that should be advertised to the legacy router. In this scenario, you can define the 0.0.0.0/0 prefix-list with Aggregate option.




Option 2: "Export Route Control Subnet" Scope with L3Out Subnets


When additional parameters such as BGP community need to be added to the advertised route, a route profile needs to be used as well. In this scenario, the recommended configuration is to use a route profile with a custom name (not "default-export") with Type "Match Prefix AND Routing Policy.” Then apply it to the L3Out subnet with "Export Route Control Subnet" scope or to the L3Out EPG itself.


Go to External EPG created under L3Out


Under Policy --> General --> click on "+" on subnet






Similary add 172.16.100.100/32 also as "Export Route Control Subnet"


If there are several subnets than use "Aggregate" option.




Now add 172.16.200.200/32 as "External Subnet for the External EPG" (checkmark this option & uncheck export route control subnet).


L3Out EPG needs to define the external subnet that belongs to itself via the scope "External Subnets for the External EPG."


L3 Out Transit Lab


L3 Out Transit


-----------------------------------------------------------------------------------------------------------

We are using Nexus Switch to Simulate as endpoint.

On doing "show lldp neighbour"

leaf1 Eth1/5 --> Nexus Port 120 BR Eth1/15 (Leaf 1 port)

leaf5 Eth1/24 --> Nexus Port 120 BR Eth1/14 (Leaf 5 port)

-------------------------------------------------------------------------------------------------------------

Let's check config on Nexus ports


Eth1/5


interface Ethernet1/5

no switchport

vrf member ISP1

ip address 10.0.0.2/24

ip ospf network point-to-point

ip ospf mtu-ignore

ip router ospf 1 area 0.0.0.0

no shutdown


n9k# show run vrf ISP1

interface Ethernet1/5

vrf member ISP1

vrf context ISP1

router ospf 1

vrf ISP1


Now add loopback interface


interface loopback101

  vrf member ISP1

  ip address 101.101.101.1/32

  ip ospf network point-to-point

  ip router ospf 1 area 0.0.0.0


interface loopback102

  vrf member ISP1

  ip address 102.102.102.1/32

  ip ospf network point-to-point

  ip router ospf 1 area 0.0.0.0


n9k# show ip interface brief vrf ISP1

IP Interface Status for VRF "ISP1"(53)

Interface IP Address Interface Status

Lo101 101.101.101.1 protocol-up/link-up/admin-up

Lo102 102.102.102.1 protocol-up/link-up/admin-up

Eth1/5 10.0.0.2 protocol-up/link-up/admin-up


--------------------------------------------------------------------------------------------------


Eth1/24


interface Ethernet1/24

no switchport

vrf member ISP2

ip address 172.16.0.2/24

ip ospf network point-to-point

ip ospf mtu-ignore

ip router ospf 1 area 0.0.0.0

no shutdown


n9k# show run vrf ISP2

interface Ethernet1/24

vrf member ISP2

vrf context ISP2

router ospf 1

vrf ISP2


interface loopback201

  vrf member ISP2

  ip address 201.201.201.1/32

  ip ospf network point-to-point

  ip router ospf 1 area 0.0.0.0



interface loopback202

  vrf member ISP2

  ip address 202.202.202.1/32

  ip ospf network point-to-point

  ip router ospf 1 area 0.0.0.0


n9k# show ip interface brief vrf ISP2

IP Interface Status for VRF "ISP2"(54)

Interface IP Address Interface Status

Lo201 201.201.201.1 protocol-up/link-up/admin-up

Lo202 202.202.202.1 protocol-up/link-up/admin-up

Eth1/24 172.16.0.2 protocol-up/link-up/admin-up


-------------------------------------------------------------------------------------------------------------------------------


Now the task here is to


Advertise interface Lo101 from ISP1 to ISP2

Advertise interface Lo201 from ISP2 to ISP1


External Subnet for External EPG --> Local Subnet of External EPG

For ISP1 --> 101.101.101.1/32

For ISP2 --> 201.201.201.1/32


External Route Control Subnet --> Export Route into the EPG

For ISP1 --> 201.201.201.1/32

For ISP2 --> 101.101.101.1/32





-------------------------------------------------------------------------------------------------------------------------------


n9k# show ip route vrf ISP1

1.1.1.1/32, ubest/mbest: 1/0 → Router ID set while configuring L3out OSPF

*via 10.0.0.1, Eth1/5, [110/5], 00:25:24, ospf-1, intra

10.0.0.0/24, ubest/mbest: 1/0, attached

*via 10.0.0.2, Eth1/5, [0/0], 00:25:25, direct

10.0.0.2/32, ubest/mbest: 1/0, attached

*via 10.0.0.2, Eth1/5, [0/0], 00:25:25, local

101.101.101.1/32, ubest/mbest: 2/0, attached

*via 101.101.101.1, Lo101, [0/0], 00:25:25, local

*via 101.101.101.1, Lo101, [0/0], 00:25:25, direct

102.102.102.1/32, ubest/mbest: 2/0, attached

*via 102.102.102.1, Lo102, [0/0], 00:25:25, local

*via 102.102.102.1, Lo102, [0/0], 00:25:25, direct

201.201.201.1/32, ubest/mbest: 1/0

*via 10.0.0.1, Eth1/5, [110/1], 00:25:24, ospf-1, type-2, tag 4294967295



n9k# show ip route vrf ISP2

99.99.99.99/32, ubest/mbest: 1/0 → Router ID set while configuring L3out OSPF

*via 172.16.0.1, Eth1/24, [110/5], 00:25:23, ospf-1, intra

101.101.101.1/32, ubest/mbest: 1/0

*via 172.16.0.1, Eth1/24, [110/1], 00:25:23, ospf-1, type-2, tag 4294967295

172.16.0.0/24, ubest/mbest: 1/0, attached

*via 172.16.0.2, Eth1/24, [0/0], 00:25:24, direct

172.16.0.2/32, ubest/mbest: 1/0, attached

*via 172.16.0.2, Eth1/24, [0/0], 00:25:24, local

201.201.201.1/32, ubest/mbest: 2/0, attached

*via 201.201.201.1, Lo201, [0/0], 00:25:24, local

*via 201.201.201.1, Lo201, [0/0], 00:25:24, direct

202.202.202.1/32, ubest/mbest: 2/0, attached

*via 202.202.202.1, Lo202, [0/0], 00:25:24, local

*via 202.202.202.1, Lo202, [0/0], 00:25:24, direct


Note: Leaf switch anyways learn all neighbours routes


leaf1# show ip route vrf ISP:VRF-ISP

1.1.1.1/32, ubest/mbest: 2/0, attached, direct

*via 1.1.1.1, lo5, [0/0], 11:30:15, local, local

*via 1.1.1.1, lo5, [0/0], 11:30:15, direct

10.0.0.0/24, ubest/mbest: 1/0, attached, direct

*via 10.0.0.1, eth1/15, [0/0], 11:30:14, direct

10.0.0.1/32, ubest/mbest: 1/0, attached

*via 10.0.0.1, eth1/15, [0/0], 11:30:14, local, local

99.99.99.99/32, ubest/mbest: 1/0

*via 10.0.120.70%overlay-1, [1/0], 10:30:10, bgp-65005, internal, tag 65005

101.101.101.1/32, ubest/mbest: 1/0

*via 10.0.0.2, eth1/15, [110/5], 00:33:32, ospf-default, intra

102.102.102.1/32, ubest/mbest: 1/0

*via 10.0.0.2, eth1/15, [110/5], 00:32:39, ospf-default, intra

172.16.0.0/24, ubest/mbest: 1/0

*via 10.0.120.70%overlay-1, [200/0], 10:30:02, bgp-65005, internal, tag 65005

201.201.201.1/32, ubest/mbest: 1/0

*via 10.0.120.70%overlay-1, [200/5], 00:35:17, bgp-65005, internal, tag 65005

202.202.202.1/32, ubest/mbest: 1/0

*via 10.0.120.70%overlay-1, [200/5], 00:34:58, bgp-65005, internal, tag 65005



leaf5# show ip route vrf ISP:VRF-ISP

1.1.1.1/32, ubest/mbest: 1/0

*via 10.0.120.67%overlay-1, [1/0], 10:31:25, bgp-65005, internal, tag 65005

10.0.0.0/24, ubest/mbest: 1/0

*via 10.0.120.67%overlay-1, [200/0], 10:31:25, bgp-65005, internal, tag 65005

99.99.99.99/32, ubest/mbest: 2/0, attached, direct

*via 99.99.99.99, lo2, [0/0], 10:31:26, local, local

*via 99.99.99.99, lo2, [0/0], 10:31:26, direct

101.101.101.1/32, ubest/mbest: 1/0

*via 10.0.120.67%overlay-1, [200/5], 00:34:49, bgp-65005, internal, tag 65005

102.102.102.1/32, ubest/mbest: 1/0

*via 10.0.120.67%overlay-1, [200/5], 00:33:56, bgp-65005, internal, tag 65005

172.16.0.0/24, ubest/mbest: 1/0, attached, direct

*via 172.16.0.1, eth1/14, [0/0], 10:31:19, direct

172.16.0.1/32, ubest/mbest: 1/0, attached

*via 172.16.0.1, eth1/14, [0/0], 10:31:19, local, local

201.201.201.1/32, ubest/mbest: 1/0

*via 172.16.0.2, eth1/14, [110/5], 00:36:33, ospf-default, intra

202.202.202.1/32, ubest/mbest: 1/0

*via 172.16.0.2, eth1/14, [110/5], 00:36:15, ospf-default, intra


------------------------------------------------------------------------------------------------------------------


Now in order to communicate between two external EPGs we need a contract


Let’s add a contract in both Ex EGP to allow communication to each other.




n9k# ping 201.201.201.1 vrf ISP1 source 101.101.101.1

PING 201.201.201.1 (201.201.201.1) from 101.101.101.1: 56 data bytes

64 bytes from 201.201.201.1: icmp_seq=0 ttl=252 time=1.6 ms

64 bytes from 201.201.201.1: icmp_seq=1 ttl=252 time=1.22 ms

64 bytes from 201.201.201.1: icmp_seq=2 ttl=252 time=1.202 ms

64 bytes from 201.201.201.1: icmp_seq=3 ttl=252 time=1.196 ms

64 bytes from 201.201.201.1: icmp_seq=4 ttl=252 time=1.263 ms

--- 201.201.201.1 ping statistics ---

5 packets transmitted, 5 packets received, 0.00% packet loss

round-trip min/avg/max = 1.196/1.296/1.6 ms



n9k# ping 101.101.101.1 vrf ISP2 source 201.201.201.1

PING 101.101.101.1 (101.101.101.1) from 201.201.201.1: 56 data bytes

64 bytes from 101.101.101.1: icmp_seq=0 ttl=252 time=2.392 ms

64 bytes from 101.101.101.1: icmp_seq=1 ttl=252 time=1.968 ms

64 bytes from 101.101.101.1: icmp_seq=2 ttl=252 time=2.146 ms

64 bytes from 101.101.101.1: icmp_seq=3 ttl=252 time=7.412 ms

64 bytes from 101.101.101.1: icmp_seq=4 ttl=252 time=1.617 ms

--- 101.101.101.1 ping statistics ---

5 packets transmitted, 5 packets received, 0.00% packet loss

round-trip min/avg/max = 1.617/3.107/7.412 ms











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