WTF Are all those Checkboxes? (ACI L3 Outs) – Part 1 of ???

Here is a screenshot of some of said checkboxes in case you don’t know what the hell I’m going on about:



If you still don’t know what these things are, these are part of an L3 Out in ACI, specifically these are options that are configurable on the “subnet” of a “network” on an L3 Out. Essentially these control the import/export of prefixes for the L3 out. Since I literally always forget which one does what I figured other people probably do too,  let’s try and go through each of them and figure it out.

First off, lets outline our really really simple topology we’ll be using for this. I will be working with two leafs, and two virtual routers (CSR1000v) connected via two vPCs to a pair of UCS Fabric Interconnects. My CSRs each live in a single VLAN (two total VLANs, one per CSR) that is plumbed through UCS up into ACI (no VMM stuff, just a good ol’ fashion VLAN piped up to the leafs). So more or less I have two routers connected via vPC to my leafs, and I’m routing on a VLAN between everything. Clear as mud?

I’m going to mostly ignore the actual setup of the L3 Out itself, as I want to focus on these damn check boxes. For now, we will just start with our L3 Out “CSR-1” which is a simple OSPF area 0 L3 Out, our adjacencies between this L3 Out and CSR-1 (the router, not the L3 Out) are already up and look good.


Okay great, so we have an L3 Out, we’ve got some adjacencies now what? Well first things first, this is a “regular” area in OSPF, so we should be exchanging routes between the CSR and ACI (both CSR and ACI should be advertising their loopbacks – CSR-1 and CSR-2 are and respectively, and Leaf103 and Leaf104 are and respectively), so lets take a look on CSR-1:

CSR-1#sh ip route ospf
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
 D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
 N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
 E1 - OSPF external type 1, E2 - OSPF external type 2
 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
 ia - IS-IS inter area, * - candidate default, U - per-user static route
 o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
 a - application route
 + - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is not set is subnetted, 1 subnets
O [110/2] via, 00:00:08, GigabitEthernet2 is subnetted, 1 subnets
O [110/2] via, 00:00:08, GigabitEthernet2
O E2 [110/20] via, 00:02:21, GigabitEthernet2
                    [110/20] via, 00:02:22, GigabitEthernet2

Okay cool, so obviously OSPF is up as we are learning about the loopbacks of the Leafs. Good start. How does it look on the ACI side (note you can do all the stuff I will be doing in the GUI if you’d like but in the interest of not taking up a million pages of screenshots I’ll use the CLI)?

Leaf103# show ip route ospf vrf Carl-Testbed:Carl-Testbed
IP Route Table for VRF "Carl-Testbed:Carl-Testbed"
'*' denotes best ucast next-hop
'**' denotes best mcast next-hop
'[x/y]' denotes [preference/metric]
'%<string>' in via output denotes VRF <string>


So that’s not really what we are wanting to see is it…? We have relationships up, and it’s obviously working since the CSR gets the routes from ACI, so what gives? (Before you think it’s a stub area or something like that it’s not, this has to do with check boxes remember!?) Well out of habit I created the External EPG (under “Network” in the L3 Out) with a subnet of — why? Because usually I like to make ACI a stub area and just take a default route and be on with my life (which requires no check box foo). So what does this have to do with anything? Well lets take a look at those check boxes a little closer, we have the following options under the “Scope” heading:

  • Export Route Control Subnet
  • Import Route Control Subnet (currently grayed out)
  • External Subnets for the External EPG (checked by default)
  • Shared Route Control Subnet
  • Shared Security Import Subnet

So before we can have any traffic flowing between the CSR and ACI, we probably need to figure out why ACI is NOT getting any routes from the CSR (note right now I have one of the CSRs shut down so we are focusing just on CSR-1 at the moment). Lets take a look at that mysterious little “i”  button in the top right of the screen… it sometimes has pretty good information.



Okay so lots of words. Think we know what an IP is, but lets take a bit closer look at that one to start with… “subnet IP address to be imported from the outside into the fabric.” Okay, so that basically sounds like it may be used in route filtering, lets keep reading… “contracts associated with its parent […] are applied to the subnet.” Alright so we know have an idea that the subnet(s) configured under the “Network” in the L3 Out have at least two purposes 1) defining routes that can be imported into the fabric, and 2) this subnet is related to contracts somehow. Since we still don’t have a route from the CSR in the fabric, lets dig a bit more as to why…

Per the information page it mentions that this subnet matches a route to be imported into the fabric, well I just put which would match nothing but a default route right? So let’s try and add a subnet to the “Network” (which is a bit of a confusing name for the parent folder) that corresponds with the loopback on CSR-1:



Alrighty, subnet added (with only the default “External Subnets” box checked). Lets see if we get a route on the leaf:


Leaf103# show ip route ospf vrf Carl-Testbed:Carl-Testbed
IP Route Table for VRF "Carl-Testbed:Carl-Testbed"
'*' denotes best ucast next-hop
'**' denotes best mcast next-hop
'[x/y]' denotes [preference/metric]
'%<string>' in via output denotes VRF <string>



No? Well now we are matching an exact prefix, at least in theory, but what about those damn check boxes? If you’ll notice, there’s a box that is grayed out for “Import Route Control Subnet.” That sounds vaguely helpful right? The note from the “i” page says “Controls the import route direction.” Yeah, that seems kinda like what we want, but why is it grayed out, and more importantly how do we un-grey it out? To do that we have to take a look at the parent L3 Out object.



Back up on the top-level of the L3 Out (parent object), there are two boxes for “Route Control Enforcement” one for Import and one for Export. As far as I know Export is always checked, but Import is not checked by default. If we want to bring routes into the fabric, we will need to check this box, so I’m going to go ahead and do just that. Do note that Import control is not supported for EIGRP. With the Import Route Control box checked, our “Import Route Control Subnet” box is now check-able for our subnet, so I’ve gone ahead and ticked that box as well. In theory, we will now get the loopback from CSR-1 installed in the route table on our leaf switches… (drum roll please!):

Leaf3# show ip route ospf vrf Carl-Testbed:Carl-Testbed
IP Route Table for VRF "Carl-Testbed:Carl-Testbed"
'*' denotes best ucast next-hop
'**' denotes best mcast next-hop
'[x/y]' denotes [preference/metric]
'%<string>' in via output denotes VRF <string>, ubest/mbest: 1/0
 *via, vlan73, [110/5], 00:00:01, ospf-default, intra

Well look at that, sure enough we now install the route for the loopback. Now what if I decided that I didn’t want a /32 loopback on my CSR and did something like this:

CSR-1#show run int lo0
Building configuration...

Current configuration : 63 bytes
interface Loopback0
 ip address

Carl-Test-1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
CSR-1(config)#int lo0
CSR-1(config-if)#ip add
CSR-1(config-if)#ip ospf network point-to-point

So now my loopback is a /24, and I’ve added ospf network type of point-to-point to ensure OSPF doesn’t just insert the loopback IP as a /32 into the database. Lets pop back over and see what the leaf things about this:

Leaf3# show ip route ospf vrf Carl-Testbed:Carl-Testbed
IP Route Table for VRF "Carl-Testbed:Carl-Testbed"
'*' denotes best ucast next-hop
'**' denotes best mcast next-hop
'[x/y]' denotes [preference/metric]
'%<string>' in via output denotes VRF <string>


The leaf sure doesn’t think that’s very cool. When we first started looking at the list of check boxes, there was a whole other section that I left out, there are three boxes in the “Aggregate” section.

  • Aggregate Export
  • Aggregate Import
  • Aggregate Shared Routes

The information page is again our friend here, the very first line in the “Aggregate” section states: “when aggregation is not set, the subnets are matched exactly.” Which perfectly explains why when we changed our loopback IP we lost the route in ACI. So knowing that we can summarize, I’m going to go ahead and delete the subnet from the network, and enable “Import Route Control Subnet” as well as “Aggregate Import,” and then lets take a look at the routing table on the leaf again:

Leaf3# show ip route ospf vrf Carl-Testbed:Carl-Testbed
IP Route Table for VRF "Carl-Testbed:Carl-Testbed"
'*' denotes best ucast next-hop
'**' denotes best mcast next-hop
'[x/y]' denotes [preference/metric]
'%<string>' in via output denotes VRF <string>, ubest/mbest: 1/0
 *via, vlan73, [110/5], 00:00:16, ospf-default, intra

Bam! Alrighty so if you were paying attention earlier you may have noticed that there was a prefix in the route table on the CSR, that subnet lives in this same VRF in ACI and contains an Ubuntu VM, so lets pop over to that and see if we can ping through to our loopback.


Well it starts out looking pretty good… we can ping our default gateway, but we can’t ping our loopback. We can however ping the loopback from a leaf. Remember that box checked by default? “External Subnets for the External EPG”? Turns out there’s a bit more to that! By default in ACI VRFs are in “enforced” mode, which just means that you need to have contracts in place to permit traffic to flow between EPGs. This subnet we’ve been talking about is in fact an EPG, it is a prefix-based EPG. That little tick box that is set by default simply says “hey this subnet that I’m defining is now part of the EPG “ExternalEPG” (the name we gave our “network”).” Okay cool so it’s an EPG, and we need contracts but where do those go…


In our “Network” (what is basically an EPG!), there is a tab on the main Policy pane for contracts. If you open that tab you will have options for provide and consume. I’ve added a simple permit any contract between the EPG containing my VM and this EPG in our L3 Out, and another drum roll….


Awesome! So far we’ve covered the main Import Route Control check box, Import Route Control Subnet, and External Subnets for the External EPG “Scope” check boxes, as well as the Aggregate Import check box. We’ve still got quite a few check boxes to figure out though, so stay tuned for some more!



2 thoughts on “WTF Are all those Checkboxes? (ACI L3 Outs) – Part 1 of ???

  1. This is a great write up!

    One bit of info that I found and that you may want to include is that if your VRF is in “Unenforced” mode, you don’t have to check any boxes in order to import external routes. I’m currently building a new fabric and have built multiple L3-outs, using a catch-all, in an Unenforced VRF and they import external OSPF routes without checking “Route Control Enforcement- Import” in the main level of the L3-out. In an “Enforced” VRF, “Policy Control Enforcement Direction- Ingress” is on by default, which I believe is why you had to check all those Import boxes to import routes into the fabric.
    You know the old saying, “there’s more than one way to skin a cat” which is an understatement in regards to ACI.



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