Bridging

This chapter contains the following sections:

Bridged Interface to an External Router

As shown in the figure below, when the leaf switch interface is configured as a bridged interface, the default gateway for the tenant VNID is the external router.
Figure 1. Bridged External Router

The ACI fabric is unaware of the presence of the external router and the APIC statically assigns the leaf switch interface to its EPG.

Bridge Domains and Subnets

A bridge domain (fvBD) represents a Layer 2 forwarding construct within the fabric. The following figure shows the location of bridge domains in the management information tree (MIT) and their relation to other objects in the tenant.

Figure 2. Bridge Domains


A bridge domain must be linked to a VRF instance (also known as a context or private network). With the exception of a Layer 2 VLAN, it must have at least one subnet (fvSubnet) associated with it. The bridge domain defines the unique Layer 2 MAC address space and a Layer 2 flood domain if such flooding is enabled. While a VRF instance defines a unique IP address space, that address space can consist of multiple subnets. Those subnets are defined in one or more bridge domains that reference the corresponding VRF instance.

The options for a subnet under a bridge domain or under an EPG are as follows:

  • Public: The subnet can be exported to a routed connection.

  • Private: The subnet applies only within its tenant.

  • Shared: The subnet can be shared with and exported to multiple VRF instances in the same tenant or across tenants as part of a shared service. An example of a shared service is a routed connection to an EPG present in another VRF instance in a different tenant. This enables traffic to pass in both directions across VRF instances. An EPG that provides a shared service must have its subnet configured under that EPG (not under a bridge domain), and its scope must be set to advertised externally, and shared between VRF instances.


    Note

    Shared subnets must be unique across the VRF instance involved in the communication. When a subnet under an EPG provides a Layer 3 external network shared service, such a subnet must be globally unique within the entire Cisco Application Centric Infrastructure (ACI) fabric.

Bridge domain packet behavior can be controlled in the following ways:

Packet Type

Mode

ARP

You can enable or disable ARP Flooding; without flooding, ARP packets are sent with unicast.

Note

 

If the limitIpLearnToSubnets in fvBD is set, endpoint learning is limited to the bridge domain only if the IP address is in a configured subnet of the bridge domain or an EPG subnet that is a shared service provider.

Unknown Unicast

L2 Unknown Unicast, which can be Flood or Hardware Proxy.

Note

 

When the bridge domain has L2 Unknown Unicast set to Flood, if an endpoint is deleted the system deletes it from both the local leaf switches as well as the remote leaf switches where the bridge domain is deployed, by selecting Clear Remote MAC Entries. Without this feature, the remote leaf continues to have this endpoint learned until the timer expires.

Modifying the L2 Unknown Unicast setting causes traffic to bounce (go down and up) on interfaces to devices attached to EPGs associated with this bridge domain.

Unknown IP Multicast

L3 Unknown Multicast Flooding

Flood: Packets are flooded on ingress and border leaf switch nodes only. With N9K-93180YC-EX, packets are flooded on all the nodes where a bridge domain is deployed.

Optimized: Only 50 bridge domains per leaf are supported. This limitation is not applicable for N9K-93180YC-EX.

L2 Multicast, Broadcast, Unicast

Multi-Destination Flooding, which can be one of the following:

  • Flood in BD: Flood in bridge domain

  • Flood in Encapsulation: Flood in encapsulation

  • Drop: Drop the packets


Note
Beginning with Cisco APIC release 3.1(1), on the Cisco Nexus 9000 series switches (with names ending with EX and FX and onwards), the following protocols can be flooded in encapsulation or flooded in a bridge domain: OSPF/OSPFv3, BGP, EIGRP, LACP, ISIS, IGMP, PIM, ST-BPDU, ARP/GARP, RARP, and ND.

Bridge domains can span multiple switches. A bridge domain can contain multiple subnets, but a subnet is contained within a single bridge domain. If the bridge domain (fvBD) limitIPLearnToSubnets property is set to yes, endpoint learning will occur in the bridge domain only if the IP address is within any of the configured subnets for the bridge domain or within an EPG subnet when the EPG is a shared service provider. Subnets can span multiple EPGs; one or more EPGs can be associated with one bridge domain or subnet. In hardware proxy mode, ARP traffic is forwarded to an endpoint in a different bridge domain when that endpoint has been learned as part of the Layer 3 lookup operation.

Bridge Domain Options

A bridge domain can be set to operate in flood mode for unknown unicast frames or in an optimized mode that eliminates flooding for these frames. When operating in flood mode, Layer 2 unknown unicast traffic is flooded over the multicast tree of the bridge domain (GIPo). For the bridge domain to operate in optimized mode you should set it to hardware-proxy. In this case, Layer 2 unknown unicast frames are sent to the spine-proxy anycast VTEP address.


Caution

Changing from unknown unicast flooding mode to hw-proxy mode is disruptive to the traffic in the bridge domain.

If IP routing is enabled in the bridge domain, the mapping database learns the IP address of the endpoints in addition to the MAC address.

The Layer 3 Configurations tab of the bridge domain panel allows the administrator to configure the following parameters:

  • Unicast Routing: If this setting is enabled and a subnet address is configured, the fabric provides the default gateway function and routes the traffic. Enabling unicast routing also instructs the mapping database to learn the endpoint IP-to-VTEP mapping for this bridge domain. The IP learning is not dependent upon having a subnet configured under the bridge domain.

  • Subnet Address: This option configures the SVI IP addresses (default gateway) for the bridge domain.

  • Limit IP Learning to Subnet: This option is similar to a unicast reverse-forwarding-path check. If this option is selected, the fabric will not learn IP addresses from a subnet other than the one configured on the bridge domain.


Caution

Enabling Limit IP Learning to Subnet is disruptive to the traffic in the bridge domain.

Disabling IP Learning per Bridge Domain

You can disable IP dataplane learning for a bridge domain. The MAC learning still occurs in the hardware, but the IP learning only occurs from the ARP/GARP/ND processes. This functionality was introduced in the Cisco APIC 3.1 releases primarily for service graph policy-based redirect (PBR) deployments. We do not recommend disabling IP learning per bridge domain and it is not supported except when used with PBR.

See the following guidelines and limitations for disabling IP learning per bridge domain:

  • Layer 3 multicast is not supported because the source IP address is not learned to populate the S,G information in the remote top-of-rack (ToR) switches.

  • As the DL bit is set in the iVXLAN header, the MAC address is also not learned from the data path in the remote TORs. It results in flooding of the unknown unicast traffic from the remote TOR to all TORs in the fabric where this BD is deployed. It is recommended to configure the BD in proxy mode to overcome this situation if endpoint dataplane learning is disabled.

  • ARP should be in flood mode and GARP based detection should be enabled.

  • When IP learning is disabled, Layer 3 endpoints are not flushed in the corresponding VRF. It may lead to the endpoints pointing to the same TOR forever. To resolve this issue, flush all the remote IP endpoints in this VRF on all TORs.

  • On Cisco ACI switches with Application Leaf Engine (ALE), the inner MAC address is not learned from the VXLAN packets.

  • When dataplane learning is disabled on a BD, the existing local endpoints learned via dataplane in that BD are not flushed. If the data traffic is flowing, the existing local endpoints do not age out.

When IP learning is disabled, you have to enable the Global Subnet Prefix check option in System > System Settings > Fabric Wide Setting > Enforce Subnet Check in the Online Help.

Creating a Tenant, VRF, and Bridge Domain Using the GUI

If you have a public subnet when you configure the routed outside, you must associate the bridge domain with the outside configuration.

Procedure

Step 1

On the menu bar, choose Tenants > Add Tenant.

Step 2

In the Create Tenant dialog box, perform the following tasks:

  1. In the Name field, enter a name.

  2. Click the Security Domains + icon to open the Create Security Domain dialog box.

  3. In the Name field, enter a name for the security domain. Click Submit.

  4. In the Create Tenant dialog box, check the check box for the security domain that you created, and click Submit.

Step 3

In the Navigation pane, expand Tenant-name > Networking, and in the Work pane, drag the VRF icon to the canvas to open the Create VRF dialog box, and perform the following tasks:

  1. In the Name field, enter a name.

  2. Click Submit to complete the VRF configuration.

Step 4

In the Networking pane, drag the BD icon to the canvas while connecting it to the VRF icon. In the Create Bridge Domain dialog box that displays, perform the following tasks:

  1. In the Name field, enter a name.

  2. Click the L3 Configurations tab.

  3. Expand Subnets to open the Create Subnet dialog box, enter the subnet mask in the Gateway IP field and click OK.

  4. Click Submit to complete bridge domain configuration.

Step 5

In the Networks pane, drag the L3 icon down to the canvas while connecting it to the VRF icon. In the Create Routed Outside dialog box that displays, perform the following tasks:

  1. In the Name field, enter a name.

  2. Expand Nodes And Interfaces Protocol Profiles to open the Create Node Profile dialog box.

  3. In the Name field, enter a name.

  4. Expand Nodes to open the Select Node dialog box.

  5. In the Node ID field, choose a node from the drop-down list.

  6. In the Router ID field, enter the router ID.

  7. Expand Static Routes to open the Create Static Route dialog box.

  8. In the Prefix field, enter the IPv4 or IPv6 address.

  9. Expand Next Hop Addresses and in the Next Hop IP field, enter the IPv4 or IPv6 address.

  10. In the Preference field, enter a number, then click UPDATE and then OK.

  11. In the Select Node dialog box, click OK.

  12. In the Create Node Profile dialog box, click OK.

  13. Check the BGP, OSPF, or EIGRP check boxes if desired, and click NEXT. Click OK to complete the Layer 3 configuration.

To confirm L3 configuration, in the Navigation pane, expand Networking > VRFs.

Creating a Tenant, VRF, and Bridge Domain Using the NX-OS Style CLI

This section provides information on how to create tenants, VRFs, and bridge domains.

Note
Before creating the tenant configuration, you must create a VLAN domain using the vlan-domain command and assign the ports to it.

Procedure

Step 1

Create a VLAN domain (which contains a set of VLANs that are allowable in a set of ports) and allocate VLAN inputs, as follows:

Example:

In the following example ("exampleCorp"), note that VLANs 50 - 500 are allocated. 

apic1# configure
apic1(config)# vlan-domain dom_exampleCorp
apic1(config-vlan)# vlan 50-500
apic1(config-vlan)# exit

Step 2

Once the VLANs have been allocated, specify the leaf (switch) and interface for which these VLANs can be used. Then, enter "vlan-domain member" and then the name of the domain you just created.

Example:

In the following example, these VLANs (50 - 500) have been enabled on leaf 101 on interface ethernet 1/2-4 (three ports including 1/2, 1/3, and 1/4). This means that if you are using this interface, you can use VLANS 50-500 on this port for any application that the VLAN can be used for. 

apic1(config-vlan)# leaf 101
apic1(config-vlan)# interface ethernet 1/2-4
apic1(config-leaf-if)# vlan-domain member dom_exampleCorp
apic1(config-leaf-if)# exit
apic1(config-leaf)# exit

Step 3

Create a tenant in global configuration mode, as shown in the following example:

Example:


apic1(config)# tenant exampleCorp

Step 4

Create a private network (also called VRF) in tenant configuration mode as shown in the following example:

Example:


apic1(config)# tenant exampleCorp
apic1(config-tenant)# vrf context exampleCorp_v1
apic1(config-tenant-vrf)# exit

Step 5

Create a bridge domain (BD) under the tenant, as shown in the following example:

Example:


apic1(config-tenant)# bridge-domain exampleCorp_b1
apic1(config-tenant-bd)# vrf member exampleCorp_v1
apic1(config-tenant-bd)# exit

Note

 

In this case, the VRF is "exampleCorp_v1".

Step 6

Allocate IP addresses for the BD (ip and ipv6), as shown in the following example.

Example:

apic1(config-tenant)# interface bridge-domain exampleCorp_b1
apic1(config-tenant-interface)# ip address 172.1.1.1/24
apic1(config-tenant-interface)# ipv6 address 2001:1:1::1/64
apic1(config-tenant-interface)# exit

What to do next

The next section describes how to add an application profile, create an application endpoint group (EPG), and associate the EPG to the bridge domain.

Creating a Tenant, VRF, and Bridge Domain Using the REST API

Procedure

Step 1

Create a tenant.

Example:

 POST https://apic-ip-address/api/mo/uni.xml
<fvTenant name="ExampleCorp"/>
When the POST succeeds, you see the object that you created in the output.

Step 2

Create a VRF and bridge domain.

Note

 

The Gateway Address can be an IPv4 or an IPv6 address. For more about details IPv6 gateway address, see the related KB article, KB: Creating a Tenant, VRF, and Bridge Domain with IPv6 Neighbor Discovery .

Example:

 URL for POST: https://apic-ip-address/api/mo/uni/tn-ExampleCorp.xml

<fvTenant name="ExampleCorp">
   <fvCtx name="pvn1"/>
   <fvBD name="bd1">
      <fvRsCtx tnFvCtxName="pvn1"/>
      <fvSubnet ip="10.10.100.1/24"/>
   </fvBD>      
</fvTenant>

Note

 

If you have a public subnet when you configure the routed outside, you must associate the bridge domain with the outside configuration.

Configuring an Enforced Bridge Domain

An enforced bridge domain configuration entails creating an endpoint in a subject endpoint group (EPG) that can only ping subnet gateways within the associated bridge domain. With this configuration, you can then create a global exception list of IP addresses that can ping any subnet gateway.

Figure 3. Enforced Bridge Domain

Note

  • The exception IP addresses can ping all of the bridge domain gateways across all of your VRF instances.

  • A loopback interface configured for an L3Out does not enforce reachability to the IP address that is configured for the subject loopback interface.

  • When an eBGP peer IP address exists in a different subnet than the subnet of the L3Out interface, you must add the peer subnet to the allowed exception subnets. Otherwise, eBGP traffic is blocked because the source IP address exists in a different subnet than the L3Out interface subnet.

  • For a BGP prefixed-based peer, you must add the peer subnet to the list of allowed exception subnets. For example, if 20.1.1.0/24 is configured as BGP prefixed-based peer, you must add 20.1.1.0/24 to the list of allowed exception subnets.

  • An enforced bridge domain is not supported with the Management tenant, regardless if the VRF instances are in-band or out-of-band, and any rules to control the traffic to these VRF instances should be configured using regular contracts.

Configuring an Enforced Bridge Domain Using the NX-OS Style CLI

This section provides information on how to configure your enforced bridge domain using the NX-OS style command line interface (CLI).

Procedure

Step 1

Create and enable the tenant:

Example:

In the following example ("cokeVrf") is created and enabled. 

apic1(config-tenant)# vrf context cokeVrf

apic1(config-tenant-vrf)# bd-enforce enable

apic1(config-tenant-vrf)# exit

apic1(config-tenant)#exit

Step 2

Add the subnet to the exception list.

Example:

apic1(config)#bd-enf-exp-ip add1.2.3.4/24

apic1(config)#exit
You can confirm if the enforced bridge domain is operational using the following type of command:
apic1# show running-config all | grep bd-enf

bd-enforce enable
bd-enf-exp-ip add 1.2.3.4/24

Example

The following command removes the subnet from the exception list:
apic1(config)# no bd-enf-exp-ip 1.2.3.4/24

apic1(config)#tenant coke

apic1(config-tenant)#vrf context cokeVrf

What to do next

To disable the enforced bridge domain run the following command:
apic1(config-tenant-vrf)# no bd-enforce enable

Configuring an Enforced Bridge Domain Using the REST API

Procedure

  Command or Action Purpose

Step 1

Create a tenant.

Example:

 POST https://apic-ip-address/api/mo/uni.xml
<fvTenant name="ExampleCorp"/>
When the POST succeeds, you see the object that you created in the output.

Step 2

Create a VRF and bridge domain.

Example:

 URL for POST: https://apic-ip-address/api/mo/uni/tn-ExampleCorp.xml

<fvTenant name="ExampleCorp">
   <fvCtx name="pvn1"/>
   <fvBD name="bd1">
      <fvRsCtx tnFvCtxName="pvn1" bdEnforceEnable="yes"/>
      <fvSubnet ip="10.10.100.1/24"/>
   </fvBD>      
</fvTenant>

For adding an exception IP, use the following post:

https://apic-ip-address/api/node/mo/uni/infra.xml 

<bdEnforceExceptionCont>
<bdEnforceExceptIp ip="11.0.1.0/24"/> 
</bdEnforceExceptionCont>

Note

 
If you have a public subnet when you configure the routed outside, you must associate the bridge domain with the outside configuration.

Note

 
The Gateway Address can be an IPv4 or an IPv6 address. For more about details IPv6 gateway address, see the related KB article, KB: Creating a Tenant, VRF, and Bridge Domain with IPv6 Neighbor Discovery .

Configuring Flood in Encapsulation for All Protocols and Proxy ARP Across Encapsulations

Cisco Application Centric Infrastructure (ACI) uses the bridge domain as the Layer 2 broadcast boundary. Each bridge domain can include multiple endpoint groups (EPGs), and each EPG can be mapped to multiple virtual or physical domains. Each EPG can also use different VLAN encapsulation pools in each domain. Each EPG can also use different VLAN or VXLAN encapsulation pools in each domain.

Ordinarily, when you put multiple EPGs within bridge domains, broadcast flooding sends traffic to all the EPGs in the bridge domain. Because EPGs are used to group endpoints and manage traffic to fulfill specific functions, sending the same traffic to all the EPGs in the bridge domain is not always practical.

The flood in encapsulation feature helps to consolidate bridge domains in your network. The feature does so by enabling you to control broadcast flooding to endpoints within the bridge domain based on the encapsulation of the virtual or physical domain that the EPGs are associated with.

Flood in encapsulation requires the bridge domain to be configured with a subnet and with IP routing because in order to allow communication between endpoints of different EPGs in the same bridge domain Cisco ACI performs proxy ARP.

Using multiple VLANs in tunnel mode can introduce a few challenges. In a typical deployment using Cisco ACI with a single tunnel, as illustrated in the following figure, there are multiple EPGs under one bridge domain. In this case, certain traffic is flooded within the bridge domain (and thus in all the EPGs), with the risk of MAC address learning ambiguities that can cause forwarding errors.

Figure 4. Challenges of Cisco ACI with VLAN Tunnel Mode

In this topology, the fabric has a single tunnel network defined that uses one uplink to connect with the Cisco ACI leaf node. Two user VLANs, VLAN 10 and VLAN 11 are carried over this link. The bridge domain is set in flooding mode as the servers' gateways are outside the Cisco ACI cloud. ARP negotiations occur in the following process:

  • The server sends one ARP broadcast request over the VLAN 10 network.

  • The ARP packet travels through the tunnel network to the external server, which records the source MAC address, learned from its downlink.

  • The server then forwards the packet out its uplink to the Cisco ACI leaf switch.

  • The Cisco ACI fabric sees the ARP broadcast packet entering on access port VLAN 10 and maps it to EPG1.

  • Because the bridge domain is set to flood ARP packets, the packet is flooded within the bridge domain and thus to the ports under both EPGs as they are in the same bridge domain.

  • The same ARP broadcast packet comes back over the same uplink.

  • The external server sees the original source MAC address from this uplink.

Result: the external device has the same MAC address learned from both the downlink port and uplink port within its single MAC forwarding table, causing traffic disruptions.

Recommended Solution

The Flood in Encapsulation option is used to limit flooding traffic inside the bridge domain to a single encapsulation. When two EPGs share the same bridge domain and Flood in Encapsulation is enabled, the EPG flooding traffic does not reach the other EPG.

Beginning with Cisco Application Policy Infrastructure Controller (APIC) release 3.1(1), on the Cisco Nexus 9000 series switches (with names ending with EX and FX and onwards), all protocols are flooded in encapsulation. Also when enabling Flood in Encapsulation for any inter-VLAN traffic, Proxy ARP ensures that the MAC flap issue does not occur, and it limits all flooding (ARP, GARP, and BUM) to the encapsulation. This applies for all EPGs under the bridge domain where it is enabled.


Note

Before Cisco APIC release 3.1(1), these features are not supported (Proxy ARP and all protocols being included when flooding within encapsulation). In an earlier Cisco APIC release or earlier generation switches (without EX or FX on their names), if you enable Flood in Encapsulation it does not function, no informational fault is generated, but Cisco APIC decreases the health score by 1.

The recommended solution is to support multiple EPGs under one bridge domain by adding an external switch. This design with multiple EPGs under one bridge domain with an external switch is illustrated in the following figure.

Figure 5. Design with Multiple EPGs Under one Bridge Domain with an External Switch

Within the same bridge domain, some EPGs can be service nodes and other EPGs can have flood in encapsulation configured. A Load Balancer resides on a different EPG. The load balancer receives packets from the EPGs and sends them to the other EPGs (there is no proxy ARP and flood within encapsulation does not take place).

If you want to add flood in encapsulation only for selected EPGs, using the NX-OS style CLI, enter the flood-on-encapsulation enable command under EPGs.

If you want to add flood in encapsulation for all EPGs, you can use the multi-destination encap-flood CLI command under the bridge domain.

Using the CLI, flood in encapsulation configured for an EPG takes precedence over flood in encapsulation that is configured for a bridge domain.

When both bridge domains and EPGs are configured, the behavior is described as follows:

Table 1. Behavior When Both Bridge Domains and EPGs Are Configured

Configuration

Behavior

Flood in encapsulation at the EPG and flood in encapsulation at the bridge domain

Flood in encapsulation takes place for the traffic on all VLANs within the bridge domain.

No flood in encapsulation at the EPG and flood in encapsulation at the bridge domain

Flood in encapsulation takes place for the traffic on all VLANs within the bridge domain.

Flood in encapsulation at the EPG and no flood in encapsulation at the bridge domain

Flood in encapsulation takes place for the traffic on that VLAN within the EPG of the bridge domain.

No flood in encapsulation at the EPG and no flood in encapsulation at the bridge domain

Flooding takes place within the entire bridge domain.

Multi-Destination Protocol Traffic

The EPG/bridge domain level broadcast segmentation is supported for the following network control protocols:

  • OSPF

  • EIGRP

  • LACP

  • IS-IS

  • BGP

  • IGMP

  • PIM

  • STP-BPDU (flooded within EPG)

  • ARP/GARP (controlled by ARP Proxy)

  • ND

Flood in Encapsulation Limitations

The following limitations apply when using flood in encapsulation for all protocols:

  • Flood in encapsulation does not work in ARP unicast mode.

  • Neighbor Solicitation (NS/ND) is not supported for this release.

  • You must enable per-port CoPP with flood in encapsulation.

  • Flood in encapsulation is supported only in bridge domain in flood mode and ARP in flood mode. Bridge domain spine proxy mode is not supported.

  • IPv4 Layer 3 multicast is not supported.

  • IPv6 is not supported.

  • Virtual machine migration to a different VLAN has momentary issues (60 seconds).

  • A load balancer acting as a gateway is supported, for example, in one to one communication between virtual machines and the load balancer in non-proxy mode. No Layer 3 communication is supported. The traffic between virtual machines and the load balancer is on Layer 2. However, if intra-EPG communication passes through the load balancer, then the load balancer changes the SIP and SMAC; otherwise it can lead to a MAC flap. Therefore, Dynamic Source Routing (DSR) mode is not supported in the load balancer.

  • Setting up communication between virtual machines through a firwall, as a gateway, is not recommended because if the virtual machine IP address changes to the gateway IP address instead of the firewall IP address, then the firewall can be bypassed.

  • Prior releases are not supported (even interoperating between prior and current releases).

  • Prior to the 3.2(5) release, the proxy ARP and flood in encapsulation features are not supported for VXLAN encapsulation.

  • A mixed-mode topology with Application Leaf Engine (ALE) and Application Spine Engine (ASE) is not recommended and is not supported with flood in encapsulation. Enabling them together can prevent QoS priorities from being enforced.

  • Flood in encapsulation is not supported with remote leaf switches and Cisco ACI Multi-Site.

  • Flood in encapsulation is not supported for Common Pervasive Gateway (CPGW).

  • Flood in encapsulation is not supported on EPGs where microsegmentation is configured.

  • If you configure the flood in encapsulation on all EPGs of a bridge domain, ensure that you configure the flood in encapsulation on the bridge domain as well.

  • IGMP snooping is not supported with flood in encapsulation.

  • There is a condition that causes Cisco ACI to flood in the bridge domain (instead of the encapsulation) packets that are received on an EPG that is configured for flood in encapsulation. This happens regardless of whether the administrator configured flood in encapsulation directly on the EPG or on the bridge domain. The condition for this forwarding behavior is if the ingress leaf node has a remote endpoint for the destination MAC address while the egress leaf node does not have a corresponding local endpoint. This can happen due to reasons such as an interface flapping, an endpoint flush due to STP TCN, learning being disabled on the bridge domain due to an excessive amount of moves, and so on.

  • A Layer 3 gateway must be in the Cisco ACI fabric.