Route import and export, route summarization, and route community match

Subnet route scope options

For routed subnets, the scope options are available:

• Export Route Control Subnet: Controls the export route direction.

• Import Route Control Subnet: Controls the import route direction.

  Note

  Import route control is supported for BGP and OSPF, but not EIGRP.

• External Subnets for the External EPG (Security Import Subnet): Specifies which external subnets have contracts applied as part of a specific external L3Out EPG (l3extInstP). For a subnet under the l3extInstP to be classified as an external EPG, the scope on the subnet should be set to "import-security". Subnets of this scope determine which IP addresses are associated with the l3extInstP. Once this is determined, contracts determine with which other EPGs that external subnet is allowed to communicate. For example, when traffic enters the ACI switch on the Layer 3 external outside network (L3extOut), a lookup occurs to determine which source IP addresses are associated with the l3extInstP. This action is performed based on Longest Prefix Match (LPM) so that more specific subnets take precedence over more general subnets.

• Shared Route Control Subnet: In a shared service configuration, only subnets that have this property enabled will be imported into the consumer EPG Virtual Routing and Forwarding (VRF). It controls the route direction for shared services between VRFs.

• Shared Security Import Subnet: Applies shared contracts to imported subnets. The default specification is External Subnets for the external EPG.

Routed subnets can be aggregated. When aggregation is not set, the subnets are matched exactly. For example, if 11.1.0.0/16 is the subnet, then the policy will not apply to a 11.1.1.0/24 route, but it will apply only if the route is 11.1.0.0/16. However, to avoid a tedious and error prone task of defining all the subnets one by one, a set of subnets can be aggregated into one export, import or shared routes policy. At this time, only 0/0 subnets can be aggregated. When 0/0 is specified with aggregation, all the routes are imported, exported, or shared with a different VRF, based on the selection option:

• Aggregate Export: Exports all transit routes of a VRF (0/0 subnets).

• Aggregate Import: Imports all incoming routes of given L3 peers (0/0 subnets).

  Note

  Aggregate import route control is supported for BGP and OSPF, but not for EIGRP.

• Aggregate Shared Routes: If a route is learned in one VRF but needs to be advertised to another VRF, the routes can be shared by matching the subnet exactly, or can be shared in an aggregate way according to a subnet mask. For aggregate shared routes, multiple subnet masks can be used to determine which specific route groups are shared between VRFs. For example, 10.1.0.0/16 and 12.1.0.0/16 can be specified to aggregate these subnets. Or, 0/0 can be used to share all subnet routes across multiple VRFs.

Note

Routes shared between VRFs function correctly on Generation 2 switches (Cisco Nexus N9K switches with "EX" or "FX" on the end of the switch model name, or later; for example, N9K-93108TC-EX). On Generation 1 switches, however, there may be dropped packets with this configuration, because the physical ternary content-addressable memory (TCAM) tables that store routes do not have enough capacity to fully support route parsing.

Route summarization simplifies route tables by replacing many specific addresses with an single address. For example, 10.1.1.0/24, 10.1.2.0/24, and 10.1.3.0/24 are replaced with 10.1.0.0/16. Route summarization policies enable routes to be shared efficiently among border leaf switches and their neighbor leaf switches. BGP, OSPF, or EIGRP route summarization policies are applied to a bridge domain or transit subnet. For OSPF, inter-area and external route summarization are supported. Summary routes are exported; they are not advertised within the fabric. In the example above, when a route summarization policy is applied, and an EPG uses the 10.1.0.0/16 subnet, the entire range of 10.1.0.0/16 is shared with all the neighboring leaf switches.

Note

When two L3extOut policies are configured with OSPF on the same leaf switch, one regular and another for the backbone, a route summarization policy configured on one L3extOut is applied to both L3extOut policies because summarization applies to all areas in the VRF.

As illustrated in the figure below, route control profiles derive route maps according to prefix-based and community-based matching.

The route control profile (rtctrtlProfile) specifies what is allowed. The Route Control Context specifies what to match, and the scope specifies what to set. The subject profile contains the community match specifications, which can be used by multiple l3extOut instances. The subject profile (SubjP) can contain multiple community terms each of which contains one or more community factors (communities). This arrangement enables specifying the boolean operations:

• Logical or among multiple community terms

• Logical and among multiple community factors

For example, a community term called northeast could have multiple communities that each include many routes. Another community term called southeast could also include many different routes. The administrator could choose to match one, or the other, or both. A community factor type can be regular or extended. Care should be taken when using extended type community factors, to ensure there are no overlaps among the specifications.

The scope portion of the route control profile references the attribute profile (rtctrlAttrP) to specify what set-action to apply, such as preference, next hop, community, and so forth. When routes are learned from an l3extOut, route attributes can be modified.

The figure above illustrates the case where an l3extOut contains a rtctrtlProfile. A rtctrtlProfile can also exist under the tenant. In this case, the l3extOut has an interleak relation policy (L3extRsInterleakPol) that associates it with the rtctrtlProfile under the tenant. This configuration enables reusing the rtctrtlProfile for multiple l3extOut connections. It also enables keeping track of the routes the fabric learns from OSPF to which it gives BGP attributes (BGP is used within the fabric). A rtctrtlProfile defined under an L3extOut has a higher priority than one defined under the tenant.

The rtctrtlProfile has two modes: combinable, and global. The default combinable mode combines pervasive subnets (fvSubnet) and external subnets (l3extSubnet) with the match/set mechanism to render the route map. The global mode applies to all subnets within the tenant, and overrides other policy attribute settings. A global rtctrtlProfile provides permit-all behavior without defining explicit (0/0) subnets. A global rtctrtlProfile is used with non-prefix based match rules where matching is done using different subnet attributes such as community, next hop, and so on. Multiple rtctrtlProfile policies can be configured under a tenant.

rtctrtlProfile policies enable enhanced default import and default export route control. Layer 3 Outside networks with aggregated import or export routes can have import/export policies that specify supported default-export and default–import, and supported 0/0 aggregation policies. To apply a rtctrtlProfile policy on all routes (inbound or outbound), define a global default rtctrtlProfile that has no match rules.

Note

While multiple l3extOut connections can be configured on one switch, all Layer 3 outside networks configured on a switch must use the same rtctrtlProfile because a switch can have only one route map.

The protocol interleak and redistribute policy controls externally learned route sharing with ACI fabric BGP routes. Set attributes are supported. Such policies are supported per L3extOut, per node, or per VRF. An interleak policy applies to routes learned by the routing protocol in the L3extOut. Currently, interleak and redistribute policies are supported for OSPF v2 and v3. A route control policy rtctrtlProfile has to be defined as global when it is consumed by an interleak policy.