As network engineers, we often encounter scenarios where a design that worked perfectly on one platform behaves differently when migrating to another. This is especially true when moving to a carrier-grade platform like a Cisco Network Convergence System (NCS), which operates with a strict and predictable routing logic.

A recent forum discussion highlighted a classic issue: an overlapping IP addressing scheme and the “disappearing” aggregate route. Let’s break down the problem and walk through a definitive solution.

The Scenario: A Forum Deep Dive

A network administrator, let’s call him Lester, was migrating a cloud infrastructure from a DPDK-based router to a Cisco NCS. His IP addressing scheme was designed for efficiency and scale:

• Cloud VMs: A large number of /24 prefixes are used for a cloud cluster. The virtual machines within this cluster use the .1 address of each /24 as their default gateway. This gateway IP is configured on a Bridge-Group Virtual Interface (BVI). For example, 192.168.1.1/24 is on interface BVI1.
• Bare-Metal Users: From within these same /24 blocks, more specific prefixes (like /29s and /31s) are carved out and assigned to bare-metal clients. Each specific prefix is configured on a dedicated VLAN subinterface, which acts as the gateway for that client. For example, 192.168.1.10/29 might be on interface GigabitEthernet0/0/0.100.

The Problem: On the old router, this setup worked seamlessly. However, on the Cisco NCS, a strange behavior occurred. When the subinterfaces with the specific /29 or /31 routes were in an up state, the parent /24 route associated with BVI1 would disappear from the routing table (show route). If those subinterfaces were shut down, the /24 route would immediately reappear.

Lester’s critical constraint is that re-architecting the IP addressing scheme is not an option due to the thousands of end-user devices (VMs and bare-metal servers) with statically configured settings.

The “Why”: Understanding Cisco’s Routing Logic

The forum contributors correctly identified the root cause: Longest Prefix Match (LPM) and the behavior of connected routes. This isn’t a bug; it’s the intended, correct, and safe behavior of a robust routing information base (RIB).

1. Connected Routes: When you configure an IP address on an active interface, the router automatically installs a “connected” route into its RIB. These routes have an administrative distance of 0, making them the most believable source of routing information.

2. The Golden Rule - Longest Prefix Match: When a router needs to forward a packet, it looks for all matching routes in its table. If it finds multiple matches, it will always choose the route with the longest, most specific subnet mask. A /31 is more specific than a /29, which is more specific than a /24.

3. RIB Superset Logic: The Cisco IOS XR RIB is designed to be efficient and unambiguous. When it sees multiple connected interfaces with overlapping IP addresses, it installs the most specific prefixes. It sees the /29 and /31 on the subinterfaces as explicit, specific paths. The broader /24 on the BVI is seen as a less-specific “summary” or “superset” of these connected routes. To avoid ambiguity and maintain a clean RIB, the router will often suppress the installation of the less-specific connected route when more-specific connected routes from the same range exist. The DPDK router likely had a more permissive RIB that allowed these overlapping connected routes to coexist.

The router essentially concludes: “I have a direct connection to 192.168.1.8/29 and 192.168.1.20/31. Therefore, I already know how to reach these parts of the 192.168.1.0/24 network. I will not install the less-specific connected /24 to avoid redundancy.” The problem is that this leaves the rest of the /24 (the parts not covered by the specifics) unreachable.

Solutions and Implementation Guide

Given the constraint of not being able to re-address, we need a solution that works with the existing IP plan.

Solution 1: The “Clean” but Impractical Approach

One forum user suggested re-partitioning the /24 to eliminate overlaps. This would involve taking the /24, removing the blocks used for /29s and /31s, and assigning the remaining, non-contiguous subnets to the BVI. While this is the cleanest design from a network architecture perspective, it is not feasible for Lester, as it would require changing the IP and subnet mask on thousands of VMs.

Solution 2: The Static Route Override (Recommended)

This is the most direct and least disruptive solution. If the router won’t install the /24 as a connected route automatically, we can instruct it to do so manually. We will create a static route for the entire /24 and point it directly to the BVI interface.

This tells the router, “Regardless of any other connected interfaces, I explicitly want a route for the entire 192.168.1.0/24 network to be associated with BVI1.”

Configuration (IOS XR):

! Enter global configuration mode
configure

! Define the static route
router static
 address-family ipv4 unicast
  192.168.1.0/24 BVI1

! Commit the changes
commit

How This Works:

• This command manually injects the 192.168.1.0/24 prefix into the RIB.
• The Longest Prefix Match rule still applies perfectly for forwarding. Packets destined for an IP within the /29 block will still be forwarded out the specific subinterface.
• However, the router now also has a valid /24 route that it can advertise to its neighbors and use for any traffic within the /24 that doesn’t match a more specific entry. This solves the “disappearing route” problem.

Solution 3: The L2 Subinterface with a Centralized Gateway (Alternative Design)

Another valid, though more architectural, approach is to change the nature of the subinterfaces. Instead of having them be Layer 3 gateways, you can convert them to Layer 2 transport interfaces and bridge them all to the single BVI.

In this model, the BVI becomes the only Layer 3 gateway for everyone. The subinterfaces simply act as L2 paths to get traffic into the correct bridge-domain, where the BVI handles all routing.

This is a significant design change and may not be suitable if bare-metal clients are configured to use the IP on their subinterface as a gateway. However, if all clients (VMs and bare-metal) can be configured to use the BVI’s .1 IP as their gateway, this simplifies the L3 topology.

Verification Steps

After applying the static route (Solution 2), use these commands to verify the fix:

1. Check for all routes: The primary test is to see if the /24 now coexists with the more specific routes.

   show route
   

   You should now see entries for the /24, /29, and /31 prefixes simultaneously.

2. Check for the specific /24 route:

   show route 192.168.1.0/24
   

   This should show the static route you just configured, pointing to BVI1.

3. Check interface status: Ensure all relevant interfaces are up.

   show ip interface brief
   

Conclusion

Migrating to high-performance platforms like the Cisco NCS often means adapting to a more structured and robust operational logic. The case of the disappearing overlapping route is a perfect example. While initially confusing, it reflects the router’s strict adherence to routing principles designed to prevent ambiguity.

For networks with immovable IP addressing schemes, the static route pointed to the BVI interface provides a powerful and precise tool. It allows you to achieve your desired routing