High Availability Options — BGP/ECMP vs. Azure ILB

When deploying F5 Distributed Cloud (XC) Customer Edge (CE) nodes in cloud environments, choosing the high availability (HA) ingress pattern is a key architectural decision.

This page compares the two patterns implemented by this repository. For reusable guidance on Virtual Sites versus clustering, route withdrawal, ECMP path limits, and persistence, start with Customer Edge high availability.

This deployment demonstrates both primary HA patterns in a single multi-cloud environment:

Dynamic BGP / ECMP Routing (Azure Route Server / AWS VPC / FRR ToR): Used for the Rest of World (eastus), AWS (us-east-2), and On-Premise KVM paths.
Azure Internal Load Balancer (ILB) with HA Ports: Used as the Canadian regional demonstration variation (enable_canada_ilb = true).

1. The Architectural Inflection Point

Observed 2026-08-08.

The table below summarizes the core differences between dynamic BGP/ECMP routing and Azure Internal Load Balancer ingress:

Architectural Metric	Pattern A: BGP / ECMP via Azure Route Server	Pattern B: Azure Internal Load Balancer (ILB)
Ingress Mechanism	CEs advertise `/32` host routes via eBGP (ASN `64512`) to Azure Route Server (ASN `65515`). ARS updates VNet system route tables with equal-cost next hops.	Azure Standard ILB listens on a static VNet private IP (`10.250.1.10`) and balances traffic across CE management NICs (`eth0`/SLO).
Health Detection & Node Removal	This deployment uses BGP session state and hold timers; BFD is disabled. When a route is withdrawn or the session times out, ARS removes that next hop after convergence.	Azure ILB Health Probe (`Tcp/65500` Site Console or `Tcp/80`). When a node fails consecutive probes, ILB removes it from the backend address pool.
Port & Protocol Handling	All ports destined for the `/32` VIP route natively to the CEs without port translation or rule limits.	Azure ILB HA Ports rule (`protocol = "All"`, `frontend_port = 0`, `backend_port = 0`) forwards all TCP/UDP traffic to backend CEs.
Subnet & Footprint Requirements	Requires a dedicated `/27` subnet named `RouteServerSubnet` (`10.0.4.0/27`) with no Network Security Group or route table.	No dedicated subnet required. Frontend IP sits directly in existing management (`snet-hub-management`) or internal subnets.
BGP Configuration on CE	Requires `xcsh_bgp` resources bound to the explicit `eth0` SLO interface on each CE site object.	No BGP configuration required. CE nodes require no BGP peering resources or BGP neighbor definitions.
Standing Cost (Azure)	Azure Route Server standing charge (~$0.60/hour, ~$430/month).	Azure Standard ILB standing charge (~$0.025/hour, ~$18/month + data processing).

2. Pattern A Drill-Down: Dynamic eBGP / ECMP Routing

Architecture & Traffic Flow

In the eBGP/ECMP architecture, Customer Edge nodes act as BGP speakers peering with Azure Route Server or a containerized Top-of-Rack (ToR) BGP router.

               Client Request
                     |
         +-----------v-----------+
         | VNet System Route     | (Equal-Cost Next Hops)
         | 10.250.0.10/32 -> ECMP|
         +-----+-----+-----+-----+
               |     |     |
         +-----+     |     +-----+
         |           |           |
      CE-01       CE-02       CE-03
     (eastus)    (eastus)    (eastus)

Host Route Advertisement: Each CE node advertises the HTTP Load Balancer VIP (10.250.0.10/32) via eBGP (ASN 64512) to the Azure Route Server peer IPs (10.0.4.4, 10.0.4.5, ASN 65515).
VNet Route Table Programming: Azure Route Server receives the host route from all active CEs and programs the VNet effective route table with multiple equal-cost next hops (nextHopType: VirtualNetworkGateway).
Data-Plane Forwarding: Client traffic to 10.250.0.10 is distributed across all healthy CE nodes by the Azure SDN fabric using 5-tuple hash ECMP.
Route Withdrawal: If CE-02 closes its BGP session, or the hold timer expires, Azure Route Server removes that path after routing convergence. This repository has not measured that failover interval, so the architecture is not evidence of a specific recovery time.

Key Verification Commands for Pattern A

# Query learned routes on Azure Route Server peering
az network routeserver peering list-learned-routes \
  --name "eastus01-bgp" \
  --routeserver "$(terraform output -raw route_server_name)" \
  -g "$(terraform output -raw resource_group_name)" \
  -o table

# Inspect effective route table on client NIC
az network nic show-effective-route-table \
  -g "$(terraform output -raw resource_group_name)" \
  -n "$(terraform output -raw client_nic_name)" \
  --query "value[?addressPrefix[0]=='$(terraform output -raw vip)/32']" \
  -o json

Observed output from healthy ARS BGP/ECMP deployment:

[
  {
    "addressPrefix": [
      "10.250.0.10/32"
    ],
    "nextHopIpAddress": [
      "10.0.1.4",
      "10.0.1.5",
      "10.0.1.6"
    ],
    "nextHopType": "VirtualNetworkGateway",
    "state": "Active"
  }
]

3. Pattern B Drill-Down: Azure Internal Load Balancer (ILB)

Pattern B Architecture & Traffic Flow

In the Azure ILB architecture variation (enable_canada_ilb = true), Azure’s Standard Load Balancer acts as the L4 ingress mechanism for the Canadian Customer Edge cluster.

               Client Request
                     |
         +-----------v-----------+
         | Azure Internal LB     | (Frontend IP: 10.250.1.10)
         | HA Ports Rule         |
         +-----+-----+-----+-----+
               |     |     |
         +-----+     |     +-----+
         |           |           |
      CE-CA-01   CE-CA-02    CE-CA-03
  (canadacentral)(canadacentral)(canadacentral)

Frontend Private IP: Standard Azure ILB (azurerm_lb.ca_ilb) provisions private IP 10.250.1.10 in snet-hub-management.
Backend Address Pool: The management NICs (eth0/SLO) of all Canadian CEs (CE-CA-01, CE-CA-02, CE-CA-03) are registered in azurerm_lb_backend_address_pool.ca_ce_backend.
Health Probing: The ILB probe (azurerm_lb_probe.ca_site_console) sends TCP SYN requests to port 65500 (Site Console / VPM service) every 5 seconds.
HA Ports Forwarding: The HA Ports rule (azurerm_lb_rule.ca_ha_ports) forwards all incoming TCP/UDP traffic on frontend 10.250.1.10 across healthy backend CE NICs with floating_ip_enabled = true.
Node Failure & Failover: If CE-CA-02 stops answering TCP/65500 probes for 10 seconds (2 consecutive probe failures), the Azure ILB marks CE-CA-02 unhealthy and redirects all new connections to CE-CA-01 and CE-CA-03.

Key Verification Commands for Pattern B

# Query Azure Internal Load Balancer status
az network lb show \
  -g "$(terraform output -raw resource_group_name)" \
  -n "$(terraform output -raw ca_ilb_id | cut -d/ -f9)" \
  -o json

# Query health probe status on backend pool
az network lb probe show \
  -g "$(terraform output -raw resource_group_name)" \
  --lb-name "$(terraform output -raw ca_ilb_id | cut -d/ -f9)" \
  -n "site-console-probe" \
  -o json

Observed output from healthy Canadian Azure ILB deployment:

{
  "backendPools": [
    "ca-ce-backend-pool"
  ],
  "frontendIp": "10.200.1.10",
  "name": "mcn-ce-ha-ca-ilb",
  "probes": [
    {
      "name": "site-console-probe",
      "port": 65500,
      "protocol": "Tcp"
    }
  ],
  "sku": "Standard"
}

4. Decision Matrix: When to Choose BGP/ECMP vs. ILB

Criteria / Constraint	Recommended Pattern	Architectural Justification
Strict BGP-Free Network Policy	Azure ILB	Many enterprise cloud landing zones prohibit BGP peering with Azure Route Server or virtual network gateways.
Cost Optimization	Azure ILB	Replaces ~$430/month Azure Route Server standing charge with ~$18/month Standard ILB.
Subnet Constrained Environment	Azure ILB	Does not require carving out a dedicated `/27` `RouteServerSubnet`.
Multi-Cloud Consistency (AWS & Azure)	BGP / ECMP	Uses identical BGP host route advertisement mechanics across AWS VPC, Azure ARS, and On-Premise ToR routers.
Native routed ingress	BGP / ECMP	Packets route to a CE next hop without an intermediate Azure load-balancer rule. This row does not claim Direct Server Return behavior.

5. Verification & Teardown

To verify both HA ingress patterns side-by-side:

cd terraform
# Verify ROW BGP/ECMP VIP health
curl -s -o /dev/null -w "%{http_code}\n" -H "Host: $(terraform output -raw lb_domain)" "http://$(terraform output -raw vip)/"

# Verify Canadian ILB VIP health
curl -s -o /dev/null -w "%{http_code}\n" -H "Host: $(terraform output -raw ca_lb_domain)" "http://$(terraform output -raw ca_vip)/"

To tear down the environment:

cd terraform
terraform destroy -auto-approve

Engineering SpecificationFull technical specification covering all four network paths.

Prove it healthyDetailed four-step health check sequence for all regions.