— W H ITE PA PER

Cross-subnet roaming in ABB broadband wireless mesh networksBroadband wireless mesh networks from ABB support two types of cross-subnet roaming: Border Gateway Protocol (BGP)-Based Cross-Subnet Roaming and Tunnel-Based Cross-Subnet Roaming. BGP-Based Cross-Subnet Roaming uses one or more BGP routers in an operator’s core network to enable client mobility and nomadicity across IP subnets in the broadband wireless mesh network. Tunnel-Based Cross-Subnet Roaming enables a similar capability using tunnels between wireless mesh routers configured as gateways (i.e., having an upstream wired connection to an external network such as an intranet or the internet) in the broadband wireless network.

This paper describes both mechanisms for implementing cross-subnet roaming in ABB broadband wireless mesh networks. Please note that, when the network operator’s core network supports BGP and cross-subnet roaming is required, ABB strongly recommends using BGP-Based Cross-Subnet Roaming. It offers higher performance, lower latency and is much more scalable than Tunnel-Based Cross-Subnet Roaming when a BGP router is already in place in the core network. Even network operators not currently using BGP should consider the pros and cons of implementing BGP compared to activating Tunnel-Based Cross-Subnet Roaming.

Cross-subnet roaming: General capabilitiesBroadband wireless mesh networks from ABB support seamless, session-persistent roaming for IEEE 802.11 clients at vehicular speeds for as long as they stay within the mesh coverage area. Clients, including those that have established an IPsec/VPN connection, can move between nodes, gateways and subnets without losing connection(s).

BGP-Based Cross-Subnet Roaming This section outlines the operation of BGP-Based Cross-Subnet Roaming.

Figure 1 illustrates an ABB wireless broadband mesh network with three gateways. Gateways A and B are connected directly to a wired Ethernet switch (Switch 1) while Gateway C is connected indirectly to Switch 2 via a wireless point-to-multipoint (PTMP) link.

A BGP router (BGP Router) is located upstream from all of the gateways in the network. Note that the BGP router must be the gateways’ immediate upstream neighbor router. Note also that there can be more than one BPG router in the system – the figure shows only one for simplicity.

A client is connected to an ABB broadband wireless mesh router that is associated with Cluster A. (A cluster is a group of wireless mesh routers that share a common gateway.) This client’s IP address belongs to the subnet that also includes both the wired and wireless interfaces of Gateway A and Gateway B.

White Paper Cross-subnet roaming in ABB broadband wireless mesh networks 2

Figure 1: Client connects to Cluster A

In Figure 2, the client has moved and is now connected to an ABB broadband wireless mesh router that is associated with Cluster B. Again, note that the client’s IP address belongs to the subnet that also includes both the wired and wireless interfaces of Gateway A and Gateway B. Gateway A and Gateway B communicate such that they know the client has moved to Cluster B. The client’s sessions, including TCP, VPN and application sessions, are transparently preserved during and after the move. This transition is handled entirely by the gateways; no use of BGP is required at this point.

In Figure 3, the example client has moved again, this time to Cluster C. Note that, while the wired and wireless interfaces for Gateway C belong to a subnet different than Gateways A and B, the IP address of the example client does not change.

Gateway C has been pre-configured to permit clients from the 192.168.1.0/24 subnet to join Cluster C even though Cluster C is in the 192.168.2.0/24 subnet. This pre-configuration also enables Gateway C to notify its BGP neighbor, the BGP Router, if a client from the 192.168.2.0/24 subnet joins Cluster C. Please see the section “Configuring BGP-Based Cross-Subnet Roaming” for more details.

When the client, whose IP address is 192.168.1.110, associates with a wireless mesh router in Cluster C, it is permitted to join the cluster because of the pre-configuration of Gateway C. Gateway C then uses BGP to communicate with the BGP Router, informing it that packets destined for 192.168.1.110 should be sent to Gateway C rather that Gateway B. The BGP Router will act accordingly. The client’s sessions, including TCP, VPN and application sessions, are transparently preserved during and after the move.

Figure 2: Client moves to Cluster B

White Paper Cross-subnet roaming in ABB broadband wireless mesh networks 3

As this example shows, ABB wireless broadband mesh networks can easily and efficiently support cross-subnet roaming using BGP.

Configuring BGP-Based Cross-Subnet RoamingConfiguration of BGP-Based Cross-Subnet Roaming is simple. In each gateway, the name and IP address of its BGP neighbor (BGP Router, in this example), the Autonomous System Number (ASN) of the gateway the (local ASN) and the BGP Router (the remote ASN), and the applicable virtual local area network (VLAN) ID are config-ured. Note that, because ABB’s BGP-Based Cross-Subnet Roaming use Exterior BGP (EBGP), as opposed to Inte-rior BGP (IBGP), the local and remote ASNs must be different. After this information is added, the list of foreign subnets (e.g., in the illustration above, the 192.168.1.0/24 is a foreign subnet for Gateway C) to be supported for BGP-Based Cross-Subnet Roaming is entered into each gateway.

To implement BGP-Based Cross-Subnet Roaming, BGP Router must be configured to recognize Gateways A, B and C as BGP neighbors.

Extending BGP-Based Cross-Subnet Roaming to fixed, nomadic and mobile nodes While the example above explains BGP-Based Cross-Subnet Roaming in the context of a mobile client, the same functionality applies to mobile, nomadic and fixed nodes. BGP-Based Cross-Subnet Roaming treats the IP addresses of nodes in the same manner that it treats IP addresses of clients.

While the need for mobile and nomadic nodes to roam is obvious, the applicability of BGP-Based Cross-Subnet Roaming fixed nodes is less so. A fixed node, even if physically stationary, can migrate from one cluster to another, and, therefore, from one IP subnet to another, if interference causes a mesh link to become suboptimal or if the backhaul connection to its gateway fails. See the white paper Mesh OS: Foundation of the ABB Wireless mesh network architecture for more details regarding how ABB broadband wireless mesh networks route around interference and failures. Use of BGP-Based Cross-Subnet Roaming will maintain connectivity as well as TCP, VPN and application sessions for wired and wireless clients connected to a node that moves from one cluster and subnet to another.

Tunnel-Based Cross-Subnet RoamingThis section outlines the operation of Tunnel-Based Cross-Subnet Roaming.

Figure 4 again illustrates an ABB wireless broadband mesh network with three gateways. As with the figures above, Gateways A and B are connected directly to Switch 1 while Gateway C is connected indirectly to Switch 2 via a wireless PTMP. Note that, in contrast to the figures above, no upstream routers are shown. This is because Tunnel-Based Cross-Subnet Roaming does not require outside resources to operate. All functionality needed to implement Tunnel-Based Cross-Subnet Roaming is supported by the gateways in the wireless mesh network.

Figure 3: Client moves to Cluster C

White Paper Cross-subnet roaming in ABB broadband wireless mesh networks 4

As with the previous example, a client is connected to an ABB broadband wireless mesh router that is associated with cluster A. This client’s IP address belongs to the subnet that also includes both the wired and wireless interfaces of gateway A and gateway B.

In Figure 5, the client has moved and is now connected to an ABB broadband wireless mesh router that is associated with Cluster B. The operation is the same as described for Figure 2, above.

Figure 4: Client connects to Cluster A

Figure 5: Client moves to Cluster B

In Figure 6, the example client has moved again, this time to Cluster C. Note that, while the wired and wireless interfaces for Gateway C belong to a subnet different than Gateways A and B, the IP address of the example client does not change. Gateway C uses a tunnel to Gateway B to exchange information about supported subnets and client devices. This enables Gateway B to use the tunnel to forward the client’s traffic to Gateway C. The client’s sessions, including TCP, VPN and application sessions, are transparently preserved during and after the move. Please see the section “Configuring Tunnel-Based Cross-Subnet Roaming” for more details regarding configuration.

Note that Figure 6 illustrates one of the main drawbacks of Tunnel-Based Cross-Subnet Roaming. Even though the client is no longer connected to Gateway B’s cluster and, therefore, the client cannot be reached directly via

White Paper Cross-subnet roaming in ABB broadband wireless mesh networks 5

Gateway B, traffic destined for the client must still traverse Gateway B. This U-turn, that is, traffic destined for the client being sent to and subsequently sent from Gateway B, increases latency and can cause congestion on the backhaul link, potentially harming throughput.

Figure 6: Client moves to Cluster C

As this example shows, ABB wireless broadband mesh networks can support cross-subnet roaming using tunnels. However, because of the U-turn inherent in the use of tunnels, the tunnel-based approach suffers from higher latency and lower performance.

Configuring Tunnel-Based Cross-Subnet RoamingEach gateway in the roaming domain must be configured with a list of the IP addresses and subnet masks of all gateways in the roaming domain.

Comparing cross-subnet roaming capabilitiesThe primary advantage of Tunnel-Based Cross-Subnet Roaming is that it does not require outside resources to operate. All functionality needed to implement Tunnel-Based Cross-Subnet Roaming is supported by the gateways in the wireless mesh network.

The primary advantage of BGP-Based Cross-Subnet Roaming is that it offers higher performance, lower latency and better scalability. The BGP-based solution eliminates the U-turn of the tunnel-based solution that increases latency and can cause congestion on the backhaul link.

Another advantage of BGP-Based Cross-Subnet Roaming is, under certain failure scenarios, higher reliability. For Tunnel-Based Cross-Subnet Roaming to function, the home gateway, Gateway B in the example above, must be operational at all times while the client is roaming. If the mesh router acting as the home gateway, or its backhaul connection, fails, Tunnel-Based Cross-Subnet Roaming will cease operation. In contrast, with BGP-Based Cross-Subnet Roaming, only the gateway of the cluster to which the client is currently connected is in the data path. Therefore, continued operation of gateways for clusters to which the client was previously connected is unnecessary for BGP-Based Cross-Subnet Roaming to function.

Interaction between BGP-Based and Tunnel-Based Cross-Subnet RoamingBGP-Based and Tunnel-Based Cross-Subnet Roaming are not designed to be used together. The interaction between them has not been tested by ABB Wireless. The consequences of using them simultaneously are not known. Customers must pick one or the other. Do not use BGP-Based and Tunnel-Based Cross-Subnet Roaming in a network at the same time.

—We reserve the right to make technical changes or modify the contents of this document without prior notice. With regard to purchase orders, the agreed particulars shall prevail. ABB does not accept any responsibility whatsoever for potential errors or possible lack of information in this document.

We reserve all rights in this document and in the subject matter and illustrations contained therein. Any reproduction, disclosure to third parties or utilization of its contents – in whole or in parts – is forbidden without prior written consent of ABB. Copyright© 2018 ABBAll rights reserved

—ABB Wireless3055 Orchard DriveSan Jose, CA 95134wireless.sales@nam.abb.com

abb.com/unwired

Use of mobile mesh routers in cross-subnet roaming environmentsWhen mobile mesh routers from ABB are employed in cross-subnet roaming, make-before-break connections are used when mobile mesh routers transition from being connected to one fixed wireless broadband mesh router to being connected to another in the coverage area. Using ABB mobile mesh routers, handoffs employ make-before-break connections, reducing handoff times and making the persistence of client connections even more reliable. When mobile routers are used, mobile clients connect to them rather than to the fixed broadband wireless mesh infrastructure.

Mobile mesh routers also transmit at higher power than typical wireless clients. Higher transmit power enables more reliable and longer distance connections to the fixed routers in the mesh.

Summary and conclusionsBoth BGP-Based Cross-Subnet Roaming and Tunnel-Based Cross-Subnet Roaming enable wireless clients to move across subnets while transparently preserving TCP, VPN and application sessions. BGP-Based Cross-Sub-net Roaming is strongly recommended because it is more scalable and offers better performance. Tunnel-Based Cross-Subnet Roaming may be used in networks that do not support BGP routing.

4C

AE

00

06

31 -

10

00

- 0

6.2

018