In Chapter 2, you saw that an MPLS deployment in a router-only network does not increase the overall convergence time of the network after a network failure (the
convergence time does increase following recovery of a link—see Chapter 2 for more details). On the other hand, the convergence in ATM networks can change considerably when deploying MPLS. In a traditional ATM network, the convergence time consisted of the following components:
• An edge router had to detect an adjacent router failure through ATM signaling, ATM operation-and-maintenance (OAM) cells, or routing protocol timeouts (dead timer or hold timer).
• The edge router detecting the adjacent router failure immediately propagated the change in network topology to all other routers.
• In link-state protocols, all the routers had to recompute a new network topology, usually after a slight delay.
When the ATM network is migrated toward MPLS, the convergence time of the network consists of the following components:
• An LSR must detect an adjacent LSR failure. This process is usually very quick because the adjacent LSRs linked with point-to-point links and the physical layer indicates line failure very quickly.
• The LSR must propagate change in network topology to other LSRs. This process takes longer in MPLS networks because the number of routing devices between the edges of the ATM network has increased. All ATM switches that were transparent to IP routing in traditional ATM networks now act as IP routers.
• All LSRs, including ATM switches, must recalculate the new network topology and change their routing tables.
• If the next-hop for a destination has changed, an ATM edge-LSR must request new labels for these destinations. Other ATM-LSRs must propagate these label requests across the ATM-LSR domain, more so if VC merge is not used and each request must be propagated all the way across the ATM network to the egress ATM edge-LSR. This is an extra step that is not needed in traditional ATM networks.
When comparing the convergence of a traditional ATM-based IP backbone with the MPLS-based IP+ATM backbone, you can see that the convergence time in the MPLS-MPLS-based
backbone usually increases because the extra steps were not performed in the traditional IP backbone. The other benefits of MPLS usually outweigh this concern, but the increased convergence time is still a parameter that you must take into account when planning the migration of your ATM backbone toward an MPLS-enabled IP+ATM backbone.
Summary
In this chapter, we've discussed the specifics of running MPLS across ATM networks. The MPLS architecture allows MPLS to be deployed in ATM networks with no hardware
upgrades to the ATM switches.
Note
A hardware upgrade is usually needed to support VC merge functionality in the ATM switches because the traditional ATM switches have no equivalent function.
ATM switches do need new control software in the control processors that support MPLS signaling. Some switches cannot support the increased demands, resulting in the need for an external controller (Label Switch Controller) that provides MPLS support for such a switch.
The MPLS forwarding and label allocation procedures were slightly modified to support the ATM environment:
• Cell-based label switching is performed purely based on VPI/VCI values in ATM cell headers to support the existing ATM infrastructure. The top-of-stack MPLS label is thus encoded in the ATM cell header.
• Even though the top-of-stack label is moved into the ATM cell header, the MPLS stack in the labeled packet is still intact because it is needed to support additional MPLS functionality such as MPLS experimental bits or the TTL field. The label in the top entry of the MPLS label header is set to 0 because it is not used across an ATM network.
• Label distribution in an ATM network is based on downstream-on-demand procedures to minimize VC usage across LC-ATM interfaces.
• Traditional ATM switches must request a label from the downstream LSR before they can allocate a label to an upstream LSR and establish inbound-to-outbound VPI/VCI mapping in the ATM switching matrix. A new label must be requested from the downstream LSR for each upstream request to prevent cell interleave problems.
• Advanced ATM switches support VC merge, additional cell buffering that prevents cell interleave problems. These switches can use the same downstream label for all
upstream neighbors, resulting in significant savings of VCs used across LC-ATM interfaces.
The downstream-on-demand label distribution in ATM networks also affects the
convergence time of ATM-based MPLS networks. The overall convergence time usually increases because new labels must be requested and allocated following the convergence of an IP routing protocol.
Chapter 4. Running Frame-mode MPLS Across Switched WAN Media
The previous two chapters showed that you can deploy MPLS using different modes of operation. Chapter 2, "Frame-mode MPLS Operation," details how MPLS operates across framed interfaces, and Chapter 3, "Cell-mode MPLS Operation," shows how MPLS operates natively across ATM media.The Layer 2 infrastructure, which provides the media over which Frame-mode MPLS can operate, often can be supplied through the use of switched WAN technology, such as Frame Relay or ATM. You can run MPLS in Cell-mode across ATM but not when using Frame Relay or when the ATM structure is built using traditional ATM Forum PVCs. This means that it must be possible to run Frame-mode MPLS across these types of interfaces so you can deploy MPLS end-to-end across the network.
This chapter considers the deployment of MPLS across Frame Relay interfaces and ATM PVCs. It also considers the use of Frame-mode and Cell-mode MPLS across the same physical interface; this functionality can be useful during a migration to the MPLS architecture, as you'll learn in Chapter 6, "MPLS Migration and Configuration Example."