Scalability and Flexibility of IP-based Forwarding

To understand all the issues that affect the scalability and the flexibility of traditional IP packet forwarding networks, you must start with a review of some of the basic IP forwarding mechanisms and their interaction with the underlying infrastructure (local- or wide-area networks). With this information, you can identify any drawbacks to the existing approach and perhaps provide alternative ideas on how this could be improved.

Network Layer Routing Paradigm

Traditional network layer packet forwarding (for example, forwarding of IP packets across the Internet) relies on the information provided by network layer routing protocols (for example, Open Shortest Path First [OSPF] or Border Gateway Protocol [BGP]), or static routing, to make an independent forwarding decision at each hop (router) within the network. The forwarding decision is based solely on the destination unicast IP address. All packets for the same destination follow the same path across the network if no other equal-cost paths exist. Whenever a router has two equal-cost paths toward a destination, the packets toward the destination might take one or both of them, resulting in some degree of load sharing.


Enhanced Interior Gateway Routing Protocol (EIGRP) also supports non?equal-cost load sharing although the default behavior of this protocol is equal-cost. You must configure EIGRP variance for non?equal-cost load balancing. Please see EIGRP Network Design Solutions (ISBN 1-57870-165-1), from Cisco Press for more details on EIGRP.

Load sharing in Cisco IOS can be performed on a packet-by-packet or source-destination-pair basis (with Cisco Express Forwarding [CEF] switching) or on a destination basis (most of the other switching methods).

Routers perform the decision process that selects what path a packet takes. These network layer devices participate in the collection and distribution of network-layer information, and perform Layer 3 switching based on the contents of the network layer header of each packet. You can connect the routers directly by point-to-point links or local-area networks (for example, shared hub or MAU), or you can connect them by LAN or WAN switches (for example, Frame Relay or ATM switches). These Layer 2 (LAN or WAN) switches unfortunately do not have the capability to hold Layer 3 routing information or to select the path taken by a packet through analysis of its Layer 3 destination address. Thus, Layer 2 (LAN or WAN) switches cannot be involved in the Layer 3 packet forwarding decision process. In the case of the WAN environment, the network designer has to establish Layer 2 paths manually across the WAN network. These paths then forward Layer 3 packets between the routers that are connected physically to the Layer 2 network.

LAN Layer 2 paths are simple to establish?all LAN switches are transparent to the devices connected to them. The WAN Layer 2 path establishment is more complex. WAN Layer 2 paths usually are based on a point-to-point paradigm (for example, virtual circuits in most WAN networks) and are established only on request through manual configuration. Any routing device (ingress router) at the edge of the Layer 2 network that wants to forward Layer 3 packets to any other routing device (egress router) therefore needs to either establish a direct connection across the network to the egress device or send its data to a different device for transmission to the final destination.

Consider, for example, the network shown in Figure 1-1.

Figure 1-1. Sample IP Network Based on ATM Core


The network illustrated in Figure 1-1 is based on an ATM core surrounded by routers that perform network layer forwarding. Assuming that the only connections between the routers are the ones shown in Figure 1-1, all the packets sent from San Francisco to or via Washington must be sent to the Dallas router, where they are analyzed and sent back over the same ATM connection in Dallas to the Washington router. This extra step introduces delay in the network and unnecessarily loads the CPU of the Dallas router as well as the ATM link between the Dallas router and the adjacent ATM switch in Dallas.

To ensure optimal packet forwarding in the network, an ATM virtual circuit must exist between any two routers connected to the ATM core. Although this might be easy to achieve in small networks, such as the one in Figure 1-1, you run into serious scalability problems in large networks where several tens or even hundreds of routers connect to the same WAN core.

The following facts illustrate the scalability problems you might encounter:

  • Every time a new router is connected to the WAN core of the network, a virtual circuit must be established between this router and any other router, if optimal routing is required.


In Frame Relay networks, the entire configuration could be done within the Layer 2 WAN core and the routers would find new neighbors and their Layer 3 protocol addresses through the use of LMI and Inverse ARP. This also is possible on an ATM network through the use of Inverse ARP, which is enabled by default when a new PVC is added to the configuration of the router, and ILMI, which can discover PVCs dynamically that are configured on the local ATM switch.

  • With certain routing protocol configurations, every router attached to the Layer 2 WAN core (built with ATM or Frame Relay switches) needs a dedicated virtual circuit to every other router attached to the same core. To achieve the desired core redundancy, every router also must establish a routing protocol adjacency with every other router attached to the same core. The resulting full-mesh of router adjacencies results in every router having a large number of routing protocol neighbors, resulting in large amounts of routing traffic. For example, if the network runs OSPF or IS-IS as its routing protocol, every router propagates every change in the network topology to every other router connected to the same WAN backbone, resulting in routing traffic proportional to the square of the number of routers.


Configuration tools exist in recent Cisco IOS implementations of IS-IS and OSPF routing protocols that allow you to reduce the routing protocol traffic in the network. Discussing the design and the configuration of these tools is beyond the scope of this book (any interested reader should refer to the relevant Cisco IOS configuration guides).

  • Provisioning of the virtual circuits between the routers is complex, because it's very hard to predict the exact amount of traffic between any two routers in the network. To simplify the provisioning, some service providers just opt for lack of service guarantee in the network?zero Committed Information Rate (CIR) in a Frame Relay network or Unspecified Bit Rate (UBR) connections in an ATM network.

The lack of information exchange between the routers and the WAN switches was not an issue for traditional Internet service providers that used router-only backbones or for traditional service providers that provided just the WAN services (ATM or Frame Relay virtual circuits). There are, however, several drivers that push both groups toward mixed backbone designs:

  • Traditional service providers are asked to offer IP services. They want to leverage their investments and base these new services on their existing WAN infrastructure.

  • Internet service providers are asked to provide tighter quality of service (QoS) guarantees that are easier to meet with ATM switches than with traditional routers.

  • The rapid increase in bandwidth requirements prior to the introduction of optical router interfaces forced some large service providers to start relying on ATM technology because the router interfaces at that time did not provide the speeds offered by the ATM switches.

It is clear, therefore, that a different mechanism must be used to enable the exchange of network layer information between the routers and the WAN switches and to allow the switches to participate in the decision process of forwarding packets so that direct connections between edge routers are no longer required.

Differentiated Packet Servicing

Conventional IP packet forwarding uses only the IP destination address contained within the Layer 3 header within a packet to make a forwarding decision. The hop-by-hop destination-only paradigm used today prevents a number of innovative approaches to network design and traffic-flow optimization. In Figure 1-2, for example, the direct link between the San Francisco core router and the Washington core router forwards the traffic entering the network in any of the Bay Area Points-of-Presence (POPs), although that link might be congested and the links from San Francisco to Dallas and from Dallas to Washington might be only lightly loaded.

Figure 1-2. Sample Network that Would Benefit from Traffic Engineering


Although certain techniques exist to affect the decision process, such as Policy Based Routing (PBR), no single scalable technique exists to decide on the full path a packet takes across the network to its final destination. In the network shown in Figure 1-2, the policy-based routing must be deployed on the San Francisco core router to divert some of the Bay Area to Washington traffic toward Dallas. Deploying such features as PBR on core routers could severely reduce the performance of a core router and result in a rather unscalable network design. Ideally, the edge routers (for example, the Santa Clara POP in Figure 1-2) can specify over which core links the packets should flow.


Several additional issues are associated with policy-based routing. PBR can lead easily to forwarding loops as a router configured with PBR deviates from the forwarding path learned from the routing protocols. PBR also is hard to deploy in large networks; if you configure PBR at the edge, you must be sure that all routers in the forwarding path can make the same route selection.

Because most major service providers deploy networks with redundant paths, a requirement clearly exists to allow the ingress routing device to be capable of deciding on packet forwarding, which affects the path a packet takes across the network, and of applying a label to that packet that indicates to other devices which path the packet should take.

This requirement also should allow packets that are destined for the same IP network to take separate paths instead of the path determined by the Layer 3 routing protocol. This decision also should be based on factors other than the destination IP address of the packet, such as from which port the packet was learned, what quality of service level the packet requires, and so on.

Independent Forwarding and Control

With conventional IP packet forwarding, any change in the information that controls the forwarding of packets is communicated to all devices within the routing domain. This change always involves a period of convergence within the forwarding algorithm.

A mechanism that can change how a packet is forwarded, without affecting other devices within the network, certainly is desirable. To implement such a mechanism, forwarding devices (routers) should not rely on IP header information to forward the packet; thus, an additional label must be attached to a forwarded packet to indicate its desired forwarding behavior. With the packet forwarding being performed based on labels attached to the original IP packets, any change within the decision process can be communicated to other devices through the distribution of new labels. Because these devices merely forward traffic based on the attached label, a change should be able to occur without any impact at all on any devices that perform packet forwarding.

External Routing Information Propagation

Conventional packet forwarding within the core of an IP network requires that external routing information be advertised to all transit routing devices. This is necessary so that packets can be routed based on the destination address that is contained within the network layer header of the packet. To continue the example from previous sections, the core routers in Figure 1-2 would have to store all Internet routes so that they could propagate packets between Bay Area customers and a peering point in MAE-East.


You might argue that each major service provider also must have a peering point somewhere on the West coast. That fact, although true, is not relevant to this discussion because you can always find a scenario where a core router with no customers or peering partners connected to it needs complete routing information to be able to forward IP packets correctly.

This method has scalability implications in terms of route propagation, memory usage, and CPU utilization on the core routers, and is not really a required function if all you want to do is pass a packet from one edge of the network to another.

A mechanism that allows internal routing devices to switch the packets across the network from an ingress router toward an egress router without analyzing network layer destination addresses is an obvious requirement.

    Part 2: MPLS-based Virtual Private Networks