Many large service providers have already dropped this approach because it does not result in the cost reduction or increase in switching speed that pure IP-based networks bring.
@38396 The IP routing tricks try to shift the traffic load to underutilized links by artificially lowering their cost thus making them look more attractive to routing protocols like OSPF or IS-IS. Fine-tuning the link costs in a complex network to achieve good traffic distribution is almost impossible so this approach works only in niche situations. Significantly better results can be achieved with Border Gateway Protocol (BGP) thanks to a rich set of attributes it can carry with every IP route. Note that BGP was originally designed to support various routing policies so you could implement rudimentary traffic engineering as yet another routing policy.
Virtual circuits implemented with IP-over-IP tunnels (using a variety of technologies) are approximately as complex as routing protocol cost-tuning and so are better avoided (although they could still represent a valuable temporary fix). MPLS traffic engineering (MPLS TE) on the other hand is a complete implementation of traffic engineering technology rivaling the features available in advanced ATM or Frame Relay networks. For example:
- The MPLS TE network tracks available resources on each link using extensions to IP routing protocols (only OSPF and IS-IS are supported as MPLS TE needs full visibility of network topology which is not available with any other routing protocol).
- Whenever a new tunnel (the MPLS TE terminology for virtual circuit) needs to be established the head-end router computes the end-to-end path through the network based on the reported state of available resources.
- The tunnel establishment request is signaled hop-by-hop from the tunnel head-end to the tunnel tail router reserving resources on every hop.
- After the tunnel is established the new path is seamlessly integrated with the routing protocols running in the network.
The support for MPLS TE is available in high-end and midlevel routers from multiple vendors. It's therefore highly advisable that you consider the requirements of MPLS TE (OSPF or IS-IS for example) in your network design. If you implement the basic infrastructure needed by MPLS TE during the network deployment you'll have it ready to use when you need to shift the traffic to cope with unexpected increases in bandwidth usage or delayed deployment of higher-speed links.
MPLS traffic engineering essentials
MPLS (Multi-protocol Label Switching) is the end result of the efforts to integrate Layer 3 switching better known as routing with Layer 2 WAN backbones primarily ATM.. Even though the IP+ATM paradigm is mostly gone today because of the drastic shift to IP-only networks in the last few years MPLS retains a number of useful features from Layer 2 technologies. One of the most notable is the ability to send packets across the network through a virtual circuit called Label Switched Path or LSP in MPLS terminology.
NOTE: While the Layer 2 virtual circuits are almost always bidirectional (although the traffic contracts in each direction can be different) the LSPs are always unidirectional. If you need bidirectional connectivity between a pair of routers you have to establish two LSPs.
The LSPs in MPLS networks are usually established based on the contents of IP routing tables in core routers. However there is nothing that would prevent LSPs being established and used through other means provided that:
- All the routers along the path agree on a common signaling protocol.
- The router where the LSP starts (head-end router) and the router where the LSP ends (tail-end router) agree on what's traveling across the LSP.
NOTE: The other routers along the LSP do not inspect the packets traversing the LSP and are thus oblivious to their content; they just need to understand the signaling protocol that is used to establish the LSP.