IS--IS may form multiple equal cost paths between points. This is true of any link state protocol. In the absence of any explicit support to take advantage of this, a path may be chosen arbitrarily. Techniques have been utilized to divide traffic somewhat evenly among the available paths. These techniques have been referred to as Equal Cost Multipath (ECMP). An unequal division of traffic among the available paths is generally preferable. Routers generally have no knowledge of traffic loading on distant links and therefore have no basis to optimize the allocation of traffic.
Optimized Mulitpath is a extension to IS--IS, utilizing additional Type/Length/Value (TLV) tuples to distribute loading information. An algorithm to adjust forwarding, gradually enough to insure stability yet provide reasonably fast adjustment when needed, is provided in the related document OSPF--OMP \cite{draft-ietf-ospf-omp-01}. The IS--IS encapsulation and minor differences in the dynamics of ISIS--OMP relative to OSPF--OMP are described here.
IS-IS Optimized Multipath (ISIS-OMP) is one of three related internet-drafts. The three are OSPF-OMP, ISIS-OMP and MPLS-OMP. There is now one set of web pages that provides a complete description of OMP, including tutorials, simulations, and comparisons of simulations results to directed connection techniques (VC or LSP layout). A description of recent changes to both the internet draft and the simulations is available.
The IS-IS Optimized Multipath (OMP) draft is based on OSPF-OMP. Both IS-IS and OSPF are link state protocol. The dynamics of IS-IS OMP is only slightly different from OSPF-OMP. The difference is due to the need to pack IS-IS Link State PDU (LSPDU) into no more than 256 LSPDU fragments. As a result, when one link's loading information must be flooded, other link loading information in the same LSPDU fragment is also updated. Simulations of the most drastic case where all link information is in a single LSPDU fragment are planned but not yet available.
This page provides a link to the latest ISIS-OMP internet-draft in text, postscript, PDF, and HTML format.
This document is available as a postscript file or, as a plain text file or, in HTML. The source is in latex. The HTML version was produced by latex2html available at a Lawrence Livermore National Laboratory archive.