10.9. Issues

One of the consequences of defining different OSPF TE extensions is that the frequency and the volume of routing updates may increase significantly as compared with regular OSPF. With traffic engineering extensions, link state advertisements capture resource availability. This information must be refreshed frequently enough to maintain fidelity. At the same time, they should not be generated too frequently, for instance, to reflect minor changes in resource status. A configurable thresholding scheme can be used whereby a node would generate an LSA update only if the resource information changes “significantly.” This modification reduces the number of TE LSA updates.

In the absence of any change in the network state, the TE LSAs are refreshed at regular refresh intervals of thirty minutes, just like other LSAs. In addition to regular refreshes, LSAs need to be updated to reflect changes in the network state (topology and resource information). The following configurable mechanisms can be used to reduce the number of TE LSA updates:

• Relative change based triggers: Under this scheme, an update is triggered when the relative difference between the current and the previously advertised link state exceeds a certain threshold expressed as a percentage value. For example, anytime the percentage of available bandwidth in a TE link increases or decreases by more than the specified thresholds, an update may be generated.

• Absolute change based triggers: This scheme differs from the relative change based scheme described above in that the measure of change is absolute. For example, in this scheme an update is triggered when the number of available component links in a link bundle crosses a certain configurable constant.

• Timer based triggers: Timer based triggers may be used to generate updates at fixed intervals or used to enforce minimum spacing between two consecutive updates. The latter is referred to as a hold-down timer and is used in conjunction with threshold or absolute change based triggers to control the volume of link state updates.

Figure 10-7 shows bandwidth requirement for optical TE and router LSA updates for OSPF areas of different sizes under different connection holding times [Sengupta+02]. In this figure, we assume that an optical TE LSA is generated as soon as there is a change in link state due to change in resource availability (e.g., note that this an extreme situation reflecting the upper bound on the bandwidth requirement). Figure 10-8 shows the bandwidth usage for networks of different sizes and for different triggering thresholds. These results show that bandwidth requirement for LSA update is modest even under extreme conditions, with very small connection holding time and in very large networks.