Selective Packet Discard

So far, this chapter has covered queue management for user data on the network. What about data that is critical for maintaining the network itself, such as routing updates or interface keepalives? Cisco routers automatically send packets that are critical to internetwork control with an IP precedence of 6 or above. The routers perform selective packet discard (SPD) for packets that are not critical to routing and interface stability.

You do not need to perform any configuration to enable SPD functionality. However, a more aggressive mode can be configured via the ip spd mode aggressive global configuration command. When aggressive mode is configured, all IP packets that fail basic sanity checks, such as those with bad checksums or TTLs, will be dropped aggressively as an extra protection against bad IP packet spoofing. The show ip spd command displays whether aggressive mode is enabled.

When the IP input queue reaches SPD minimum threshold, which is tuned via the ip spd queue min-threshold n command, all packets that are subject to aggressive drop policy are dropped immediately, whereas normal IP packets (not high-priority packets) are dropped with increasing probability as the length of the IP input queue grows. When the IP input queue reaches SPD maximum threshold, specified by the ip spd queue max-threshold n command, all normal IP packets are dropped at 100 percent. The default SPD minimum threshold is 10, whereas the default maximum threshold is 75. The default values for min and max threshold have been carefully selected by Cisco, and for most purposes, you will not need to modify them.

Managing congestion when it occurs is always tricky. What works in some instances may not work in others. Moreover, most congestion-management techniques have very little or no intelligence about one of the most ubiquitous forms of Internet traffic—TCP data flows. Congestion-avoidance algorithms introduce this intelligence.

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