Information Technology Reference
In-Depth Information
depth of 75, only high-priority packets are accepted, until the input queue reaches an
overloaded depth of 175. Packets considered high-priority are IP Precedence 6 traffic
(BGP), IGP packets, and L2 keepalives.
The second extension is the SPD Extended Headroom, which allows further extension to
the input queue beyond the combined value of the input hold queue and the SPD Headroom.
The default value for the SPD Extended Headroom is 10, which results in the input queue's
having a maximum depth of 185. The SPD Extended Headroom is only for IGP packets and
L2 keepalives. These packets are crucial to maintaining a stable network.
The complete picture of the input queue is shown in Figure 3-3.
Figure 3-3
Input Hold Queue Layout
Input Hold Queue
SPD Headroom
Extended SPD Headroom
0
75
175
185
Normal IP Traffic,
BGP, OSPF, IS-IS,
L2 Keepalives
BGP, OSPF, IS-IS,
L2 Keepalives
IS-IS, OSPF,
L2 Keepalives
The maximum input queue actually consists of three values: the input hold queue, the SPD
Headroom, and the SPD Extended Headroom. Each is more specific and is tailored toward
preserving higher-priority traffic from being crowded out.
The default size of the SPD Headroom has been increased in Cisco IOS Release 12.0(19)S
to accommodate the needs of large-scale BGP networks. The new default, 1000, is specific
to 12.0S and is the same as the standard suggested size for the input hold queue. This is
because in the worst-case scenario of tuning the input hold queue, it was determined that
1000 TCP ACKs was a reasonable expectation. If the input hold queue is full of regular IP
traffic for some reason, the SPD Headroom of 1000 is sufficient to hold the influx of TCP
ACK messages.
The sizing of the SPD Headroom is to prevent packet loss for high-priority traffic. Aggressive
sizing of the SPD Headroom is not a problem, because only high-priority traffic is allowed
to use the SPD Headroom to increase the input queue depth beyond the configured input
hold queue.
