Information Technology Reference
In-Depth Information
•
Number of BGP sessions terminated on the router
—There is a single TCP session
for each BGP peering session.
The following hold queue sizing formula helps you determine the worst-case scenario:
Hold
Queue
Size
Window Size
2 * MSS
=
----------------------------------* Peer Count
This is a worst-case formula for the number of acknowledgments that can be sent to a single
BGP router at one time. The window size divided by the MSS times 2 provides the maximum
number of TCP segments that can be unacknowledged at any point. Cisco IOS software
sends one TCP ACK per two TCP segments, so the maximum number of acknowledgments
is one-half the maximum number of segments that can be outstanding. The maximum num-
ber of outstanding acknowledgments per peer can be multiplied by the route's peer count
to obtain the maximum number of outstanding TCP ACK messages for a particular BGP
router.
Table 3-1 shows results for various TCP MSS values.
Table 3-1
Wo rst-Case Input Queue Values for BGP Traffic
TCP MSS (in Bytes)
Window Size (in Bytes)
Session Count
Hold Queue Size
536
16000
50
700
1460
16000
50
200
4430
16000
50
50
536
16000
100
1400
1460
16000
100
400
4430
16000
100
100
The values in Table 3-1 are the worst-case values for the BGP packets. This does not include
any other traffic destined for the route processor that also needs a place in the hold queue.
The common recommendation is to set the input hold queue to a value of 1000 in heavy
BGP environments. This accounts for additional traffic, such as management traffic and
other control plane traffic.
SPD
The SPD feature is a queue-management mechanism that operates on the input hold queues
for traffic destined for the route processor. The SPD process can distinguish between high-
and normal-priority traffic, allowing it to better manage system resources in the input
queue. The SPD function is specifically for managing input queue congestion.
