Cisco Cisco IOS Software Release 12.0(13)S7
Behavioral Change for Buffer Recarving
Controlling Buffer Recarving
3
Behavioral Change for Buffer Recarving
Understanding Buffer Carving Mechanism
The Route Processor (RP) runs the buffer carve code to determine the buffer specifications to be sent
down to line cards. Line cards apply these specifications on their packet buffers. Route Processor buffer
carve code can be invoked under the following conditions:
down to line cards. Line cards apply these specifications on their packet buffers. Route Processor buffer
carve code can be invoked under the following conditions:
•
Router power-on
•
MTU changes on an LC interface, done from the RP
•
Online insertion and removal (OIR) event; for example, insertion or removal of a line card
Controlling Buffer Recarving
Each line card on Cisco 12000 series Internet router has two sets of packet memories. One of the packet
memories is on the ingress side (tofab) and the other is on the egress side(frfab).
memories is on the ingress side (tofab) and the other is on the egress side(frfab).
The Route Processor calculates buffer specifications and sends it down to the line cards on their
power-on. The line cards blindly carve their packet memories based on these specifications. Changes in
the buffer specification can happen when the MTU distribution changes on the router, triggering
recarving of buffers on line cards (LC). Currently by default, buffer recarving occurs in any of the
following cases:
power-on. The line cards blindly carve their packet memories based on these specifications. Changes in
the buffer specification can happen when the MTU distribution changes on the router, triggering
recarving of buffers on line cards (LC). Currently by default, buffer recarving occurs in any of the
following cases:
•
Case 1: Change that results from increasing or decreasing the maximum MTU configured on the
router.
router.
•
Case 2: Change that results from adding or removing new buffer pool within the maximum slot
MTU, with no change in the maximum MTU on the router.
MTU, with no change in the maximum MTU on the router.
•
Case 3: Change that results from increasing or decreasing the existing pool buffer length while the
maximum MTU on the router remains the same.
maximum MTU on the router remains the same.
•
Case 4: Change that results from increasing or decreasing the maximum slot MTU, which may
change the maximum MTU on the router.
change the maximum MTU on the router.
Network Traffic Disruption
Recarves on line card result in disruption of the traffic flow; the worst-case recarve will result in all line
card Tofab and Fr Fab packet memories getting recarved, resulting in a router-wide traffic drop.
Recarving in
card Tofab and Fr Fab packet memories getting recarved, resulting in a router-wide traffic drop.
Recarving in
is acceptable to ensure the biggest packet flowing through the router does not get
dropped. Similarly,
is acceptable because it ensures that the biggest packets on the ingress side
do not get dropped. However,
and
can be disabled using the
command.