Dell PowerConnect M6220, User Configuration Manual

Page: 961 / 1294

Configuring OSPF and OSPFv3
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LSA Pacing
OSPF refreshes each self-originated LSA every 30 minutes. Because a router
tends to originate many LSAs at the same time, either at startup or when
adjacencies are formed or when routes are first learned, LSA refreshes tend to
be grouped. Further, Area Border Routers (ABRs) attached to the same area
tend to originate summary LSAs into the area at the same time. This behavior
leads to periodic bursts of LS Update packets. Update bursts can lead to high
CPU utilization, packet loss, and retransmission, if a receiver cannot absorb
all packets in a burst. These losses occur primarily in two places: 1) at the
Class of Service (CoS) queue where the hardware queues packets to the CPU,
and 2) when a message buffer is allocated for an incoming packet.
This feature makes changes to OSPFv2 to improve the efficiency of LSA
flooding, with the expectation that the improvements will greatly reduce or
eliminate the packet drops caused by bursts in OSPF control packets. The
changes are as follows:
• Introduce LSA transmit pacing, limiting the rate of LS Update packets
that OSPF can send
• Introduce LSA refresh groups, so that OSPF efficiently bundles LSAs into
LS Update packets when periodically refreshing self-originated LSAs
To configure LSA transmit pacing, use the timers pacing flood command in
router config mode:
ABR-R0(config)#router ospf
ABR-R0(config-router)#timers pacing flood 50
This will cause LSA Update packets to be sent at no less than a 50 millisecond
interval.
When OSPF refreshes LSAs, it considers all self-originated LSAs whose age is
from 1800 to 1800 plus the pacing group size. Grouping LSAs for refresh
allows OSPF to combine refreshed LSAs into a minimal number of LS
Update packets. Minimizing the number of Update packets makes LSA
distribution more efficient. To configure an LSA Refresh window, use the
timers pacing lsa-group command in router-config mode:
ABR-R0(config)#router ospf
ABR-R0(config-router)#timers pacing lsa-group 300
This sets the LSA Refresh window to 2100 seconds or about 35 minutes.