Cisco 7604 Configuration Manual page 533

Chapter 36 Configuring Denial of Service Protection
ICMP Unreachable (Unicast Only)
In an ICMP unreachable attack, a device is flooded with a large number of packets that contain a
destination address that is unreachable from the flooded device (in this case, the MSFC). The ICMP
unreachable rate limiter allows you to rate limit the packets that are sent to the MSFC containing
unreachable addresses.
This example shows how to rate limit the packets that are sent to the MSFC because of an ACL drop to
10000 pps and a burst of 100:
Router(config)# mls rate-limit unicast ip icmp unreachable acl-drop 10000 100
This example shows how to rate limit the packets that require generation of ICMP-unreachable messages
because of a FIB miss to 80000 pps and burst to 70:
Router(config)# mls rate-limit unicast ip icmp unreachable no-route 80000 70
The four rate limiters, ICMP unreachable no route, ICMP unreachable ACL drop, IP errors, and IP RPF
failure, share a single rate-limiter register. If any of these limiters are enabled, all of the limiters in this
group will share the same value and sometimes the same state (for example, ON/ON/ON). When
verifying the rate limiters, if the members of this register are enabled through another feature, an
ON-Sharing status (instead of an ON status) is displayed. The exception is the TTL failure rate limiter:
its value shares the same value as the other members in the register if you have manually enabled the
feature.
FIB (CEF) Receive Cases (Unicast Only)
The FIB receive rate limiter provides the capability to rate limit all packets that contain the MSFC IP
address as the destination address. The rate limiters do not discriminate between good frames and bad
frames.
Do not enable the FIB receive rate limiter if you are using CoPP. The FIB receive rate limiter overrides
Note
the CoPP policies.
This example shows how to rate limit the traffic to 25000 pps with a burst of 60:
Router(config)# mls rate-limit unicast cef receive 25000 60
FIB Glean (Unicast Only)
The FIB glean rate limiter does not limit ARP traffic, but provides the capability to rate limit traffic that
requires address resolution (ARP) and requires that it be sent to the MSFC. This situation occurs when
traffic enters a port and contains the destination of a host on a subnet that is locally connected to the
MSFC, but no ARP entry exists for that destination host. In this case, because the MAC address of the
destination host will not be answered by any host on the directly connected subnet that is unknown, the
"glean" adjacency is hit and the traffic is sent directly to the MSFC for ARP resolution. This rate limiter
limits the possibility of an attacker overloading the CPU with such ARP requests.
This example shows how to rate limit the rate at which this traffic is sent to the MSFC to 20000 pps and
a burst of 60:
Router(config)# mls rate-limit unicast cef glean 20000 60
OL-4266-08
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
Understanding How DoS Protection Works
36-17