Cisco Cisco 1900 2900 3900 Series 4-Port Gigabit Ethernet HS-WIC with PoE
© 2015 Cisco and/or its affiliates. All rights reserved. This document is Cisco/Verizon Public Information.
Page 26 of 26
Frequenty Asked Questions
Q) If encryption is enabled (GETVPN, DMVPN, etc.) with QoS, QoS reorders packets. Can packets be dropped by
the anti-replay algorithm in the crypto function?
A) While this is possible, it is not seen in most deployments. The anti-replay window (if enabled) defaults to a 64
packet sliding window, which is normally wide enough to accommodate this packet reordering. The packets most
likely to be affected are those in the default/best-effort class(es). In the unlikely case this is suspected, the
following 2 documents discuss the issue and how to expand the anti-replay sliding window.
www.cisco.com/c/en/us/support/docs/ip/internet-key-exchange-ike/116858-problem-replay-00.html
www.cisco.com/c/en/us/td/docs/ios/sec_secure_connectivity/configuration/guide/convert/sec_ipsec_data_plane_15_1_book/sec_ipsec_antireplay.html#wp1051449
Q) Why is a site survey so important for LTE QoS?
A) Site surveys are important for all LTE deployments. The radio signal reception strength and quality have a
significant impact on the modulation chosen between the LTE modem and network, which directly impacts the
throughput. Site surveys are available as a professional service, or can be done using a smart phone with texting,
using the no-charge ISR app referenced under “LTE QoS Planning Guidelines” above.
Q) Can I use both precedence and DSCP values is QoS policies?
A) Yes. Precedence only takes into consideration the 3 most significant bits in the ToS byte. DSCP takes into
consideration the 6 most significant bits. If a class map matches precedence 3 for example, it would match any IP
packet with precedence 3, CS3, AF31, AF32, AF33. This can be useful for example in the 2
nd
level policy, making
the configuration shorter. Similarly, precedence 5 would match on both DSCP CS5 and EF. DSCP is used to
provide more granular classification, as is used the above example for the 3
rd
level policies.
Printed in USA
C07-720264-00 01/13