This recipe describes how to use SNMP to determine the traffic offered and drop rate for a QoS policy configured on a Cisco router.
The steps here describe which OIDs to interrogate in the CISCO-CLASS-BASED-QOS-MIB
STEP 1:
Get the SNMP ifIndex for the interface where the QoS policy is attached:
CiscoRouter#sh snmp mib ifmib ifindex | i GigabitEthernet0/1
GigabitEthernet0/1: Ifindex = 22
(or by using SNMP):
root@Linux# snmpwalk -v2c -c communityname 10.0.200.12 1.3.6.1.2.1.2.2 | grep -i GigabitEthernet0/1
RFC1213-MIB::ifDescr.22 = STRING: “GigabitEthernet0/1” Read more…
The TCPDUMP filter to match a DSCP is a little tricky to construct. I need to explain how the filter works and also what TCPDUMP sees in the ToS Byte of the IP header and then how to filter on just the DiffServ extensions.
The IP ToS Byte (DSCP and IP ECN)
FIRST THE ANSWER:
(in case it is not important to understand how the filter is constructed)
– an example filtering on DSCP decimal 20 (AF22):
tcpdump -i eth0 (ip and (ip[1] & 0xfc) >> 2 == 20) -vvv
– An example filtering on DSCP Hex 14 (also decimal 20 or AF22):
tcpdump -i eth0 (ip and (ip[1] & 0xfc) >> 2 == 0x14) –vvv
Read more…
RFC 2597 defines four Assured Forwarding groups, denoted by the letters “AF” followed by two digits: Read more…
Traffic Shaping involves in queueing traffic rather than dropping it. Traffic Shaping terminology is: Read more…
This table compares QoS values for IP precedence, Differentiated Services Code Point (DSCP) and Multiprotocol Label Switching (MPLS) Experimental (EXP) values along with standard values used in Cisco IOS Software for configuration. Read more…
To understand how to interpret the show policy-map interface command, you first need to understand the concept of congestion.Conceptually, congestion is defined by the Cisco IOS software configuration guide as: “During periods of transmit congestion at the outgoing interface, packets arrive faster than the interface can send them.” Read more…
Planning for Voice over IP requires an understanding of the various headers added when transporting packetised voice, espcially over an IPSec VPN: Read more…
1P1Q3T – ingress queuing
As switch platforms have architectures based on oversubscription, they have been engineered to guarantee QoS by protecting critical traffic trying to access the backplane/stack-ring via ingress queuing. Read more…
The Cisco 2960, 2970, 3560 and 3750 Catalyst switches support a 1p3q3t que model which provides 3 thresholds in which to drop traffic. Read more…
Deciding upon a QoS Classification and Marking strategy can be a difficult task. Cisco have provided certain recommendations which may be implemented as a baseline QoS strategy and then altered over time: Read more…