RE: IGMP on Cat 3550

From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Fri Aug 22 2003 - 14:20:45 GMT-3

Jonathan,

        This (badly worded) question is referring to WRED.

The following output is the default values when random-detection is
configured on an interface:

Router#sh queueing int e0/0
Interface Ethernet0/0 queueing strategy: random early detection (WRED)
    Exp-weight-constant: 9 (1/512)
    Mean queue depth: 0

    Class Random drop Tail drop Minimum Maximum
Mark
    (Prec) pkts/bytes pkts/bytes threshold threshold
probability
        0 0/0 0/0 20 40
1/10
        1 0/0 0/0 22 40
1/10
        2 0/0 0/0 24 40
1/10
        3 0/0 0/0 26 40
1/10
        4 0/0 0/0 28 40
1/10
        5 0/0 0/0 31 40
1/10
        6 0/0 0/0 33 40
1/10
        7 0/0 0/0 35 40
1/10
     rsvp 0/0 0/0 37 40
1/10

        WRED works as follows: When a packet arrives on the interface
running WRED, the average queue size for the packet's particular IP
precedence or DSCP is calculated. If the average queue size for this
class is lower than the 'Minimum threshold' value for that class, the
packet is sent. If the average queue size for the class is between the
'Minimum threshold' value and the 'Maximum threshold' value, the packet
is either sent or dropped based on the 'Mark probablility'. In the
default case, 1 out of 10 packets will be randomly dropped. If the
average queue size exceeds the 'Maximum threshold', the packet is
dropped.

        As you can see from the above values, as the IP precedence value
increases, as does the 'Minimum threshold'. This implies that packets
with a higher IP precedence value are less likely to be dropped, since
there must be consistently more of them in the queue over packets with
lower precedence values.

        Specifically for your question, we would have to assume that web
traffic is already marked with a higher precedence value than other
traffic. If not, web traffic would have to be manually marked with a
precedence value higher than other traffic. For normal user data, IP
prec 5 (critical) should be the highest value set. Precedence 6 and 7
should be reserved for actual network traffic (routing protocol updates,
keepalives, etc).

HTH,

Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com

Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-334-8987
Direct: 708-362-1418 (Outside the US and Canada)

-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Jonathan V Hays
Sent: Friday, August 22, 2003 10:38 AM
To: ccielab@groupstudy.com
Subject: IGMP on Cat 3550

I would appreciate any ideas on how to approach this QoS problem. 

---
VLAN 3--e0-R21-s0--F.R--s0-R22-e0-VLAN 103

Users on VLAN 3 (192.168.1.0) need uninterrupted access to a web-based
application on VLAN 103 (172.16.1.0). The R21-R22 link is experiencing
congestion on the frame relay circuit. Configure so that packets with
higher precedence are given preference and packets with lower precedence
are dropped when congestion is experienced.
---
There is no definition of what "higher precedence" or "lower precedence"
refers to. I guess you could do an access list for the web traffic. I'm
not sure what kind of QoS to apply here. I would think you'd want it
outbound on R21 and R22 but I'm not sure how to configure for generic
"higher" and "lower" precedence.

Please advise.

Jonathan

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