From: HP-France,ex2 ("SANCHEZ-MONGE,ANTONIO)
Date: Wed Apr 21 2004 - 18:34:22 GMT-3
Hi Ken,
---> what command (show/debug) can I use to determine if traffic leaving
this interface is marked?
#show policy interface
[...]
QoS Set
ip dscp af21
Packets marked 12
---> Also, if I want to mark all ICMP traffic generated by the router with a
particular DSCP value, how can I do this?
You can:
1) Use a policy map on all the outgoing interfaces. If you match with an
access list traffic will be marked. If you match with NBAR, the traffic will
NOT be marked since NBAR works with CEF and locally originated traffic is
process switched.
2) With the "ip local-policy" redirect ICMP traffic to the loopback
interface and use a policy map to mark there.
---> Is it only appropriate to use a single implementation (legacy TOS vs.
DSCP) throughout your entire domain?
You can implement the policy the way you want. There are some
recommendations in RFCs (sorry I'm too lazy to check the numbers now) that
advise to use the first 3 bits of DSCP as precedence and the next 2 as drop
probability. The RFCs also define EF and CS# for highest priority and legacy
precedence, respectively. I said "you can implement the policy the way you
want", but some Cisco mechanisms like WFQ and WRED follow by default the
interpretation suggested by the RFCs, so you just need to beware of that.
Basically legacy precedence is fine unless you want more granularity and/or
playing with drop probability. IOS is quite flexible, maybe too ;)
Best luck in your exam, you'll get it for sure.
Cheers,
Ato.
-----Original Message-----
From: Kenneth Wygand [mailto:KWygand@customonline.com]
Sent: miircoles, 21 de abril de 2004 20:39
To: ccielab@groupstudy.com
Subject: Interface QoS statistics & More questions for the experts...
If I set DSCP on all telnet traffic out port FastEthernet0/0, what command
(show/debug) can I use to determine if traffic leaving this interface is
marked? I'm using an outbound service policy on the interface.
Also, if I want to mark all ICMP traffic generated by the router with a
particular DSCP value, how can I do this? I tried doing this with a
local-policy route-map, but I did not have the option of setting DSCP
through this method (I can set IP Precedence or IP TOS, but the TOS is "max
reliability/throughput, min delay/monetary-cost"). Are these TOS values
considered "legacy" as compared with DSCP.
This is one thing that confuses me. There are 8 bits in the IP header for
TOS. When flagging "max reliability/throughput, min delay/monetary-cost",
how does the receiving device know that this is not a DSCP value? There is
no "switch" that tells the receiving device which implementation is used, so
is there any correlation? I know DSCP is backwards compatible with IP
precedence due to the assignment of the first three bits in the DSCP value
as "priority". I don't understand the TOS/DSCP correlation though. Is it
only appropriate to use a single implementation (legacy TOS vs. DSCP)
throughout your entire domain?
Sorry for the conglomerate of questions here... just beginning the mental
breakdown before exam time on Monday... errr....
Kenneth E. Wygand
Systems Engineer, Project Services
CISSP #37102, CCNP, CCDP, ACSP, Cisco IPT Design Specialist, MCP, CNA,
Network+, A+
Custom Computer Specialists, Inc.
"I am not really smart. I just stick with problems longer." -Albert Einstein
Custom Computer Specialists, Inc.
"Celebrating 25 Years of Excellence"
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