Hi Experts,,
Recently I was asked to plan and deploy CoS for of our customer, I spent a complete weekend for initial testing of design on my LAB network, as it was my first experience to CoS so it took lot of time and efforts in order to attain desired results. I want to share few important things to kept in mind for implementing CoS especially guys who are new to CoS like me..
1. Junos does not classify egress traffic initiated by itself and ingress traffic destined to it at least on EX 4200 switch
2. It only apply classifier to transit traffic
3. If we want that CoS information must be retained at upper or core layer applied earlier by Edeg switch or lower layer then at edge switch we must have to apply rewrite rule either default or customized
4. One important thing must be kept in mind that if we apply customized classifier and default rewrite rule then code point must be matched at each stage
5. While planning bandwidth limit or burst size in traffic policer keep in mind that 1000 (not 1024) is being used as multiplier for k , m or g values
6. If we want to ensure that CoS information are being applied exactly as per our desire to data packets we can verify it monitoring traffic at egrees and ingress interfaces of edge and Core layer switch respectively by using monitor traffic interface ge-0/0/* extensive command and find ToS value (in Hex) in appropiate packet captured then ToS value can be mapped to CoS value by using following link http://www.tucny.com/Home/dscp-tos
7. Capture taken at ingress interface of upper or core switch showing tos0xb8 valve which is equivalent to CoS (01110 , 0x2e or ef mean packet traffic is classified as expedited-forwarding class at ingress interface of upper layer or core switch , as at egress interface of lower layer or edge switch rewrite rule was applied to achieve the result.
00:1e:ec:69:c0:87 > 3c:8a:b0:1e:bb:01, ethertype IPv4 (0x0800), length 74: (tos 0xb8, ttl 128, id 10746, offset 0, flags [none], proto: ICMP (1), length: 60) 192.168.1.3 > 192.168.1.1: ICMP echo request, id 1, seq 8404, length 40
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Kashif Nawaz JNCIP-Sec , JNCIS-Ent, JNCIS-Sec, JNCIA-Junos