MPLS QoS – TOS Reflection

MPLS QoS – TOS Reflection

The default behavior for an ingress LSR is to copy the first 3 bits of the DSCP value to the EXP header. This is known as TOS Reflection. Here we take a look at how to verify and test this behavior. In future posts, we will look more in depth and the different models that exists for MPLS QoS.

CE1 has the following configuration:

access-list 1 permit 192.168.0.1
!
class-map match-all LOOP0
match access-group 1
!
policy-map MARK
class LOOP0
set ip precedence 5
!
interface Serial1/0
 service-policy output MARK

PE1 does not have any QoS Applied, but we can see through “debug mpls packets” that it will indeed copy the Precedence value of 5 into both MPLS labels that get applied to packets sent from CE1′s loopback to CE2:

CE1#ping 192.168.0.2 source lo 0 re 1
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 192.168.0.2, timeout is 2 seconds:
Packet sent with a source address of 192.168.0.1
!
Success rate is 100 percent (1/1), round-trip min/avg/max = 120/120/120 ms
CE1#
P#
*Mar  1 00:18:52.679: MPLS: Se1/1: recvd: CoS=5, TTL=254, Label(s)=17/19
*Mar  1 00:18:52.683: MPLS: Se1/0: xmit: CoS=5, TTL=253, Label(s)=19
*Mar  1 00:18:52.755: MPLS: Se1/0: recvd: CoS=5, TTL=254, Label(s)=18/20
*Mar  1 00:18:52.755: MPLS: Se1/1: xmit: CoS=5, TTL=253, Label(s)=20

We can also verify the EXP value with the traceroute command:

CE1#trace 192.168.0.2 sou lo 0
Type escape sequence to abort.
Tracing the route to 192.168.0.2
1 200.0.0.2 32 msec 88 msec 8 msec
2 200.0.0.6 [MPLS: Labels 17/19 Exp 5] 56 msec 24 msec 28 msec
3 200.0.0.13 [AS 100] [MPLS: Label 19 Exp 5] 24 msec 32 msec 16 msec
4 200.0.0.14 [AS 100] 68 msec *  132 msec
CE1#

Regards, Bryan (Post by Bryan Bartik)

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Tags: ccie lab, CCIE R&S 4.0 Lab, CCIE R&S Lab, MPLS, QoS