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MPLS QoS Priority

Unanswered Question
Aug 28th, 2006
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Hi everybody,


I've seen guys saying that ICMP has low priority on MPLS Domain and normaly i can lost some packet when I ping some router on mpls domain, but I'd like to know when it is true, because I did mpls course and I didnt see nothing about that. ICMP has low priority just when I pratice MPLS TE on my network or it is include on mpls's struture.

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Overall Rating: 4 (2 ratings)
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mheusinger Tue, 08/29/2006 - 00:52
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Hi,


MPLS and QoS are two different things. In fact implementing MPLS does not change the QoS settings on router interfaces. So whether ICMP is low priority or high priority is solely defined by the QoS policy implemented by the ISP.


From a technical point of view, a LSR will not even know it is transporting ICMP, because this is "hidden" behind some labels used for forwarding.


MPLS TE also does not change anything here, because it is "only" a more complex and tunable way of setting up label switched pathes. From an IP point of view MPLS TE is a modification to the destination based forwarding by including more requirements for path selection. So MPLS TE is a control plane feature.

QoS is something different, as it is basically a data plane feature (assuming DiffServ and setting DiffServ aware MPLS TE aside).


Hope this helps! Please rate all posts.


Regards, Martin

santos.alberto Tue, 08/29/2006 - 14:14
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hi there,


I understood that mpls and qos are two differents ways and if I Implement MPLS without QoS policy, ICMP is gonna have same priority that http's traffic, am i right?

cuz my ISP is still telling me that I can't use ping to make a baisc troubleshoting, cuz its normaly lost packet in mpls domain, I asked him about qos policy on ISP's network and he didnt know anything about that.

swaroop.potdar Tue, 08/29/2006 - 14:50
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Hi Santos,


What you heard is partially true.


1) ICMP has low processing priority and also as you ISP said should not be used for troubleshooting. Now a days you have inbuilt Control Plane Policing where you can do police or completely drop ICMP packets which are not from known destinations. But again this behaviour has nothing to do with MPLS in specific. Its general.


Also you may like to go through this link for academic purpose.

http://www.cisco.com/en/US/products/sw/iosswrel/ps1831/products_tech_note09186a00800a6057.shtml#performance



2) As far as MPLS domain goes, If you ISP hasnt done any ratelimiting or ICMP discard at the edge then the packets shouldnt be lost or discarded in the Domain. Because whatever happens happends at the edge. As the devices in the CORE dont know whether the CE packet is a ICMP or HTTP or any other application packet.


HTH-Cheers,

Swaroop

santos.alberto Tue, 08/29/2006 - 15:29
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I got it, if I tuning my PE with qos, ratelimit, its gonna be droped independent that is a mpls domain or whatever, but I was confused about relationship between mpls and icmp, but they told me about the packets loss but he didnt tell me why. yeah I know that ping is basic troubleshooting, but normaly, its the first step that you do., thabk all guys here for help

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