Label tracking...

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Mar 20th, 2008
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Hi


We have probes running on our links between our P-Node and PE routers. These probes are supposed to be picking up the different VRFs. I may be incorrect, but with MPLS, tagging of packets take place at every hop to identify the VRF it belongs to; and then decapsulation takes place? To my knowledge, when decoding the packets; i have come across that the probe looks at the inner most label (source label) and builds a table of all labels transversing the network (insane!). The problem with this is that labels are dynamically and randomly assigned by routers.

Anyway, the probe does seem to pick up the different VRFs but seem to have a problem picking up VRFs relating to traffic encapsulated by GTP.

The amount of traffic on the link is disproportional to the amount being captured/monitored in the different VRFs.


Could it be a label problem? Are labels assigned to the header of a gtp packet or within?


Not looking for possible solutions; just ideas...


thanks.

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Jon Marshall Thu, 03/20/2008 - 07:32
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Hi


"I may be incorrect, but with MPLS, tagging of packets take place at every hop to identify the VRF it belongs to; and then decapsulation takes place?"


I don't believe it does do this. A VPN label is added to the packet and then a further label which is used to label switch the packet from the ingress PE to the egress PE is applied.


While the packet is being switched by the P routers only the top label is used. The VPN label is only used when the packet arrives at the egress PE.


Hope i haven't misunderstood your question


Jon

cisco_trouble Wed, 04/02/2008 - 07:06
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Thanks,


Yes, you understood my question. I just need clarification. For e.g. 5 routers, five labels to a packet; the most significant label wouldbe the top label; which will be removed; and so forth until it reaches its destination where the VPN label is then read?

Jon Marshall Wed, 04/02/2008 - 07:25
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5 routers does not equal 5 labels.


In an MPLS VPN network there will typically be 2 labels on each packet. So


H1 -> CE1 -> PE1 -> P1 -> P2 -> P3 -> PE2 -> CE2 -> H2


H1 and H2 are in the same MPLS VPN.


H1 sends an IP packet to H2. PE1 puts 2 labels on the packet. The first label it attaches is for the VPN. This label is not read or changed until the packet reaches PE2.


The second label is the label used to get the packet to PE2. So PE1 attaches a label to get to P1 and the packet is label switched to P1. P1 receives the packet, removes the top label only and adds a new label to get the packet to P2. P2 does same.


When packet arrives at PE2, PE2 removes the top label and then the second label which is the VPN label and then forwards the packet to CE2.


Just to be correct PE2 would probably not have to remove the topmost label due to PHP (Penultimate Pop Hopping).


Jon

cisco_trouble Thu, 04/03/2008 - 00:03
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At any point from H1 to H2; 2 labels would be attached to the packet. A probe attached between the P-nodes e.g. P1 and P2, would pick up the exact labels if it were attached between P2 and P3.


Thanks Jon, you clarified things for me.



cisco_trouble Thu, 04/03/2008 - 00:05
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At any point from H1 to H2; 2 labels would be attached to the packet. A probe attached between the P-nodes e.g. P1 and P2, would pick up the exact labels if it were attached between P2 and P3.


Thanks Jon, you clarified things for me.



shivlu jain Thu, 03/27/2008 - 09:05
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dear always remeber one thing that we are having 2 types of labels tags on the packet one is IGP label and another is VPN label and when the packet is processed form the PE to P it always look for the top label which is of IGP label and when it reaches to the second last router in the network it performs the PHP and remove the IGP label and only send the packet with vpn label, Once the packet is receved by that PE the vpn label is removed and the packet is enetered in the vrf table and forwarded to CE as ip packet.


regards

shivlu.

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