RE: Traceroute issue - MPLS CE router replys 2 hops on ingress

From: Jerry Wang (jerry-wang@rogers.com)
Date: Wed Aug 31 2005 - 17:02:26 GMT-3

The CE1 ingress and egress interfaces have different IP addresses. But unfortuately I do not have access to the CE & PE routers, although this is a lab environment.
 
But you might be right - the "CE1 egress_int" address could be the IP address of the PE. It might be a simple reverse of IP addresses between the CE1 and PE1 link.
 
But I still wonder is it possible, by any chance, that a single router will reflect 2 traceroute hops on its ingress and egress interfaces?
 
Thanks for your help!

Abbas Elzein <Abbas.Elzein@telus.com> wrote:
Are you seeing the same IP address on both the CE1 ingress and egress interfaces?? if not.. the "CE1 egress_int" that address could be the IP address of the PE receiving the your IP packets before they get labelled and sent through the MPLS cloud..... Just guessing...
 
AE

---------------------------------
From: Jerry Wang [mailto:jerry-wang@rogers.com]
Sent: Wednesday, August 31, 2005 1:53 PM
To: Abbas Elzein; ccielab@groupstudy.com
Subject: RE: Traceroute issue - MPLS CE router replys 2 hops on ingress & egress interfaces

Thanks Abbas,
 
Yes I agree the TTL propagation must be disabled on the PEs, this explains why the traceroute output does not reflect PEs. But I still do not understand why CE1 counts 2 hops for traceroute? See the 1st and 2nd hops, can you please explain how this happens?
 
1. CE1_ingress_int
2. CE1_egress_int
3. CE2_ingress_int
4. R2_ingress_int
 
Thanks and best regards
 
Jerry

Abbas Elzein <Abbas.Elzein@telus.com> wrote:
Most likely the TTL propagation is disabled on the PEs.. Disabling TTL propagation allows the structure of the MPLS network to be hidden from customers, but not the provider.

-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Jerry Wang
Sent: Wednesday, August 31, 2005 12:16 PM
To: ccielab@groupstudy.com
Subject: Traceroute issue - MPLS CE router replys 2 hops on ingress & egress interfaces

Hi Guys,

Please help to explain the following traceroute issue I found in my lab:

I have 2 Routers R1 & R2 on two sites, both are connecting to MPLS/VPN cloud. When I do traceroute from R1 to R2, I found the first 2 hops reflect the ingress and egress interfaces on the same CE1 router, which is connecting to R1. Please see following details:

Traceroute path (on R1, do traceroute to R2's loopback address):

R1 -- CE1 -- PE1 -- PE2 -- CE2 -- R2

Tracing output:

1. CE1_ingress_int
2. CE1_egress_int
3. CE2_ingress_int
4. R2_ingress_int

FYI: the /30 link route between CE1 & PE1 is not advertised to R1. not sure if this affects or not.

I have no access to to the MPLS CEs & PEs , I assume it might be related to CE1's MPLS traceroute configuration. Or it might be related to the ttl propagation between the IP & MPLS domain?

Your help is highly appreciated!

Thanks and best regards

Juntao
Network Engineer

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