From: Scott Morris (smorris@internetworkexpert.com)
Date: Fri Oct 24 2008 - 13:20:16 ARST
Got it. The difference is whether it can go someplace else or not (short
answer).
A loopback is a terminating interface. Therefore the router KNOWS it has to
be the one to process everything and wants a pure IP packets. It will send
it to the next hop as untagged, and then R3 should see it as Pop Tag.
On the other hand, the serial link (or any other "real" interface) is
non-terminating. There MAY be other devices there, and even if there are no
MPLS peers, the logic is that there COULD be in the future, so it wants to
be the end of the MPLS stack (in case it's a VRF interface) but keep the
thinking attached. So it will send this to a neighbor as Pop Tag, and R2
would send it to R3 with some numerical label in it.
HTH,
Scott Morris, CCIE4 #4713, JNCIE-M #153, JNCIS-ER, CISSP, et al.
CCSI/JNCI-M/JNCI-ER
Senior CCIE Instructor
smorris@internetworkexpert.com
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PS. Sorry for the delay. Been crazy busy. :)
_____
From: Ovais Iqbal [mailto:ovais.iqball@yahoo.com]
Sent: Thursday, October 16, 2008 11:23 PM
To: smorris@internetworkexpert.com
Cc: ccielab@groupstudy.com
Subject: RE: MPLS Query !
Dear Scott, thanks for the feedback, i have attatched the diagram so that
its easy for all to understand.
My configuration is very basic.
1) Assigned the IPs on the interfaces shown
2) Run OSPF on all the interface shown
3) Run mpls ip on
R1 s0, R2 s0
R2 s1, R3 s1
Now when i am doing show mpls forwarding-table on R2, i am seeing this
R2#sh mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
32 Untagged 10.0.0.1/32 0 Se1/0 point2point
33 Pop tag 100.0.0.0/8 0 Se1/0 point2point
R2#
So this is my confusion, i am getting two different outgoing tags for
interfaces with same configuration ( except that 1 is loopback the other is
serial).
Pls keep in mind that R1 se1/1 is NOT connected to any router, i have used
no keepalive to turn its status to up, i can also ping it from R2 and R3.
Kindly clear my confusion. if you require any further show results pls tell
me.
Thanks in advance
--- On Fri, 10/17/08, Scott Morris <smorris@internetworkexpert.com> wrote:
From: Scott Morris <smorris@internetworkexpert.com>
Subject: RE: MPLS Query !
To: "'Ovais Iqbal'" <ovais.iqball@yahoo.com>, ccielab@groupstudy.com
Date: Friday, October 17, 2008, 1:59 AM
I'd consider that it's either a peer that you are not exchanging LDP
with
(got the route but no associated tag) or perhaps you are exchanging LDP with
the neighbor but just not for that FEC.
What does your configuration of the various routers look like?
Scott
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Ovais Iqbal
Sent: Thursday, October 16, 2008 11:31 AM
To: ccielab@groupstudy.com
Subject: MPLS Query !
hi all, how are you, i just have a simple MPLS query see i have 3 routers
connected serially to each other.
on R1 what i did, i made a loopback, a serial interface serial 1/0 with
no
keepalive to take it to up state and another serial link serial 1/2
connected to R2.
I just enabled mpls ip on serial connected to R2 and ran ospf on all
links.This is wat i got when i ran show mpls forwarding-table on R1
for loopback my outgoing label or vc was UNTAGGED !!!!!
for my serial (no keepalive) it was POP TAG !!! Why is that so ??
where will i see untagged ?? i heard someone saying that we will see
untagged on interface that are not running mpls, but i m not running mpls on
serial1/0 then why am i seeing pop tag on serial 1/0 ?? shouldnt it be
UNTAGGED ?
Kindly clear my confusion ? pls explain me when i should be seeing untagged
and when pop tag, i am a totally new bie so pls explain Thanks
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