Re: Simple MPLS : Difference between "untagged" and "pop tag"

Hi Carlos,

I guess we are both right!!!!!!

I just tested it out on GNS3:

Example:
Just two neighbors, R1 f0/0 --- f0/0 R2
Very simple topology, connected by F0/0

R1 config:

interface FastEthernet0/0
 ip address 172.16.1.1 255.255.255.252
 duplex auto
 speed auto
 mpls ip
end

R1#show run int f0/1
Building configuration...

Current configuration : 93 bytes
!
interface FastEthernet0/1
 ip address 10.0.0.1 255.255.255.0
 duplex auto
 speed auto
end

R1#show run | sec ospf
router ospf 1
 log-adjacency-changes
 network 10.0.0.1 0.0.0.0 area 0
 network 172.16.1.1 0.0.0.0 area 0

------------------

R2 config:

interface FastEthernet0/0
 ip address 172.16.1.2 255.255.255.252
 duplex auto
 speed auto
 mpls ip
end

R2#show run | sec ospf
router ospf 1
 log-adjacency-changes
 network 172.16.1.2 0.0.0.0 area 0

-----------------------

IMPLICIT-NULL:

R2#show mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
16 *Pop tag* 10.0.0.0/24 0 Fa0/0 172.16.1.1

As expected!!!

EXPLICIT-NULL:

R2#show mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
16 *Untagged* 10.0.0.0/24 0 Fa0/0 172.16.1.1

--------------------

Tested on:
GNS3 router model: Cisco 2691
IOS: (C2691-ADVENTERPRISEK9-M), Version 12.4(5a)

It might show correct label in other IOS versions. this might be just some
kind of cosmetic bug.

Regards,

On Sun, Dec 26, 2010 at 8:22 PM, Carlos G Mendioroz <tron_at_huapi.ba.ar>wrote:

> Hmm, no.
> Explicit-null would actually use the null (0) tag on the last MPLS hop.
> Explicit-null will just show as 0, AFAIK.
>
> -Carlos
>
> George Goglidze wrote:
>
>> I can't check it right now, but I think "explicit-null" label would
>> indicate "Untagged" as well in the LFIB.
>> Please correct me if I'm wrong.
>>
>> Regards,
>>
>> On Sun, Dec 26, 2010 at 2:05 PM, Carlos G Mendioroz <tron_at_huapi.ba.ar<mailto:
>> tron_at_huapi.ba.ar>> wrote:
>>
>> Pop tag means that the destination (network) is connected to a LSR,
>> i.e., a router that this router knows that supports MPLS.
>> This may be because he is receiving LDP from it indicating
>> "implicit-null" as the tag for this prefix.
>>
>> Untagged means that the destination is known by some means (routing
>> protocol?) but the next hop is not MPLS aware (this router is not
>> receiving MPLS tag info for this prefix) so it will remove the stack
>> before forwarding.
>>
>> In your example, the 11.1.68.0/24 <http://11.1.68.0/24> is locally
>>
>> connected, and the "next hop" is this router itself.
>> The 11.1.6.6/32 <http://11.1.6.6/32> is connected to another router
>>
>> and there is no tag info
>> for this prefix. May be the loopback is a /24 ?
>>
>> -Carlos
>>
>>
>> Manouchehr Omari wrote:
>>
>> Pop tag means remove the top label and send the remaining payload.
>> Untagged means the outgoing interface is not MPLS enabled remove
>> the label
>> and send the packet without label.
>>
>> I recommend you to read "MPLS Fundamentals" which cover these
>> tips in
>> detail.
>>
>> HTH
>>
>>
>> On Sat, Dec 25, 2010 at 9:33 PM, Lefe Meedorf
>> <feeldefreedom_at_gmail.com <mailto:feeldefreedom_at_gmail.com>>wrote:
>>
>>
>> Hi,
>>
>> In my simple MPLS setup, I see following entires in "LFIB".
>>
>> R5#show mpls forwarding-table
>> Local Outgoing Prefix Bytes tag Outgoing
>> Next Hop
>> tag tag or VC or Tunnel Id switched interface
>> 16 Untagged 11.1.6.6/32 <http://11.1.6.6/32> 0
>> Se0/0 point2point
>> 17 Pop tag 11.1.68.0/24 <http://11.1.68.0/24> 0
>>
>> Se0/0 point2point
>>
>> What is the difference between "Untagged" and "Pop tag" ?
>> 11.1.6.6 is R6 (i.e. PE) Loopback 0.
>> 11.1.68.0/24 <http://11.1.68.0/24> is network between R6 and
>>
>> CE router.
>>
>> Topology
>> ---(R5)----(R6)----(CE)
>>
>> R6 Lo : 11.1.6.6/32 <http://11.1.6.6/32>
>> R6/CE : 11.1.68.0/24 <http://11.1.68.0/24>
>>
>>
>> --
>> /Lefe Meedorf/
>>
>>
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>>
>>
>>
>> -- Carlos G Mendioroz <tron_at_huapi.ba.ar <mailto:tron_at_huapi.ba.ar
>> >>
>>
>> LW7 EQI Argentina
>>
>>
>>
>> Blogs and organic groups at http://www.ccie.net
>>
>> _______________________________________________________________________
>> Subscription information may be found at:
>> http://www.groupstudy.com/list/CCIELab.html
>>
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>>
>>
>>
> --
> Carlos G Mendioroz <tron_at_huapi.ba.ar> LW7 EQI Argentina

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Received on Sun Dec 26 2010 - 22:49:33 ART