In my knolwledge Implicit and Explicit Null is when LSR Penultimate will
send only packet IP, hence does not perform swap; instead it performs a
pop the label that is necessary to reduce the load on last hop router
LSR, then LER will strips off the label and forwards the final IP packet
using usual IP rules, forwarding toward cpe.
Implicit are used for prevent two lookups: altough Label lookup is more
simpler than IP lookup, neverthless two lookups Label plus IP are more
intensive than 1 lookup.
However implicit/explicit nulls are generated for prefixes in RIB
learned and connected/aggregate otherwise IP router will have table
incompleted.
For example:
R1#show ip route 10.10.10.0
Routing entry for 10.10.10.0/24
Known via "ospf 100", distance 110, metric 81, type intra area
Last update from 172.16.30.1 on Serial1/0, 00:09:46 ago
Routing Descriptor Blocks:
* 172.16.30.1, from 192.168.6.6, 00:01:45 ago, via Serial1/0
Route metric is 81, traffic share count is 1
r1#show mpls forwarding-table 10.10.10.0 detail
Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or Tunnel Id Switched interface
2093 No Label 10.10.10.0/24 0 Se1/0 out
point2point
MAC/Encaps=4/4, MRU=1504, Label Stack{}
FF030021
No output feature configured
At this time as usual LFIB come in game, bulding outgoing interface plus
l2 header express in code hex, knowing next hop and l2 header router
will forward ip packet through label lookup without decode ip header.
if I am not wrong 0x0021 should be IP packet; Outgoing interface
contain L2 header plus mpls label stack, in my case is empty,
NO label when outgoing interface MPLS won't enabled or LDP of next hop
router has not advertised any label for that prefix.
Pop label if some router has advertised some implicit/explicit label
for that prefix.
If the prefix is a some range summary outgoing will showed as a punt
interface (Null0)
The situation is a little complicated with label stack VPN: here there
are 2 label top label or igp label and inner label or vpn label
PHP remove top label and forward as just vpn label
However implicit-null are advertised whenever you have to perform an IP
lookup toward LER
At least This is my opinion.
On Sun, Apr 7, 2013, at 06:57 PM, Narendra Naukwal wrote:
> Yes, you are correct about the source of confusion :)
> I was gng through various sites in trying to prepare for the written
> exam.
> However, I am not sure if Implicit Null be advertised for any other
> prefix apart connected & summary.
>
> Thanks
>
> ________________________________
> From: Nicola Arnoldi <[email protected]>
> To: Narendra Naukwal <[email protected]>
> Cc: "[email protected]" <[email protected]>
> Sent: Sunday, 7 April 2013 9:41 PM
> Subject: Re: [OSL | CCIE_RS] MPLS - PHP
>
>
> http://cciebible.com/ccie-rs-350-001-qas-implement-mpls-layer-3-vpns-1-5/
>
>
>
>
> On Sun, Apr 7, 2013 at 5:00 PM, Nicola Arnoldi <[email protected]>
> wrote:
>
> Yes, re-reading your problem clearly shows that I was wrong, and your
> assumption is totally correct.
> >You advertise an implicit-null whenever you have to perform an IP lookup at
> >the E-LSR.
> >
> >
> >After a bit of Googling, I can guess your question comes from the various
> >exam dumps you can find on the Internet, which claim that the statement "PHP
> >is used only for directly connected subnets or aggregate routes" is wrong.
> >
> >
> >That's a good spot, however.
> >
> >
> >
> >
> >
> >On Sun, Apr 7, 2013 at 4:49 PM, Nicola Arnoldi <[email protected]>
> >wrote:
> >
> >try reading this
> >>http://blog.ioshints.info/2011/07/penultimate-hop-popping-php-demystified.html
> >>
> >>
> >>
> >>
> >>On Sun, Apr 7, 2013 at 4:15 PM, Narendra Naukwal <[email protected]>
> >>wrote:
> >>
> >>Hi,
> >>>
> >>>Thanks for looking into, however, I am not sure if i understand it
> >>>completly.
> >>>My question is For what routes will a PE advertise an IMPLICIT NULL label?
> >>>
> >>>I know it does advertises Implicit null (LDP value 3 or TDP Value 1) for
> >>>prefixes that need an IP lookup on the PE itslef.
> >>>The only route types that come to my mind are route 'Connected'on PE &
> >>>Routes 'Aggregated'for PE.
> >>>
> >>>By the way a router will generate a MPLS label for every prefix installed
> >>>in RIB via IGP. This I beleive has nothing to do with implicit null.
> >>>
> >>>For Example:
> >>>R3----mpls----R1---R2 (2.2.2.2)
> >>>| |
> >>>(3.3.3.3) (1.1.1.1)
> >>>
> >>>MPLS is not enabled on the link between R1 & R2 & OSPF is the configured
> >>>IGP. R1 learns about 2.2.2.2 via OSPF
> >>>
> >>>R1#sh ip route ospf
> >>> 2.0.0.0/32 is subnetted, 1 subnets
> >>>O 2.2.2.2 [110/11] via 12.0.0.2, 00:20:44, FastEthernet0/0
> >>> 3.0.0.0/32 is subnetted, 1 subnets
> >>>O 3.3.3.3 [110/11] via 13.0.0.3, 00:20:13, FastEthernet0/1
> >>>
> >>>Although MPLS is not enabled between R1 & R2, however, R1 still genarates
> >>>& advertises a label to R3 (100 in this case).
> >>>
> >>>R1#sh mpls forwarding-table
> >>>Local Outgoing Prefix Bytes tag Outgoing Next Hop
> >>>tag tag or VC or Tunnel Id switched interface
> >>>100 Untagged 2.2.2.2/32 0 Fa0/0 12.0.0.2
> >>>101 Pop tag 3.3.3.3/32 0 Fa0/1 13.0.0.3
> >>>
> >>>R1 advertises this to R3 & R3 uses 100 as otugoing label:
> >>>R3#sh mpls forwarding-table
> >>>Local Outgoing Prefix Bytes tag
> Outgoing Next Hop
> >>>tag tag or VC or Tunnel Id switched interface
> >>>300 100 2.2.2.2/32 0 Fa0/0 13.0.0.1
> >>>301 Pop tag 1.1.1.1/32 0 Fa0/0 13.0.0.1
> >>>
> >>>So the question remains, is there a scenario in IPv4 or VPNv4 when an EDGE
> >>>router will advertise an Implicit label aprt for connected & aggregated
> >>>routes?
> >>>
> >>>Thank You.
> >>>From: Nicola Arnoldi <[email protected]>
> >>>To: Narendra Naukwal <[email protected]>
> >>>Cc: "[email protected]" <[email protected]>
> >>>Sent: Sunday, 7 April 2013 7:10 PM
> >>>Subject: Re: [OSL | CCIE_RS] MPLS - PHP
> >>>
> >>>
> >>>
> >>>It is used as well for every route the Edge LSR learns from a PE to CE
> >>>routing protocol. No MPLS=PHP.
> >>>
> >>>
> >>>
> >>>On Sun, Apr 7, 2013 at 8:16 AM, Narendra Naukwal <[email protected]>
> >>>wrote:
> >>>
> >>>Experts,
> >>>>
> >>>>Is there a scenario wherein an Edge LSR signals an Implicit Null label to
> >>>>its directly connected Upstream neighbor for a subnet other which is not
> >>>>Connected or Summarized on the Edge LSR?
> >>>>
> >>>>Thank You.
> >>>>Naren
> >>>>_______________________________________________
> >>>>
> For more information regarding industry leading CCIE Lab training, please
> visit http://www.ipexpert.com/
> >>>>
> >>>>Are you a CCNP or CCIE and looking for a job? Check out
> >>>>http://www.platinumplacement.com/
> >>>>
> >>>>http://onlinestudylist.com/mailman/listinfo/ccie_rs
> >>>>
> >>>
> >>>
> >>>
> >>
> >
> _______________________________________________
> For more information regarding industry leading CCIE Lab training, please
> visit www.ipexpert.com
>
> Are you a CCNP or CCIE and looking for a job? Check out
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>
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