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Community Member

no label (untagged)

just need a comment on following understanding about 'No Label'

no-label is found in LFIB when the local-LSR has not received a label for a prefix from its downstream LSR and the LSR simply forwards the unlabeled packet using the LFIB without having to perform lookup in FIB...

5 REPLIES
Community Member

Re: no label (untagged)

Hi,

You will see "No label" for all prefixes received from your CE routers. The PE router's LFIB should have outgoing label as "No label" for CE prefixes because it had advertised "Implicit-Null" to its upstream routers for those prefixes. You should never see "No label" for any prefixes within the Service Provider network.

HTH.

Regards,

Amit.

Community Member

Re: no label (untagged)

Hi Amit,

thank you for ur reply...

My question was actually about the lookups the PE(ELSR) has to perform for prefixes received from CE. You are right that the PE will have 'no-Label' in its LFIB for all CE routes but will have to perform single lookup or double lookup....

Secondly following is an extract from your comments:

The PE router's LFIB should have outgoing label as "No label" for CE prefixes because it had advertised "Implicit-Null" to its upstream routers for those prefixes


consider following scenario :

100.100.100.100=CE1-PE1-P-PE2-CE2

LFIB on PE1 :

PE1#sh mpls for
Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop
tag    tag or VC   or Tunnel Id      switched   interface
16     16          10.10.10.11/32    0          Se0/2      point2point
17     Pop tag     5.5.5.0/30        0          Se0/2      point2point
20     Untagged    100.100.100.100/32[V]   \
                                     0          Se0/1      point2point

LFIB on PE2:

PE2#sh mpls for
Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop
tag    tag or VC   or Tunnel Id      switched   interface
16     17          10.10.10.10/32    0          Se0/0      point2point
17     Pop tag     4.4.4.0/30        0          Se0/0      point2point
18     Aggregate   2.2.2.0/30[V]     520
19     Aggregate   12.12.12.12/32[V] 0

100.100.100.100/32 is not visible in PE2 LFIB table, therez no implicit-null for 100.100.100.100

Community Member

Re: no label (untagged)

Hi,

If you are using MPLS VPN, the PE routers will not send Implicit-Null labels to the P/PE-routers for CE prefixes; all MPLS-enabled routers send Implicit-Null label to upstream routers for the prefixes connected to itself. The CE prefixes and labels are advertised to remote PE routers using MP-BGP. In this case, the PE router attached to the CE-router will not advertise Implicit-Null label to the remote PE-router; it will use a specific label. No CE prefixes are advertised inside the service provider network in MPLS VPN.

If you do show ip cef vrf , you will see labels imposed for CE prefixes by PE routers.

HTH.

Amit.

Hall of Fame Super Silver

Re: no label (untagged)

Hello Mukarram,

in an MPLS network we have faculty of advertising labels only for specific IP prefixes in the IP routing table.

This is done for different reasons including for scalability because the label space is in anycase limited although big.

see the following example taken from an IOS XR router where a label filter is used:

sh mpls ldp binding 10.99.87.0 255.255.255.0
Fri Feb 26 10:24:20.966 MET MEST
10.99.87.0/24, rev 11664
        Local binding: label: 16008
        Remote bindings:
            LSR: 10.80.0.26:0, label: 16003
            LSR: 10.80.0.163:0, label: 16082

this prefix is not allowed by prefix-list on next-hop devices

sh ip route 10.99.87.0
Fri Feb 26 10:24:37.301 MET MEST

Routing entry for 10.99.87.0/24
  Known via "isis core", distance 115, metric 50320, type level-2
  Installed Feb 26 09:46:37.852 for 00:37:59
  Routing Descriptor Blocks
    10.82.3.14, from 10.82.138.5, via TenGigE0/0/0/0
      Route metric is 50320
    10.82.3.18, from 10.82.138.5, via TenGigE0/1/0/0
      Route metric is 50320

the end result is unlabelled

sh mpls forw | inc 10.99.87.0              
Fri Feb 26 10:27:23.337 MET MEST
16008  Unlabelled  10.99.87.0/24      Te0/0/0/0    10.82.3.14      4029177    
       Unlabelled  10.99.87.0/24      Te0/1/0/0    10.82.3.18      772021

upstream devices have the following commands:

sh run | inc advertise
no tag-switching advertise-tags
tag-switching advertise-tags for PREFIX-LDP-out

sh ip access-lists PREFIX-LDP-out
Standard IP access list PREFIX-LDP-out
    10 permit 10.80.0.0, wildcard bits 0.0.255.255 (274114 matches)

so for that prefix 10.99.87.0/24 the initial node does not receive labels on best paths to destination and it will use IP CEF and not MPLS to send traffic towards it:

sh cef 10.99.87.0 255.255.255.0 det
Fri Feb 26 10:32:07.622 MET MEST
10.99.87.0/24, version 0, internal 0x40040001 (0x9d9279e8) [1], 0x0 (0x9cd92f30), 0x4400 (0x9d9791c0) 
Updated Feb 26 09:46:37.672
local adjacency 10.82.3.14
Prefix Len 24, traffic index 0, precedence routine (0)
  gateway array (0x9cc56d6c) reference count 1386, flags 0x400d00, source lsd (2),
                [1387 type 4 flags 0x4101001 (0x9d60d848) ext 0x0 (0x0)]
  LW-LDI[type=1, refc=1, ptr=0x9cd92f30, sh-ldi=0x9d60d848]
   via 10.82.3.14, TenGigE0/0/0/0, 15 dependencies, weight 0, class 0
    next hop 10.82.3.14
    local adjacency
     local label 16008      labels imposed {None}
   via 10.82.3.18, TenGigE0/1/0/0, 15 dependencies, weight 0, class 0
    next hop 10.82.3.18
    local adjacency
     local label 16008      labels imposed {None}

Hope to help

Giuseppe

Community Member

Re: no label (untagged)

Hello Guys,

 

LDP is up but There are no Labels for outgoing interface, could you please help me what could be the issue,

R1#sho mpls forwarding-table
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop
Label      Label      or Tunnel Id     Switched      interface
18         No Label   123.5.5.5/32     0             Et0/2      123.10.1.6
19         No Label   123.4.4.4/32     0             Et0/1      123.10.1.2
20         No Label   123.2.2.2/32     0             Et0/1      123.10.1.2
21         No Label   123.10.1.28/30   0             Et0/2      123.10.1.6

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