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The MPLS static feature enables you to statically assign local labels to an IPv4 prefix. Also, Label Switched Paths (LSPs) can be provisioned for these static labels by specifying the next-hop information that is required to forward the packets containing static label.
If there is any discrepancy between labels assigned statically and dynamically, the router issues a warning message in the console log. By means of this warning message, the discrepancy can be identified and resolved.
The advantages of static labels over dynamic labels are:
Improve security because the risk of receiving unwanted labels from peers (running a compromised MPLS dynamic labeling protocol) is reduced.
Gives users full control over defined LSPs.
Utilize system resources optimally because dynamic labeling is not processed.
Static labeling on IPv6 packets is supported.
The router does not prevent label discrepancy at the time of configuring static labels. Any generated discrepancy needs to be subsequently cleared.
Equal-cost multi-path routing (ECMP) is not supported.
Interfaces must be explicitly configured to handle traffic with static MPLS labels.
When paths of different technologies are resolved over ECMP, it results in heterogeneous ECMP, leading to severe network traffic issues. Don’t use ECMP for any combination of the following technologies:
LDP.
BGP-LU, including services over BGP-LU loopback peering or recursive services at Level-3.
VPNv4.
6PE and 6VPE.
EVPN.
Recursive static routing.
MPLS statistics is not supported.
By default, MPLS encapsulation is disabled on all interfaces. MPLS encapsulation has to be explicitly enabled on all ingress and egress MPLS interfaces through which the static MPLS labeled traffic travels.
Also, the dynamic label range needs to be defined. Any label that falls outside this dynamic range is available for manually allocating as static labels. The router does not verify statically-configured labels against the specified label range. Therefore, to prevent label discrepancy, ensure that you do not configure static MPLS labels that fall within the dynamic label range.
 Note |
For Cisco IOS XR software release 7.5.2 onwards, MPLS static supports 200G Ethernet. |
Define a dynamic label range, which in this task is set between 17000 and 18000.
Enable MPLS encapsulation on the required interface.
Setup a static MPLS LSP for a specific ingress label 24035.
Specify the forwarding information so that for packets that are received with the label, 24035, the MPLS protocol swaps labels and applies the label, 24036. After applying the new label, it forwards the packets to the next hop, 10.2.2.2, through the specified interface.
RP/0/RP0/CPU0:router(config)#mpls label range table 0 17000 18000
RP/0/RP0/CPU0:router(config)#commit
RP/0/RP0/CPU0:router(config)#mpls static
RP/0/RP0/CPU0:router(config-mpls-static)# interface HundredGigE 0/9/0/0
RP/0/RP0/CPU0:router(config-mpls-static)#address-family ipv4 unicast
RP/0/RP0/CPU0:router(config-mpls-static-af)#local-label 24035 allocate
RP/0/RP0/CPU0:router(config-mpls-static-af-lbl)#forward
RP/0/RP0/CPU0:router(config-mpls-static-af-lbl-fwd)#
RP/0/RP0/CPU0:router(config-mpls-static-af-lbl-fwd)# commit
RP/0/RP0/CPU0:router# show mpls interfaces
Mon May 12 06:21:30.937 DST
Interface LDP Tunnel Static Enabled
-------------------------- -------- -------- -------- --------
TenGigE0/0/0/5 No No Yes Yes
RP/0/RP0/CPU0:router#show mpls static local-label all
Tue Apr 22 18:21:55.764 UTC
Label VRF Type Prefix RW Configured Status
------- --------------- ------------ ---------------- --------------- --------
24035 default X-Connect NA Yes Created
RP/0/RP0/CPU0:router#show mpls label range
Mon Apr 28 19:56:00.596 IST
Range for dynamic labels: Min/Max: 17000/18000
RP/0/RP0/CPU0:router#show mpls lsd forwarding
Wed Nov 25 21:40:57.918 UTC
In_Label, (ID), Path_Info: <Type>
24035, (Static), 1 Paths
1/1: IPv4, 'default':4U, BE1.2, nh=10.20.3.1, lbl=35001, flags=0x0, ext_flags=0x0
mpls static
mpls label range
show mpls interfaces
During configuring or de-configuring static labels or a label range, a label discrepancy can get generated when:
Identify label discrepancy by using these show commands.
Router#show mpls static local-label discrepancy
Tue Apr 22 18:36:31.614 UTC
Label VRF Type Prefix RW Configured Status
------- --------------- ------------ ---------------- --------------- --------
24000 default X-Connect NA Yes Discrepancy
Router#show mpls static local-label all
Tue Apr 22 18:36:31.614 UTC
Label VRF Type Prefix RW Configured Status
------- --------------- ------------ ---------------- --------------- --------
24000 default X-Connect N/A Yes Discrepancy
24035 default X-Connect N/A Yes Created
RP/0/RP0/CPU0:router#show log
Thu Apr 24 14:18:57.655 UTC
Syslog logging: enabled (0 messages dropped, 0 flushes, 0 overruns)
Console logging: level warnings, 199 messages logged
Monitor logging: level debugging, 0 messages logged
Trap logging: level informational, 0 messages logged
Buffer logging: level debugging, 2 messages logged
Log Buffer (307200 bytes):
RP/0/RSP0/CPU0:Apr 24 14:18:53.743 : mpls_static[1043]: %ROUTING-MPLS_STATIC-7-ERR_STATIC_LABEL_DISCREPANCY :
The system detected 1 label discrepancies (static label could not be allocated due to conflict with other applications).
Please use 'clear mpls static local-label discrepancy' to fix this issue.
RP/0/RSP0/CPU0:Apr 24 14:18:53.937 : config[65762]: %MGBL-CONFIG-6-DB_COMMIT : Configuration committed by user 'cisco'.
Use 'show configuration commit changes 1000000020' to view the changes.
Label discrepancy is cleared by allocating a new label to those IP prefixes that are allocated dynamic label. The static label configuration takes precedence while clearing discrepancy. Clearing label discrepancy may result in traffic loss for the dynamic label which got cleared.
Router# clear mpls static local-label discrepancy all
Verify that the discrepancy is cleared.
Router# show mpls static local-label all
Wed Nov 25 21:45:50.368 UTC
Label VRF Type Prefix RW Configured Status
------- --------------- ------------ ---------------- --------------- --------
24000 default X-Connect N/A Yes Created
24035 default X-Connect N/A Yes Created
show mpls static local-label discrepancy
clear mpls static local-label discrepancy all
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