Untagged Labels Coming Instead Of POP Labels

Problem Description
In the given scenario loopbacks are advertised as /24 subnet. So local router will advertise loopback0 as implicit null to it adjacent neighbours with pop tag but the adjacent routers are receiving Untagged entries.


For simplicity I am using R1 to capture all the outputs. In the given outputs I am verifying loopback 0 of router 3.

FIB of R1
R1#sh mpls forwarding-tableLocal Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
18 Untagged 172.16.3.3/32 0 Fa0/0 10.1.1.1
19 Untagged 172.16.2.2/32 0 Fa0/1 10.1.2.2



FIB of R1
R1#show mpls forwarding-tableLocal Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
18 Untagged 172.16.3.3/32 0 Fa0/0 10.1.1.1
19 Untagged 172.16.2.2/32 0 Fa0/1 10.1.2.2

The loopbacks are advertised as /24 subnet mask and adjacent routers are receiving /32 entry in the routing table because by default loopbacks interface in ospf treated as stub host.

R3# sh ip ospf interface loopback 0
Loopback0 is up, line protocol is up
Internet Address 172.16.3.3/24, Area 0
Process ID 1, Router ID 172.16.3.3, Network Type LOOPBACK, Cost: 1
Enabled by interface config, including secondary ip addresses
Loopback interface is treated as a stub Host

Now if you check the adjacent router routing table loopback entry will come as /32 but it advertise the loopback (172.16.3.3/24) as implicit null. Now check the routing table entry of 172.16.3.3 and mpls binding in R1.

R1#Show ip route 172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks

O 172.16.3.3/32 [110/2] via 10.1.1.1, 03:15:04, FastEthernet0/0
O 172.16.2.2/32 [110/2] via 10.1.2.2, 03:14:54, FastEthernet0/1
C 172.16.1.0/24 is directly connected, Loopback0
10.0.0.0/30 is subnetted, 2 subnets
C 10.1.2.0 is directly connected, FastEthernet0/1
C 10.1.1.0 is directly connected, FastEthernet0/0



Show mpls ldp binding of R1tib entry: 172.16.3.0/24, rev 34(no route)
remote binding: tsr: 172.16.3.3:0, tag: imp-null
tib entry: 172.16.3.3/32, rev 31
local binding: tag: 18
remote binding: tsr: 10.1.2.2:0, tag: 20

Show mpls forwarding of R1
R1#sh mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
18 Untagged 172.16.3.3/32 0 Fa0/0 10.1.1.1
19 Untagged 172.16.2.2/32 0 Fa0/1 10.1.2.2

See carefully the output tag of 17216.3.3/32 which is Untagged. Instead of Untagged it should be pop label.

POP Label:- The router before the last LSR (the penultimate hop) pops the label and transmits the packet without the label. The last hop is called the egress LSR.
Untagged Label:- Untagged label in the LFIB when the IP prefix is a directly connected interface, a summary route or the next-hop router has not advertised the label.

You can see the “The next hop router has not advertised the label”. The next router is advertising implicit null for /24 but routing table doesn’t have any entry for the same.

Solution:- The same type of problem I faced during my CCIE lab exam. It was supposed to advertise the loopback with the exact subnet but we usually get the entry as /32. So remember this command which will resolve the whole issue.
Command is “ip ospf network point-to-point” under loopback0.

Now check the routing table, mpls binding and forwarding table of R1
Routing Table
172.16.0.0/24 is subnetted, 3 subnets
C 172.16.1.0 is directly connected, Loopback0
O 172.16.2.0 [110/2] via 10.1.2.2, 00:00:52, FastEthernet0/1
O 172.16.3.0 [110/2] via 10.1.1.1, 00:00:42, FastEthernet0/0
10.0.0.0/30 is subnetted, 2 subnets
C 10.1.2.0 is directly connected, FastEthernet0/1
C 10.1.1.0 is directly connected, FastEthernet0/0

Forwarding Table
R1#sh mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
16 Pop tag 172.16.2.0/24 0 Fa0/1 10.1.2.2
17 Pop tag 172.16.3.0/24 0 Fa0/0 10.1.1.1

Here you got the Pop tag. (Pop tag is already explained)

LDP Binding Table
tib entry: 172.16.1.0/24, rev 6
local binding: tag: imp-null
remote binding: tsr: 172.16.3.3:0, tag: 18
remote binding: tsr: 10.1.2.2:0, tag: 16


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regards
shivlu jain (TULIP)

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