MPLS Tunneling

Issue

We will configure MPLS through our backbone, EBGP as the CE-PE routing protocol and transport the CE prefixes over the MPLS backbone

ISIS will be the IGP inside the MPLS backbone

No VRF or L2VPN parameters will be configured, just enabling MPLS in the backbone and transport the routes between the CEs

Configuration

R1

R1#sh run int f0/0

interface FastEthernet0/0

 ip address 192.168.12.1 255.255.255.0

 speed 100

 full-duplex

R1#sh run int lo0

interface Loopback0

 ip address 1.1.1.1 255.255.255.255

R2

R2#sh run int f0/0

interface FastEthernet0/0

 ip address 192.168.12.2 255.255.255.0

 speed 100

 full-duplex

R2#sh run int f0/1

interface FastEthernet0/1

 ip address 192.168.23.2 255.255.255.0

 ip router isis 1

 speed 100

 full-duplex

 mpls ip

R2#sh run int lo0

interface Loopback0

 ip address 2.2.2.2 255.255.255.255

R2#sh run | sec router isis

router isis 1

 net 49.0000.0000.0000.0002.00

 is-type level-2-only

 passive-interface Loopback0

R3

R3#sh run int f0/0

interface FastEthernet0/0

 ip address 192.168.23.3 255.255.255.0

 ip router isis 1

 speed 100

 full-duplex

 mpls ip

R3#sh run int f0/1

interface FastEthernet0/1

 ip address 192.168.34.3 255.255.255.0

 ip router isis 1

 speed 100

 full-duplex

 mpls ip

R3#sh run int lo0

interface Loopback0

 ip address 3.3.3.3 255.255.255.255

R3#sh run | sec router isis

router isis 1

 net 49.0000.0000.0000.0003.00

 is-type level-2-only

 passive-interface Loopback0

mpls label protocol ldp

mpls ldp router-id Loopback0 force

R4

R4#sh run int f0/0

interface FastEthernet0/0

 ip address 192.168.34.4 255.255.255.0

 ip router isis 1

 speed 100

 full-duplex

 mpls ip

R4#sh run int f0/1

interface FastEthernet0/1

 ip address 192.168.45.4 255.255.255.0

 ip router isis 1

 speed 100

 full-duplex

 mpls ip

R4#sh run int lo0

interface Loopback0

 ip address 4.4.4.4 255.255.255.255

R4#sh run | sec router isis

router isis 1

 net 49.0000.0000.0000.0004.00

 is-type level-2-only

 passive-interface Loopback0

mpls label protocol ldp

mpls ldp router-id Loopback0 force

R5

R5#sh run int f0/0

interface FastEthernet0/0

 ip address 192.168.45.5 255.255.255.0

 ip router isis 1

 speed 100

 full-duplex

 mpls ip

R5#sh run int f0/1

interface FastEthernet0/1

 ip address 192.168.56.5 255.255.255.0

 ip router isis 1

 speed 100

 full-duplex

 mpls ip

R5#sh run int lo0

interface Loopback0

 ip address 5.5.5.5 255.255.255.255

R5#sh run | sec router isis

router isis 1

 net 49.0000.0000.0000.0005.00

 is-type level-2-only

 passive-interface Loopback0

mpls label protocol ldp

mpls ldp router-id Loopback0 force

R6

R6#sh run int f0/0

interface FastEthernet0/0

 ip address 192.168.56.6 255.255.255.0

 ip router isis 1

 speed 100

 full-duplex

 mpls ip

R6#sh run int f0/1

interface FastEthernet0/1

 ip address 192.168.19.6 255.255.255.0

 ip router isis 1

 speed 100

 full-duplex

 mpls ip

R6#sh run int lo0

interface Loopback0

 ip address 6.6.6.6 255.255.255.255

R6#sh run | sec router isis

router isis 1

 net 49.0000.0000.0000.0006.00

 is-type level-2-only

 passive-interface Loopback0

mpls label protocol ldp

mpls ldp router-id Loopback0 force

R7

R7#sh run int f0/0

interface FastEthernet0/0

 ip address 192.168.19.19 255.255.255.0

 ip router isis 1

 speed 100

 full-duplex

 mpls ip

R7#sh run int f0/1

interface FastEthernet0/1

 ip address 192.168.20.19 255.255.255.0

 speed 100

 full-duplex

R7#sh run int lo0

interface Loopback0

 ip address 19.19.19.19 255.255.255.255

R7#sh run | sec router isis

router isis 1

 net 49.0000.0000.0000.0019.00

 is-type level-2-only

 passive-interface Loopback0

R8

R8#sh run int f0/0

interface FastEthernet0/0

 ip address 192.168.20.20 255.255.255.0

 speed 100

 full-duplex

R8#sh run int lo0

interface Loopback0

 ip address 20.20.20.20 255.255.255.255

Verification

R2#sh isis neighbors

System Id      Type Interface   IP Address      State Holdtime Circuit Id

R3             L2   Fa0/1       192.168.23.3    UP    7        R3.01     

R2#sh mpls ldp neighbor

    Peer LDP Ident: 3.3.3.3:0; Local LDP Ident 2.2.2.2:0

        TCP connection: 3.3.3.3.11021 - 2.2.2.2.646

        State: Oper; Msgs sent/rcvd: 30/29; Downstream

        Up time: 00:13:18

        LDP discovery sources:

          FastEthernet0/1, Src IP addr: 192.168.23.3

        Addresses bound to peer LDP Ident:

          192.168.23.3    192.168.34.3    3.3.3.3    

R3#sh isis neighbors

System Id      Type Interface   IP Address      State Holdtime Circuit Id

R2             L2   Fa0/0       192.168.23.2    UP    24       R3.01             

R4             L2   Fa0/1       192.168.34.4    UP    7        R4.01     

R3#sh mpls ldp neighbor

    Peer LDP Ident: 4.4.4.4:0; Local LDP Ident 3.3.3.3:0

        TCP connection: 4.4.4.4.60496 - 3.3.3.3.646

        State: Oper; Msgs sent/rcvd: 42/44; Downstream

        Up time: 00:25:35

        LDP discovery sources:

          FastEthernet0/1, Src IP addr: 192.168.34.4

        Addresses bound to peer LDP Ident:

          192.168.34.4    4.4.4.4         192.168.45.4   

    Peer LDP Ident: 2.2.2.2:0; Local LDP Ident 3.3.3.3:0

        TCP connection: 2.2.2.2.646 - 3.3.3.3.11021

        State: Oper; Msgs sent/rcvd: 29/29; Downstream

        Up time: 00:12:34

        LDP discovery sources:

          FastEthernet0/0, Src IP addr: 192.168.23.2

        Addresses bound to peer LDP Ident:

          192.168.12.2    192.168.23.2    2.2.2.2      

R4#sh isis neighbors

System Id      Type Interface   IP Address      State Holdtime Circuit Id

R5             L2   Fa0/1       192.168.45.5    UP    7        R5.01             

R3             L2   Fa0/0       192.168.34.3    UP    27       R4.01              

R4#sh mpls ldp neighbor

    Peer LDP Ident: 3.3.3.3:0; Local LDP Ident 4.4.4.4:0

        TCP connection: 3.3.3.3.646 - 4.4.4.4.60496

        State: Oper; Msgs sent/rcvd: 44/42; Downstream

        Up time: 00:25:49

        LDP discovery sources:

          FastEthernet0/0, Src IP addr: 192.168.34.3

        Addresses bound to peer LDP Ident:

          192.168.23.3    192.168.34.3    3.3.3.3        

    Peer LDP Ident: 5.5.5.5:0; Local LDP Ident 4.4.4.4:0

        TCP connection: 5.5.5.5.53841 - 4.4.4.4.646

        State: Oper; Msgs sent/rcvd: 33/33; Downstream

        Up time: 00:17:03

        LDP discovery sources:

          FastEthernet0/1, Src IP addr: 192.168.45.5

        Addresses bound to peer LDP Ident:

          192.168.45.5    192.168.56.5    5.5.5.5  

R5#sh isis neighbors

System Id      Type Interface   IP Address      State Holdtime Circuit Id

R4             L2   Fa0/0       192.168.45.4    UP    28       R5.01             

R6             L2   Fa0/1       192.168.56.6    UP    8        R6.01             

R5#sh mpls ldp neighbor

    Peer LDP Ident: 192.168.56.6:0; Local LDP Ident 5.5.5.5:0

        TCP connection: 192.168.56.6.19552 - 5.5.5.5.646

        State: Oper; Msgs sent/rcvd: 41/42; Downstream

        Up time: 00:24:15

        LDP discovery sources:

          FastEthernet0/1, Src IP addr: 192.168.56.6

        Addresses bound to peer LDP Ident:

          192.168.56.6    6.6.6.6         192.168.19.6   

    Peer LDP Ident: 4.4.4.4:0; Local LDP Ident 5.5.5.5:0

        TCP connection: 4.4.4.4.646 - 5.5.5.5.53841

        State: Oper; Msgs sent/rcvd: 33/33; Downstream

        Up time: 00:17:20

        LDP discovery sources:

          FastEthernet0/0, Src IP addr: 192.168.45.4

        Addresses bound to peer LDP Ident:

          192.168.34.4    4.4.4.4         192.168.45.4  

R6#sh isis neighbors

System Id      Type Interface   IP Address      State Holdtime Circuit Id

R7             L2   Fa0/1       192.168.19.19   UP    7        R7.01             

R5             L2   Fa0/0       192.168.56.5    UP    22       R6.01             

R6#sh mpls ldp neighbor

    Peer LDP Ident: 5.5.5.5:0; Local LDP Ident 192.168.56.6:0

        TCP connection: 5.5.5.5.646 - 192.168.56.6.19552

        State: Oper; Msgs sent/rcvd: 43/42; Downstream

        Up time: 00:25:01

        LDP discovery sources:

          FastEthernet0/0, Src IP addr: 192.168.56.5

        Addresses bound to peer LDP Ident:

          192.168.45.5    192.168.56.5    5.5.5.5        

    Peer LDP Ident: 192.168.19.19:0; Local LDP Ident 192.168.56.6:0

        TCP connection: 192.168.19.19.646 - 192.168.56.6.13701

        State: Oper; Msgs sent/rcvd: 32/33; Downstream

        Up time: 00:16:43

        LDP discovery sources:

          FastEthernet0/1, Src IP addr: 192.168.19.19

        Addresses bound to peer LDP Ident:

          192.168.19.19   192.168.20.19   19.19.19.19   

R7#sh isis neighbors

System Id      Type Interface   IP Address      State Holdtime Circuit Id

R6             L2   Fa0/0       192.168.19.6    UP    26       R7.01             

R7#sh mpls ldp neighbor

    Peer LDP Ident: 192.168.56.6:0; Local LDP Ident 192.168.19.19:0

        TCP connection: 192.168.56.6.13701 - 192.168.19.19.646

        State: Oper; Msgs sent/rcvd: 34/32; Downstream

        Up time: 00:16:58

        LDP discovery sources:

          FastEthernet0/0, Src IP addr: 192.168.19.6

        Addresses bound to peer LDP Ident:

          192.168.56.6    6.6.6.6         192.168.19.6 

Now, we will establish EBGP peering between R1 and R2, R7 and R8

As well, between the PEs: R2 and R7 which will be an iBGP session

R1#sh run | sec router bgp

router bgp 1

 no synchronization

 bgp log-neighbor-changes

 network 1.1.1.1 mask 255.255.255.255

 neighbor 192.168.12.2 remote-as 100

 no auto-summary

R2#sh run | sec router bgp

router bgp 100

 no synchronization

 bgp log-neighbor-changes

 neighbor 19.19.19.19 remote-as 100

 neighbor 19.19.19.19 update-source Loopback0

 neighbor 19.19.19.19 next-hop-self

 neighbor 192.168.12.1 remote-as 1

 no auto-summary

R8#sh run | sec router bgp

router bgp 20

 no synchronization

 bgp log-neighbor-changes

 network 20.20.20.20 mask 255.255.255.255

 neighbor 192.168.20.19 remote-as 100

 no auto-summary

R7#sh run | sec router bgp

router bgp 100

 no synchronization

 bgp log-neighbor-changes

 neighbor 2.2.2.2 remote-as 100

 neighbor 2.2.2.2 update-source Loopback0

 neighbor 2.2.2.2 next-hop-self

 neighbor 192.168.20.20 remote-as 20

 no auto-summary

As the connection between R7 and R8 is EBGP session, and that connection is not advertised in the BGP session, we need to configure next-hop-self for iBGP peering inside the MPLS backbone

R1#sh ip bgp summary

BGP router identifier 1.1.1.1, local AS number 1

BGP table version is 3, main routing table version 3

2 network entries using 234 bytes of memory

2 path entries using 104 bytes of memory

3/2 BGP path/bestpath attribute entries using 372 bytes of memory

1 BGP AS-PATH entries using 24 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 734 total bytes of memory

BGP activity 2/0 prefixes, 2/0 paths, scan interval 60 secs

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

192.168.12.2    4   100      30      30        3    0    0 00:25:13        1

R1#sh ip bgp

BGP table version is 3, local router ID is 1.1.1.1

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 1.1.1.1/32       0.0.0.0                  0         32768 i

*> 20.20.20.20/32   192.168.12.2                           0 100 20 i

R2#sh ip bgp summary

BGP router identifier 2.2.2.2, local AS number 100

BGP table version is 3, main routing table version 3

2 network entries using 234 bytes of memory

2 path entries using 104 bytes of memory

3/2 BGP path/bestpath attribute entries using 372 bytes of memory

2 BGP AS-PATH entries using 48 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 758 total bytes of memory

BGP activity 2/0 prefixes, 2/0 paths, scan interval 60 secs

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

19.19.19.19     4   100      22      22        3    0    0 00:18:00        1

192.168.12.1    4     1      32      32        3    0    0 00:27:04        1

R2#sh ip bgp neighbors 192.168.12.1 advertised-routes

BGP table version is 3, local router ID is 2.2.2.2

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*>i20.20.20.20/32   19.19.19.19              0    100      0 20 i

Total number of prefixes 1

R2#sh ip bgp neighbors 19.19.19.19 advertised-routes

BGP table version is 3, local router ID is 2.2.2.2

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 1.1.1.1/32       192.168.12.1             0             0 1 i

Total number of prefixes 1

R2#sh ip bgp

BGP table version is 3, local router ID is 2.2.2.2

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 1.1.1.1/32       192.168.12.1             0             0 1 i

*>i20.20.20.20/32   19.19.19.19              0    100      0 20 i

R7#sh ip bgp summary

BGP router identifier 19.19.19.19, local AS number 100

BGP table version is 3, main routing table version 3

2 network entries using 234 bytes of memory

2 path entries using 104 bytes of memory

3/2 BGP path/bestpath attribute entries using 372 bytes of memory

2 BGP AS-PATH entries using 48 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 758 total bytes of memory

BGP activity 2/0 prefixes, 2/0 paths, scan interval 60 secs

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

2.2.2.2         4   100      23      23        3    0    0 00:18:45        1

192.168.20.20   4    20      32      32        3    0    0 00:28:21        1

R7#sh ip bg

R7#sh ip bgp neigh

R7#sh ip bgp neighbors 2.2.2.2 advert

R7#sh ip bgp neighbors 2.2.2.2 advertised-routes

BGP table version is 3, local router ID is 19.19.19.19

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 20.20.20.20/32   192.168.20.20            0             0 20 i

Total number of prefixes 1

R7#sh ip bg

R7#sh ip bgp nei

R7#sh ip bgp neighbors 192.168.20.20 advert

R7#sh ip bgp neighbors 192.168.20.20 advertised-routes

BGP table version is 3, local router ID is 19.19.19.19

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*>i1.1.1.1/32       2.2.2.2                  0    100      0 1 i

Total number of prefixes 1

R7#sh ip bg

R7#sh ip bgp

BGP table version is 3, local router ID is 19.19.19.19

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*>i1.1.1.1/32       2.2.2.2                  0    100      0 1 i

*> 20.20.20.20/32   192.168.20.20            0             0 20 i

R8#sh ip bgp summary

BGP router identifier 20.20.20.20, local AS number 20

BGP table version is 3, main routing table version 3

2 network entries using 234 bytes of memory

2 path entries using 104 bytes of memory

3/2 BGP path/bestpath attribute entries using 372 bytes of memory

1 BGP AS-PATH entries using 24 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 734 total bytes of memory

BGP activity 2/0 prefixes, 2/0 paths, scan interval 60 secs

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

192.168.20.19   4   100      32      32        3    0    0 00:28:46        1

R8#sh ip bgp

BGP table version is 3, local router ID is 20.20.20.20

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 1.1.1.1/32       192.168.20.19                          0 100 1 i

*> 20.20.20.20/32   0.0.0.0                  0         32768 i

R1#ping 20.20.20.20 source loopback 0

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 20.20.20.20, timeout is 2 seconds:

Packet sent with a source address of 1.1.1.1

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 28/30/32 ms

R8#ping 1.1.1.1 source loopback 0

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:

Packet sent with a source address of 20.20.20.20

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 28/29/32 ms

Now, if we wanted to trace the packet as it traverses through the backbone and check label assignments, we will enable debug mpls packets on the MPLS routers (R3 through R6) and initiate an ICMP packet from R8 towards R1

R3 – R6

debug mpls packets

R7

R7#sh mpls forwarding-table

Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop   

tag    tag or VC   or Tunnel Id      switched   interface              

16     Pop tag     6.6.6.6/32        0          Fa0/0      192.168.19.6

17     16          5.5.5.5/32        0          Fa0/0      192.168.19.6

18     18          4.4.4.4/32        0          Fa0/0      192.168.19.6

19     20          3.3.3.3/32        0          Fa0/0      192.168.19.6

20     Pop tag     192.168.56.0/24   0          Fa0/0      192.168.19.6

21     17          192.168.45.0/24   0          Fa0/0      192.168.19.6

22     19          192.168.34.0/24   0          Fa0/0      192.168.19.6

23     21          192.168.23.0/24   0          Fa0/0      192.168.19.6

24     23          2.2.2.2/32        0          Fa0/0      192.168.19.6

R6

*Mar  2 00:57:59.197: MPLS: Fa0/1: recvd: CoS=0, TTL=254, Label(s)=23

*Mar  2 00:57:59.201: MPLS: Fa0/0: xmit: CoS=0, TTL=253, Label(s)=23

 Note: we can see the TTL decrements

R6#sh mpls forwarding-table

Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop   

tag    tag or VC   or Tunnel Id      switched   interface             

16     Pop tag     5.5.5.5/32        0          Fa0/0      192.168.56.5

17     Pop tag     192.168.45.0/24   570        Fa0/0      192.168.56.5

18     17          4.4.4.4/32        0          Fa0/0      192.168.56.5

19     18          192.168.34.0/24   590        Fa0/0      192.168.56.5

20     19          3.3.3.3/32        0          Fa0/0      192.168.56.5

21     20          192.168.23.0/24   0          Fa0/0      192.168.56.5

22     Pop tag     19.19.19.19/32    189257     Fa0/1      192.168.19.19

23     23          2.2.2.2/32        116949     Fa0/0      192.168.56.5

R5

*Mar  2 00:57:59.429: MPLS: Fa0/1: recvd: CoS=0, TTL=253, Label(s)=23

*Mar  2 00:57:59.429: MPLS: Fa0/0: xmit: CoS=0, TTL=252, Label(s)=23

R5#sh mpls forwarding-table

Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop   

tag    tag or VC   or Tunnel Id      switched   interface             

16     Pop tag     6.6.6.6/32        0          Fa0/1      192.168.56.6

17     Pop tag     4.4.4.4/32        0          Fa0/0      192.168.45.4

18     Pop tag     192.168.34.0/24   570        Fa0/0      192.168.45.4

19     16          3.3.3.3/32        0          Fa0/0      192.168.45.4

20     17          192.168.23.0/24   0          Fa0/0      192.168.45.4

21     22          19.19.19.19/32    201541     Fa0/1      192.168.56.6

22     Pop tag     192.168.19.0/24   0          Fa0/1      192.168.56.6

23     23          2.2.2.2/32        117675     Fa0/0      192.168.45.4

R4

*Mar  2 00:58:01.133: MPLS: Fa0/1: recvd: CoS=0, TTL=252, Label(s)=23

*Mar  2 00:58:01.137: MPLS: Fa0/0: xmit: CoS=0, TTL=251, Label(s)=23

Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop   

tag    tag or VC   or Tunnel Id      switched   interface             

16     Pop tag     3.3.3.3/32        0          Fa0/0      192.168.34.3

17     Pop tag     192.168.23.0/24   0          Fa0/0      192.168.34.3

18     Pop tag     5.5.5.5/32        0          Fa0/1      192.168.45.5

19     Pop tag     192.168.56.0/24   0          Fa0/1      192.168.45.5

20     16          6.6.6.6/32        0          Fa0/1      192.168.45.5

21     21          19.19.19.19/32    200951     Fa0/1      192.168.45.5

22     22          192.168.19.0/24   0          Fa0/1      192.168.45.5

23     23          2.2.2.2/32        117103     Fa0/0      192.168.34.3

R3

*Mar  2 00:58:03.237: MPLS: Fa0/1: recvd: CoS=0, TTL=251, Label(s)=23

*Mar  2 00:58:03.237: MPLS: Fa0/0: xmit: (no label)

Note: we can see the no label keyword which is due to PHP

Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop   

tag    tag or VC   or Tunnel Id      switched   interface             

16     Pop tag     4.4.4.4/32        0          Fa0/1      192.168.34.4

17     18          5.5.5.5/32        0          Fa0/1      192.168.34.4

18     Pop tag     192.168.45.0/24   0          Fa0/1      192.168.34.4

19     19          192.168.56.0/24   0          Fa0/1      192.168.34.4

20     20          6.6.6.6/32        0          Fa0/1      192.168.34.4

21     21          19.19.19.19/32    200498     Fa0/1      192.168.34.4

22     22          192.168.19.0/24   0          Fa0/1      192.168.34.4

23     Pop tag     2.2.2.2/32        111092     Fa0/0      192.168.23.2

Then checking the reverse direction (Echo reply)

R3

*Mar  2 01:01:31.829: MPLS: Fa0/0: recvd: CoS=0, TTL=254, Label(s)=21

*Mar  2 01:01:31.829: MPLS: Fa0/1: xmit: CoS=0, TTL=253, Label(s)=21

R3#sh mpls  forwarding-table

Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop   

tag    tag or VC   or Tunnel Id      switched   interface             

16     Pop tag     4.4.4.4/32        0          Fa0/1      192.168.34.4

17     18          5.5.5.5/32        0          Fa0/1      192.168.34.4

18     Pop tag     192.168.45.0/24   0          Fa0/1      192.168.34.4

19     19          192.168.56.0/24   0          Fa0/1      192.168.34.4

20     20          6.6.6.6/32        0          Fa0/1      192.168.34.4

21     21          19.19.19.19/32    201225     Fa0/1      192.168.34.4

22     22          192.168.19.0/24   0          Fa0/1      192.168.34.4

23     Pop tag     2.2.2.2/32        111735     Fa0/0      192.168.23.2

R4

*Mar  2 01:01:29.729: MPLS: Fa0/0: recvd: CoS=0, TTL=253, Label(s)=21

*Mar  2 01:01:29.729: MPLS: Fa0/1: xmit: CoS=0, TTL=252, Label(s)=21

R4#sh mpl for

Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop   

tag    tag or VC   or Tunnel Id      switched   interface             

16     Pop tag     3.3.3.3/32        0          Fa0/0      192.168.34.3

17     Pop tag     192.168.23.0/24   0          Fa0/0      192.168.34.3

18     Pop tag     5.5.5.5/32        0          Fa0/1      192.168.45.5

19     Pop tag     192.168.56.0/24   0          Fa0/1      192.168.45.5

20     16          6.6.6.6/32        0          Fa0/1      192.168.45.5

21     21          19.19.19.19/32    201952     Fa0/1      192.168.45.5

22     22          192.168.19.0/24   0          Fa0/1      192.168.45.5

23     23          2.2.2.2/32        117924     Fa0/0      192.168.34.3

R5

*Mar  2 01:01:28.029: MPLS: Fa0/0: recvd: CoS=0, TTL=252, Label(s)=21

*Mar  2 01:01:28.029: MPLS: Fa0/1: xmit: CoS=0, TTL=251, Label(s)=22

Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop   

tag    tag or VC   or Tunnel Id      switched   interface             

16     Pop tag     6.6.6.6/32        0          Fa0/1      192.168.56.6

17     Pop tag     4.4.4.4/32        0          Fa0/0      192.168.45.4

18     Pop tag     192.168.34.0/24   570        Fa0/0      192.168.45.4

19     16          3.3.3.3/32        0          Fa0/0      192.168.45.4

20     17          192.168.23.0/24   0          Fa0/0      192.168.45.4

21     22          19.19.19.19/32    202679     Fa0/1      192.168.56.6

22     Pop tag     192.168.19.0/24   0          Fa0/1      192.168.56.6

23     23          2.2.2.2/32        118573     Fa0/0      192.168.45.4

R6

*Mar  2 01:01:27.801: MPLS: Fa0/0: recvd: CoS=0, TTL=251, Label(s)=22

*Mar  2 01:01:27.801: MPLS: Fa0/1: xmit: (no label)

Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop   

tag    tag or VC   or Tunnel Id      switched   interface             

16     Pop tag     5.5.5.5/32        0          Fa0/0      192.168.56.5

17     Pop tag     192.168.45.0/24   570        Fa0/0      192.168.56.5

18     17          4.4.4.4/32        0          Fa0/0      192.168.56.5

19     18          192.168.34.0/24   590        Fa0/0      192.168.56.5

20     19          3.3.3.3/32        0          Fa0/0      192.168.56.5

21     20          192.168.23.0/24   0          Fa0/0      192.168.56.5

22     Pop tag     19.19.19.19/32    190601     Fa0/1      192.168.19.19

23     23          2.2.2.2/32        118001     Fa0/0      192.168.56.5

As can be seen from the debug outputs above that the precedence value is 0 which is copied from the packet header to the MPLS header (CoS)

R1(config)#access-list 100 permit ip any any precedence critical

R1(config)#access-list 100 permit ip any any precedence flash   

R1(config)#access-list 100 permit ip any any precedence flash-override

R1(config)#access-list 100 permit ip any any precedence immediate    

R1(config)#access-list 100 permit ip any any precedence internet

R1(config)#access-list 100 permit ip any any precedence network

R1(config)#access-list 100 permit ip any any precedence priority

R1(config)#access-list 100 permit ip any any precedence routine

R1(config)#int f0/0

R1(config-if)#ip access-group 100 in

R1#sh access-lists 100

Extended IP access list 100

    10 permit ip any any precedence critical

    20 permit ip any any precedence flash

    30 permit ip any any precedence flash-override

    40 permit ip any any precedence immediate

    50 permit ip any any precedence internet

    60 permit ip any any precedence network

    70 permit ip any any precedence priority

    80 permit ip any any precedence routine (15 matches)

.net file

For anyone want to lab it up, please find the .net file below

autostart = False

[127.0.0.1:7200]

    workingdir = /tmp

    udp = 10001

    [[3725]]

        image = INE/c3725-adventerprisek9-mz.124-18.bin

        ram = 128

        ghostios = True

    [[ROUTER R4]]

        model = 3725

        console = 2004

        f0/0 = R3 f0/1

        f0/1 = R5 f0/0

        x = -129.0

        y = 106.0

    [[ROUTER R5]]

        model = 3725

        console = 2005

        f0/0 = R4 f0/1

        f0/1 = R6 f0/0

        x = 57.0

        y = 110.0

    [[ROUTER R6]]

        model = 3725

        console = 2006

        f0/0 = R5 f0/1

        f0/1 = R7 f0/0

        x = 61.0

        y = 14.0

    [[ROUTER R7]]

        model = 3725

        console = 2007

        f0/0 = R6 f0/1

        f0/1 = R8 f0/0

        x = 201.0

        y = -37.0

    [[ROUTER R1]]

        model = 3725

        console = 2001

        f0/0 = R2 f0/0

        x = -249.0

        y = -139.0

    [[ROUTER R2]]

        model = 3725

        console = 2002

        f0/0 = R1 f0/0

        f0/1 = R3 f0/0

        x = -246.0

        y = -37.0

    [[ROUTER R3]]

        model = 3725

        console = 2003

        f0/0 = R2 f0/1

        f0/1 = R4 f0/0

        x = -125.0

        y = 17.0

    [[ROUTER R8]]

        model = 3725

        console = 2008

        f0/0 = R7 f0/1

        x = 203.0

        y = -140.0