Only MPLS labels for loopbacks

Latest reply: Feb 25, 2014 19:07:29 2396 4 0 0

Dears,

 

I am simulating the attached topology in eNSP. The strange behaviour is that I only see MPLS labels for the router's loopbacks addresses, and not for the ethernet interfaces.

Taking R1 as an example, the output confirms the above:

<R1>display mpls lsp

-------------------------------------------------------------------------------

LSP Information: LDP LSP

-------------------------------------------------------------------------------

FEC In/Out Label In/Out IF Vrf Name

1.1.1.1/32 3/NULL -/-

5.5.5.5/32 NULL/1025 -/GE0/0/0

5.5.5.5/32 1024/1025 -/GE0/0/0

3.3.3.3/32 NULL/1026 -/GE0/0/0

3.3.3.3/32 1025/1026 -/GE0/0/0

2.2.2.2/32 NULL/3 -/GE0/0/0

2.2.2.2/32 1026/3 -/GE0/0/0

4.4.4.4/32 NULL/1027 -/GE0/0/0

4.4.4.4/32 1027/1027 -/GE0/0/0

6.6.6.6/32 NULL/1028 -/GE0/0/0

6.6.6.6/32 1028/1028 -/GE0/0/0

 

<R1>display mpls lsp include 192.168.1.0 24 verbose

<R1>

 

I have captured packets while pinging some loopback addresses and I can see MPLS label on the packet header. But when I ping PC 192.168.1.1 or any router ethernet interface (for instance, 172.16.1.10) there is no MPLS label at all, it is not label forwarding but IP forwarding.

 

Then, where is the advantage of MPLS label related to forwarding efficiency if there is only label switching for loopbacks addresses? How can this be solved?

 

Thank you in advance,

Julián

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w00226674
Created Feb 25, 2014 07:15:46 Helpful(0) Helpful(0)

Hi, this is a normal phenomenon! By default, the routes with 32-bit addresses trigger the setup of LSPs

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f80029566
Created Feb 25, 2014 15:21:50 Helpful(0) Helpful(0)

Hi w00226674,

 

Ok, then I have simulated the attached topology (topology2). CE1 and CE3 are in VPN "vpna" and PE1's GE0/0/0 and PE2's GE0/0/0 are bound to this VPN. In this topology, the PE1 output is the following:

 

<PE1>display mpls lsp

-------------------------------------------------------------------------------

LSP Information: BGP LSP

-------------------------------------------------------------------------------

FEC In/Out Label In/Out IF Vrf Name

10.1.1.0/30 1030/NULL -/- vpna

11.0.0.0/8 1031/NULL -/- vpna

22.0.0.0/8 1028/NULL -/- vpnb

10.2.1.0/30 1029/NULL -/- vpnb

-------------------------------------------------------------------------------

LSP Information: LDP LSP

-------------------------------------------------------------------------------

FEC In/Out Label In/Out IF Vrf Name

1.1.1.1/32 3/NULL -/-

2.2.2.2/32 NULL/3 -/GE0/0/2

2.2.2.2/32 1024/3 -/GE0/0/2

3.3.3.3/32 NULL/1025 -/GE0/0/2

3.3.3.3/32 1025/1025 -/GE0/0/2

 

There is no MPLS label for 33.33.33.33, but when I ping this IP from CE1 or PE1 there is two labels on the packet, see attached picture to see the labels.

 

<PE1>ping -vpn-instance vpna 33.33.33.33

PING 33.33.33.33: 56 data bytes, press CTRL_C to break

Reply from 33.33.33.33: bytes=56 Sequence=1 ttl=254 time=90 ms

Reply from 33.33.33.33: bytes=56 Sequence=2 ttl=254 time=120 ms

Reply from 33.33.33.33: bytes=56 Sequence=3 ttl=254 time=120 ms

Reply from 33.33.33.33: bytes=56 Sequence=4 ttl=254 time=120 ms

Reply from 33.33.33.33: bytes=56 Sequence=5 ttl=254 time=130 ms

--- 33.33.33.33 ping statistics ---

5 packet(s) transmitted

5 packet(s) received

0.00% packet loss

round-trip min/avg/max = 90/116/130 ms

 

According to the first output the packet would go without labels, there would be just IP forwarding, then, why are there labels 1025 and 1026 on the packet?

 

Thanks in advance.

 

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Sentinel
Created Feb 25, 2014 16:02:27 Helpful(0) Helpful(0)

Hi Julian,

 

Really for a full iunderstanding you need to do some in depth study of MPLS and MPLS VPNs but I will try and give some quick answers to the main points.

1. MPLS label forwarding does not improve switching speed, it was not designed for that, the only reason to configure MPLS is for the applications it provides such as MPLS VPNs. The original aim of MPLS (TAG switching) was better integration between ATM and IP.

2. Default on Huawei is to only generate labels for Loopback /32 networks, for BGP MPLS VPNs the next hop will always be the PE router, the label for the next hop will be used as the outer label when forwarding VPN packets. You can force the routers to generate labels for all attached interfaces if you wnat to but in a typical ISP MPLS VPN environment this would be unecessary and waste resources.

3. The second label is nothing to do with LDP so you will not see it with those commands, the inner label is assigned and advertised by MBGP. It is possible to view the label assignment but I cannot recall the commands off the top of my head.

 

Regards Nigel

 

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f80029566
Created Feb 25, 2014 19:07:29 Helpful(0) Helpful(0)

Hi Nigel,

 

Thanks for your help in my doubts. Here you are my comments:

 

1. You are right, but I thought that at the beginning MPLS was also deployed for improving swithing speed. However, later and with the deployment and improvement of ASIC technology, switching speed is not a real advantage.

 

2. Ok.

 

3. You are right and I got confused for a while about MPLS VPN. MPLS VPN uses LDP for allocating outer label and MP-BGP for allocating inner label, the command to see the inner label is

 

<PE1>display bgp vpnv4 vpn-instance vpna routing-table label

 BGP Local router ID is 172.1.1.1
 Status codes: * - valid, > - best, d - damped,
               h - history,  i - internal, s - suppressed, S - Stale
               Origin : i - IGP, e - EGP, ? - incomplete


 VPN-Instance vpna, Router ID 172.1.1.1:

 Total Number of Routes: 2


        Network           NextHop           In/Out Label

 *>i    10.3.1.0          3.3.3.3           NULL/1027
 *>i    33.0.0.0          3.3.3.3           NULL/1026     <<<<<<<<<<<<<<<<<<<<<<<<
<PE1>

 

Taking this and also the previous output into account

 

<PE1>display mpls lsp

-------------------------------------------------------------------------------

LSP Information: BGP LSP

-------------------------------------------------------------------------------

FEC In/Out Label In/Out IF Vrf Name

10.1.1.0/30 1030/NULL -/- vpna

11.0.0.0/8 1031/NULL -/- vpna

22.0.0.0/8 1028/NULL -/- vpnb

10.2.1.0/30 1029/NULL -/- vpnb

-------------------------------------------------------------------------------

LSP Information: LDP LSP

-------------------------------------------------------------------------------

FEC In/Out Label In/Out IF Vrf Name

1.1.1.1/32 3/NULL -/-

2.2.2.2/32 NULL/3 -/GE0/0/2

2.2.2.2/32 1024/3 -/GE0/0/2

3.3.3.3/32 NULL/1025 -/GE0/0/2      <<<<<<<<<<<<<<<<<<<<<<<<<<<<

3.3.3.3/32 1025/1025 -/GE0/0/2      <<<<<<<<<<<<<<<<<<<<<<<<<<<<

 

now we have labels 1025 and 1026, the labels seen in the captured packets. Now it makes sense!

 

Regards and thanks,

Julián

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