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Contributor
Posts: 55
Registered: ‎06-16-2016
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RSVP LSP MTU

Why can I send a 4000 byte ping across an LSP with an MTU of 1500? DF bit is set on the ping and I stuck a tap between two LSRs in the path to make sure that it's not being fragmented.

Super Contributor
Posts: 101
Registered: ‎03-11-2012
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Re: RSVP LSP MTU

[ Edited ]

Hi , is the protocols mpls path-mtu rsvp mtu-signaling configured ? 

try to check max mtu with command ping mpls rsvp <lsp-name> sweep. 

Super Contributor
Posts: 65
Registered: ‎06-21-2017
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Re: RSVP LSP MTU

Hi,


The MTU for an LSP can be checked on ingress using command :
>ping mpls rsvp <lsp-name> sweep

Please check the mtu from below command also on ingress mpls interface:
>show interfaces ge-0/0/0 extensive |match mtu

 

 

Regards,

Rahul

Distinguished Expert
Posts: 2,243
Registered: ‎08-21-2009
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Re: RSVP LSP MTU

Hello,

There are several possible scenarios involved with seemingly simple "send ping across LSP":

1/ You are pinging from ingress LSR

1a/ You are pinging from GRT/inet.0

1aa/ You don't have "protocols mpls traffic-engineering bgp-igp[-both-ribs]" configured - then Your ping does not go via LSP

1ab/ You have "protocols mpls traffic-engineering bgp-igp[-both-ribs]"  configured - then Your ping goes via LSP

 

1b/ You are pinging from VRF - then Your ping goes via LSP  unless You configured some fancy route leaking from GRT into VRF

 

2/ You are pinging from CE attached to ingress LSR

2a/ Your CE is in GRT - then the destination IP must resolve via that LSP for the ping to go via LSP

2b/ Your CE is in VRF - Your ping is supposed to go via LSP - unless You configured some fancy route leaking from GRT into VRF

 

Please help us to understand Your exact scenario so we can help Your further.

HTH

Thx

Alex

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Contributor
Posts: 55
Registered: ‎06-16-2016
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Re: RSVP LSP MTU

[ Edited ]

I don't have mtu-signaling configured. Output of show rsvp session name <lsp> detail shows an mtu of 1500. I haven't tried the ping sweep from the PE. I'll try that later when I get into the lab.

 

My exact scenario is that I'm pinging from CE to CE via a CCC remote-interface-switch mapped to specific LSPs.

Contributor
Posts: 30
Registered: ‎06-13-2013
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Re: RSVP LSP MTU

Hello Everyone, I actually have the same doubt, and I've never found out a clear answer.

 

For example, rsvp sessions says 1500 mtu, ping mpls sweep gives 9172. I always thought that the mtu for rsvp was only for the control plane, and that's why I assumed it was normal, but maybe someone has a better explanation.

 

> show rsvp session ingress name lsp1 extensive | match "adspec|tspec"
Tspec: rate 0bps size 0bps peak Infbps m 20 M 1500
Adspec: sent MTU 1500
Detour Tspec: rate 0bps size 0bps peak Infbps m 20 M 1500
Detour adspec: sent MTU 1500
Tspec: rate 0bps size 0bps peak Infbps m 20 M 1500
Adspec: sent MTU 1500
Detour Tspec: rate 0bps size 0bps peak Infbps m 20 M 1500
Detour adspec: sent MTU 1500
Tspec: rate 0bps size 0bps peak Infbps m 20 M 1500
Adspec: received MTU 1500
Adspec: received MTU 1500

 

> ping mpls rsvp lsp1 sweep
100! 5052! 7528! 8768! 9388. 9080! 9392. 9236. 9160! 9240. 9200. 9180. 9172! 9184. 9180. 9176.
--- lsp ping sweep result---
Maximum Transmission Unit (MTU) is 9172 bytes

Regards.
Contributor
Posts: 55
Registered: ‎06-16-2016
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Re: RSVP LSP MTU

So I did a ping sweep and....

 

88f0f2bf2507cf431bf6b63e9f15a4e5b4205bd1ce57f995016c5e75265093a5

I'm thinking that the 1500 mtu must be used for something else, but Junos documentation makes it sound like it should be affecting label switched packets. What's the point of mtu-signaling if I can just send whatever size I want (that the ports in transit allow)? Even RFC 3029 referenced by the Junos documentation shows that my 4000 byte ping should have been dropped. Here's an excerpt from section 2.6:

 

   Using the terminology defined in [5], an LSR MUST execute the
   following algorithm:

   1. Let N be the number of bytes in the label stack (i.e, 4 times the
      number of label stack entries) including labels to be added by
      this node.

   2. Let M be the smaller of the "Maximum Initially Labeled IP Datagram
      Size" or of (Path MTU - N).

   When the size of an IPv4 datagram (without labels) exceeds the value
      of M,

      If the DF bit is not set in the IPv4 header, then

         (a) the datagram MUST be broken into fragments, each of whose
             size is no greater than M, and

         (b) each fragment MUST be labeled and then forwarded.

      If the DF bit is set in the IPv4 header, then

         (a) the datagram MUST NOT be forwarded

         (b) Create an ICMP Destination Unreachable Message:

              i. set its Code field [12] to "Fragmentation Required and
                 DF Set",
             ii. set its Next-Hop MTU field [13] to M

         (c) If possible, transmit the ICMP Destination Unreachable
             Message to the source of the of the discarded datagram.
Super Contributor
Posts: 65
Registered: ‎06-21-2017
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Re: RSVP LSP MTU

Hi 

 

Thanks for sharing the output. If possible, could you please share the below output also from ingress router:

 

1.  MPLS interface:

show configuration interfaces ge-0/0/0 

show interfaces ge-0/0/0 | match mtu  

 

2. show rsvp session name <name> extensive 

 

 

 

Regards,

Rahul 

Contributor
Posts: 55
Registered: ‎06-16-2016
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Re: RSVP LSP MTU

Rahul,

 

The interface MTU is set to 9000+. Every physical interface on every router in my lab has the MTU set to 9000+. 

 

I can get the output of "show rsvp session name <name> extensive" when I get back in the lab tomorrow. If you're just looking for the sent and received MTU, then I can tell you now that it's 1500.

 

Do you believe that my pings should have been dropped?

Super Contributor
Posts: 101
Registered: ‎03-11-2012
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Re: RSVP LSP MTU

Hi , again , plz show is "protocols mpls path-mtu rsvp mtu-signaling' is configured. 

 

If it is not configured then read this

If the egress router does not support MTU signaling in RSVP, the MTU is set to 1,500 bytes by default.

and this

MTU signaling in RSVP is disabled by default.

 

So if it is not configured , rsvp will show you mtu = 1500 , but forwarding will ignore this , and will try send packets as big as possible

Super Contributor
Posts: 65
Registered: ‎06-21-2017
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Re: RSVP LSP MTU

Hi Pevangelista,

 

Thank you for sharing the output. Now since we haven't configured RSVP MTU signalling the default mtu value in "rsvp session extensive " output shall be 1500.

Wihout the MTU signalling and considering  we haven't specified MTU under family mpls, the device is using the MTU configured on the interface i.e. 9000+. As per the Juniper document "If you do not configure an MPLS MTU, the Junos OS derives the MPLS MTU from the physical interface MTU."

To confirm this you can test tweaking the MTU value on interface and ping.

 

Below is from the Juniper documentation:

 

http://www.juniper.net/documentation/en_US/junos11.4/topics/usage-guidelines/interfaces-configuring-...

 

If you do not configure an MPLS MTU, the Junos OS derives the MPLS MTU from the physical interface MTU. 

 

In other words, the formula used to determine the MPLS MTU is the following:

MPLS MTU = physical interface MTU – encapsulation overhead – 12
 

If you configure an MTU value by including the mtu statement at the [edit interfaces interface-name unit logical-unit-number family mpls] hierarchy level, the configured value is used.

 

 

 

Regards,

Rahul

 

Please mark my solution as accepted if it helped.

 

Contributor
Posts: 55
Registered: ‎06-16-2016
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Re: RSVP LSP MTU

akushner, do you have any Juniper documentation saying that forwarding ignores the default 1500 MTU? It does appear like this is the case.

 

The following from Juniper TechLibrary contradicts that though:

 

https://www.juniper.net/documentation/en_US/junos/topics/concept/mpls-mtu-signaling-rsvp.html

...

By default, the MTU for an LSP is 1,500 bytes.

If this MTU is larger than the MTU of one of the intermediate links, traffic might be dropped, because MPLS packets cannot be fragmented. Also, the ingress router is not aware of this type of traffic loss, because the control plane for the LSP would still function normally.

To prevent this type of packet loss in MPLS LSPs, you can configure MTU signaling in RSVP.

...

 

 

Contributor
Posts: 55
Registered: ‎06-16-2016
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Re: RSVP LSP MTU

I did some testing and it appears that the mtu shown in "show rsvp session extensive" is ignored with or without mtu-signaling configured.

 

I configured the mpls mtu of a P router to 2000 and configured mtu-signaling.

mpls_mtu_1.PNG

When I did show rsvp session extensive, it shows the Path MTU as 2000. But here's the output of my ping sweep:

 

 

ping mpls rsvp sweep PE1-to-PE3
 100! 5052! 7528! 8768! 9388. 9080. 8924! 9004! 9084. 9044! 9064. 9056. 9052! 9060. 9056.
--- lsp ping sweep result---
Maximum Transmission Unit (MTU) is 9052 bytes

 

 

The result is the same with or without mtu-signaling configured. I was a bit surprised that the mpls mtu didn't make a difference.

 

So I configured the interface on the ingress PE with mpls mtu of 2000.

mpls_mtu_2.PNG

 

This is what my ping sweep looks like with or without mtu signaling:

 

ping mpls rsvp sweep PE1-to-PE3
 100! 5052. 2576. 1340! 2580. 1960! 2272. 2116. 2040. 2000. 1980! 1992! 2004. 2000. 1996!
--- lsp ping sweep result---
Maximum Transmission Unit (MTU) is 1996 bytes

 

Outside of setting the mpls mtu on the ingress router, it appears that I can send whatever size I want as long as the interfaces in transit allow it.

 

If that's the case, what's the point of the RSVP LSP MTU? What's the point of mtu-signaling? It doesn't appear to affect the traffic I'm sending across the LSP.

 

Super Contributor
Posts: 65
Registered: ‎06-21-2017
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Re: RSVP LSP MTU

Hi Pevangelista,

 

It is strange that configuring the MPLS mtu did not work on transit router  and did work on Ingress. I would suggest to open a TAC case to check for such behaviour and MTU singnaling.

 

 

Regards,

Rahul

 

 

Contributor
Posts: 55
Registered: ‎06-16-2016
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Re: RSVP LSP MTU

I thought it was strange as well, but I've repeated this test using vMXs running 16.1 and got the same results. My physical lab is running 13.x.

 

I'm not going to open a TAC case but I'll hit up our RE and see what he says.