Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

MPLS MTU issues

Hello I am facing a strange issue and would be grateful to have any advice on this please.

I have an existing setup where Site A and site B are connected within an MPLS setup:

When I ping from Site B to site A with size 1500 it works no problem

ping vrf Data1 192.168.230.2 size 1500

Type escape sequence to abort.

Sending 5, 1500-byte ICMP Echos to 192.168.230.2, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 496/496/500 ms

there is a new site C that has come up with a new link as a back up of site B but when i ping it only allows ping of 1496 and not anything more.

ping vrf Data1 192.168.230.2 size 1496

Type escape sequence to abort.

Sending 5, 1496-byte ICMP Echos to 192.168.230.2, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 500/502/504 ms

ping vrf Data1 192.168.230.2 size 1497

Type escape sequence to abort.

Sending 5, 1497-byte ICMP Echos to 192.168.230.2, timeout is 2 seconds:

.....

Success rate is 0 percent (0/5)

I have compared the configs and they are setup with same MPLS mtu size show as follows

Site B

interface FastEthernet2/0

no ip address

duplex full

speed 100

mpls label protocol ldp

mpls ip

mpls mtu 1524

end

Site C

interface FastEthernet0/1

no ip address

ip flow ingress

load-interval 30

duplex full

speed 100

mpls label protocol ldp

mpls ip

mpls mtu 1524

end

I have asked the provider of the circuit and they say that the MTU sizes on their devices are 1530 same as the link from site B because the switches connected to both the above routers have system MTU as 1530.

I cannot find out what the issue could be ?

any help on this will be much appreciated.

many thanks in advance.

2 REPLIES
Cisco Employee

MPLS MTU issues

Hi Kaushik,

Since I'm not aware of the topology and the routers involved b/t source and destination, I would ask you to folow below streps to Tshoot this as a first step

1. Find the direction in which traffic is failing

ping from C to A and see if the traffic is reaching A by doing "debug icmp" or "show ip traffic | in echo"

2. Once direction is determined, follow by each hop and check the MTU (with ping tests and also verify config)

Try to focus on device/interfaces which does not come in path of B->A (working)

PE-B/A# ping hop1ip size 1500

PE-B/A# ping hop2ip size 1500

3. Check if anywhere mpls mtu is missing.  Also check for situation where we use more than one label.

Regrds,

Sudeep

New Member

MPLS MTU issues

Site A is 7206

Site B is 7206 and

Site C is 2811

1) Directions failing is both directions

When i debug the ip icmp echo

it comes back on Site A router as

*Oct 24 14:08:57.872 UTC: ICMP: dst (10.181.0.1) port unreachable rcv from 10.181.3.105

the utilization on Site C is as follows: could it be a reason why large sized packets are failing?

100 **  *       * ** *  * *    *  *      *   *         *        *   **     *

90 **  *       * ***** * *    *  *      *  *** *      * *      *   ****   *

80 #******    ** **###** **   ****      ****#*** *   ****     **  *##**  **

70 #####***   ** *######********#**   * **####***#*****#**    **  *###***##

60 #######********###########*#####*******###############***  ****#########

50 ##########**#####################***####################******##########

40 ########################################################################

30 ########################################################################

20 ########################################################################

10 ########################################################################

   0....5....1....1....2....2....3....3....4....4....5....5....6....6....7..

             0    5    0    5    0    5    0    5    0    5    0    5    0

                   CPU% per hour (last 72 hours)

                  * = maximum CPU%   # = average CPU%

I have checked the configs and compared with the Site B configs and they seem to good. I have validated with someone else as well.

How do I check if there are places where more than one label is being used?

814
Views
0
Helpful
2
Replies