MPLS MTU issues

Kaushik Ray · ‎10-24-2013

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.

Sudeep Valengattil · ‎10-24-2013

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

Kaushik Ray · ‎10-24-2013

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?