WS-X6724 MTU issue

[mazlan.alatif]

I have cisco 7606 with module WS-X6724. I have configured mpls mtu 2000 on the router interface configuration (P router).

However when we tested from MPLS VPN CE to CE ping test, we cant achive 1500 bytes end-to-end. We transfered the link to use Ge on the sup720 and it works fine (CE to CE 1500 bytes).

I have once again use port on the WS-X6724 on other router, still failed to achive CE-CE. Upon checking we found that this port dont really support MPLS MTU even it accepts the config.

Does this harware failure or IOS bugs?

--mazlan

[Harold Ritter]

Mazan,

I know there was a bug that is fixed in 12.2(18)SXE an that had for consequence that you could configure the "mpls mtu" command but it wouldn't really have any effect if the value configured was bigger than the interface mtu.

You could either upgrade or the work around is to increase the interface mtu instead of the mpls mtu.

Regards,

[mazlan.alatif]

Harold,

I have thought that increasing the interface MTU will solve the issue. However I cant increase int MTU on the 7606 since its connected to another P router which is cisco 7507 Ge (which we cant change the int MTU).

I believe this will also impact other Ethernet interfaces that the MTU cant be changed.

In my cases, we uses 7606 WS-6724 as aggreation which connected to many other routers. Many of its interface cant changed the MTU. If we run VPLS, EToM, it will 'chock' the packet size.

--mazlan

[Harold Ritter]

Mazlan,

In this case, you can increase the interface mtu on the WS-6724 and set the IP mtu to 1500, which functionaly would give you the same result as increasing the mpls mtu.

The other option is to upgrade the code on the 7600.

Regards,

[mazlan.alatif]

Harold,

The interface MTU itself is 1500, thus the IP will follow this as default, is it? How about for MPLS IP which requieres at least 1508 bytes? Ussually we will increase the MPLS MTU. In my case, WS-6724, it seems it doest really take effect of my configs, even i have increase the MPLS MTU.

(config-if)#ip mtu ?

<68-1500> MTU (bytes)

[mazlan.alatif]

harold,

I have made a testing, increase the MTU at both side. But still MPLS packet wont achive 1500 bytes from CE-CE.

Max it can go 1496 bytes end to end. Below is config at int.

P/PE1

==

interface GigabitEthernet2/10

mtu 2000

ip address 10.1.1.1 255.255.255.252

mpls mtu 1580

mpls label protocol ldp

mpls ip

end

P/PE2

interface GigabitEthernet0/1

mtu 2000

ip address 10.1.1.2 255.255.255.252

duplex auto

speed auto

media-type rj45

no negotiation auto

mpls label protocol ldp

tag-switching mtu 1580

tag-switching ip

end

--mazlan

[Harold Ritter]

Mazlan,

Are these two devices 7600? What is in between, 7500s? I suspect that the issue at this point is sitting int between these devices.

Regards,

[mazlan.alatif]

Harold,

in the tsting lab, i have connected 7206 G-1 to the 7606 and the result is still the same. if i connect the 7206 to Ge interface on the sup720 on 7606, no mtu issue. it just happen if connect it to the ws-6724 module.

i am afraid if this is bugs.

--mazlan