MPLS MTU issue in MLPP

Unanswered Question
May 7th, 2008

Hi all,

I m facing an issue in my network. I ve two links bundled in a multilink between 7206 (on PA-MC) and 1841. OSPF and MPLS are running over the multilink. L3 MPLS VPN customer VRF is terminated on 1841 (acting as PE) and trying to access a remote location. 1841 connects to the remaining network via 7206 Multilink only. L2 MTU of Multilink and E1's are 1500, MPLS MTU on multilink set to 1546 bytes. From 1841 (acting as PE), Maximum MTU i'm able to ping for customer remote end with VRF is 1496 bytes. To make it happen on 1500 bytes with Df-bit set, i ve to increase the L2 MTU of multilink by 4 bytes.

I would like to know, whether increasing the L2 MTU of multilink is the only solution available to this option or any other available.

thanks in advance

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guruprasadr Wed, 05/07/2008 - 07:43

HI ArunKumar, [Pls Rate if HELPS]

The 4" bytes added to the 1496 bytes are the VLAN Header.

With the below provided Configuration the Ping with size of 1500 works fine for my MPLS-VPN Customer running with VRF:

Current configuration : 349 bytes


interface Multilink3

bandwidth 6144

ip address xx.xx.xx.xx xx.xx.xx.xx

no ip proxy-arp

ip pim sparse-mode

no ip route-cache cef

no ip mroute-cache

ip ospf message-digest-key 1 md5 7 xxxxxxxx

mpls ip

mpls mtu 1546

no peer neighbor-route

no cdp enable

ppp chap hostname xxxx

ppp multilink

ppp multilink group 3


Router#sh int Mu 3 | in MTU

MTU 1500 bytes, BW 6144 Kbit, DLY 100000 usec,

Hope I am Informative.


Best Regards,

Guru Prasad R

guruprasadr Wed, 05/07/2008 - 07:52

HI ArunKumar, [Pls RATE if HELPS]

>> HI Again,<< More detailed Information follows:

Jumbo: Jumbo frames are frames that are bigger than the standard Ethernet frame size, which is 1518 bytes (including Layer 2 (L2) header and FCS).

Baby giants: The baby giants feature allows a switch to pass or forward packets that are slightly larger than the IEEE Ethernet MTU. Otherwise, the switch declares big frames as oversize and discards them.

As a workaround, you can enable a port for trunking in order to support baby giants. When you enable a port for 802.1q trunking, the switch automatically assumes that an extra four bytes of data are appended on, and increment the frame size of the L2 packet.

Hence, for implementations that require exactly one tag to be carried (either 802.1q or MPLS, but not both), you can configure the port as a trunk port to force the switchport to accept an extra four bytes of data. If the port were to carry multiple VLANs for VLAN-ID tagging or 802.1p prioritization, configure the port as a 802.1q trunk. However, even if VLAN tagging is not necessary, but you want the increased four-byte support, you can configure the port as a 802.1q trunk. Change the Native VLAN to be the one desired to carry the traffic. When you do so, you can accommodate an extra four bytes of data.

Configure :


Use the system mtu command to change the MTU for all 10/100 interfaces. This command only effects 10/100 interfaces.

3750(config)# system mtu 1546

3750(config)# exit

3750# reload

Use the system mtu jumbo command to change the MTU for all Gigabit Ethernet interfaces. This command only effects Gigabit Ethernet Interfaces.

3750(config)# system mtu jumbo 9000

3750(config)# exit

3750# reload

Use the show system mtu command to view the mtu sizes after reload.

Switch# show system mtu

System MTU size is 1546 bytes

Note: The MTU size of 1546 does not include the 18 bytes of the standard Ethernet header and FCS. Therefore, these switches actually support Ethernet frames up to 1564 bytes.

FYI, The above results are tested in the TEST-LAB Environment.

Hope I am Informative.


Best Regards,

Guru Prasad R


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