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New Member

Issue with a MPLS Backup Link // 802.Q

HI all,

We are deploying a MPLS network and the thing is we have an issue in the backups links of the MPLS network, the only difference between these links and the principal links is that also carry the 802.q tag, we have increased the MTU in the backup interfaces from 1508 to 1530 but there is no way to send a ping greater than 1500.... by the way the principal lines are working fine... and configs looks good.

Any idea about it?

Thanks in advance,

3 ACCEPTED SOLUTIONS

Accepted Solutions

Re: Issue with a MPLS Backup Link // 802.Q

Can you please give more info on IOS and hardware involved (routers and switches)? What did you do to proove a ping "greater than 1500" would not get through?

Did you set the interface mtu or the MPLS mtu to solve the problem?

Regards, Martin

Re: Issue with a MPLS Backup Link // 802.Q

Hello,

this might not be related to MPLS at all. Have you tried to send the ping without MPLS implemented in a test environment?

Could be a bug, but also that you have overlooked an interface MTU (including "backplane interfaces").

Regards, Martin

Re: Issue with a MPLS Backup Link // 802.Q

Hello Enrique,

the issue was to my understanding:

you had MTU set to a value (f.e. 1560), which is used for any data "above Datalink layer". This means IP and labeled packets could have the max size of 1560. Now when an IP packet with max size arrives and is labeled, then the resulting packet is larger than the MTU. Hence it can not be forwarded - there is no fragmentation for labeled packets.

Once you configure "ip mtu 1500" the LSR is instructed, that the outgoing interface allows for max 1500 Byte IP. Thus the packet will be fragmented if larger. After that labels are added and the resulting labeled packet will be smaller than interface mtu. And everything is fine, no need to drop.

The other option, by the way, is to leave MTU at default values and to increase the MTU for labeled packets.

1)

interface GigabitEthernet2/0

mtu 1530

ip mtu 1500

2)

interface GigabitEthernet2/0

mpls mtu 1530

Hope this helps! Please rate all posts.

Regards, Martin

8 REPLIES

Re: Issue with a MPLS Backup Link // 802.Q

Can you please give more info on IOS and hardware involved (routers and switches)? What did you do to proove a ping "greater than 1500" would not get through?

Did you set the interface mtu or the MPLS mtu to solve the problem?

Regards, Martin

New Member

Re: Issue with a MPLS Backup Link // 802.Q

OK, here is the information:

RouterA (PE): 7304. IOS c7300-k91p-mz.122-27.SBC2.bin

RouterB (P):7606. IOS s72033-pk9sv-mz.122-18.SXD6.bin

When I type the normal ping command I can reach the other router without problem, but an extended ping with the datagram size over 1500 don´t works and the debug ip icmp don´t shows any information, which means traffic is not leaving the router...

The interface mtu is set in both ends at 1530.

Re: Issue with a MPLS Backup Link // 802.Q

Hello,

this might not be related to MPLS at all. Have you tried to send the ping without MPLS implemented in a test environment?

Could be a bug, but also that you have overlooked an interface MTU (including "backplane interfaces").

Regards, Martin

Re: Issue with a MPLS Backup Link // 802.Q

Hello,

this might not be related to MPLS at all. Have you tried to send the ping without MPLS implemented in a test environment?

Could be a bug, but also that you have overlooked an interface MTU (including "backplane interfaces").

Regards, Martin

New Member

Re: Issue with a MPLS Backup Link // 802.Q

I removed the MPLS config from the interface and results are the same, the backup goes via Metro Ethernet... I´ll try to check the MetroEthernet links.

Thanks a lot for your help.

Regards,

New Member

Re: Issue with a MPLS Backup Link // 802.Q

mheusinger,

Thanks a lot, you were completely right, there was an issue with the MTU, the "ip mtu" was the same than "mtu", 1560. I typed the "ip mtu 1500" command in the boths subinterfaces ends and at now it works fine, I can reach the other with an extended ping and a datagram size over 1500, It´s difficult for my understand the difference between "ip mtu" and "mtu" commands...

Re: Issue with a MPLS Backup Link // 802.Q

Hello Enrique,

the issue was to my understanding:

you had MTU set to a value (f.e. 1560), which is used for any data "above Datalink layer". This means IP and labeled packets could have the max size of 1560. Now when an IP packet with max size arrives and is labeled, then the resulting packet is larger than the MTU. Hence it can not be forwarded - there is no fragmentation for labeled packets.

Once you configure "ip mtu 1500" the LSR is instructed, that the outgoing interface allows for max 1500 Byte IP. Thus the packet will be fragmented if larger. After that labels are added and the resulting labeled packet will be smaller than interface mtu. And everything is fine, no need to drop.

The other option, by the way, is to leave MTU at default values and to increase the MTU for labeled packets.

1)

interface GigabitEthernet2/0

mtu 1530

ip mtu 1500

2)

interface GigabitEthernet2/0

mpls mtu 1530

Hope this helps! Please rate all posts.

Regards, Martin

New Member

Re: Issue with a MPLS Backup Link // 802.Q

HI Martin,

Thanks a lot for the explanation, now I understand the issue and the solution.

Regards,

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