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Iam enabling MPLS (LDP) on the backbone of my network. The connected routers are 7613 at the central location and 7200 at remote location. Both sides the e1 termination are on Channelised E1 ports

The topology is attached

The moment I enable ldp neighborship bet the routers i loose connectivity to the remote location routers ethernet and loopback ips. But all the device in the remote location lan say other routers switches and pcs are reachable

Iam pinging from a vlan (noc vlan) which is behind a firewall and connected to a cisco core switch 6500

The core routers ethernet is also connected to the same switch but this falls on diff vlan called wan vlan.

Iam running hsrp bet core routers ethernet.

I suspect some mtu issues . Should i adjust mtu in the ethernet

Commands used globally are

mpls ip

mpls label protocol ldp

Interface level (serial interface)

mpls ip

mpls label protocol ldp

Iam not enabling MPLS on ethernet

Some help is appreciated


Re: MPLS MTU ISSUES - urgent


By any chance you have mpls enabled on the ethernet ports of the routers ?

Also is your switch capable of handling giant frames/jumbo frames ?

can you post the exact topology with lan devices included and also the sucessful traffic flow path details..


Community Member

Re: MPLS MTU ISSUES - urgent


I have not enabled MPLS on the ethernet interface both on the noc side.

The core router 7613 in the noc connects to 6513 physically.

But traffic entering the corerouter towards noc has to pass through logically via pix firewall 535

That is the ethernet of 7613 and wan interface of firewall are on the wan vlan in 6513 switch.

iam pinging the remote router at the branch office from one of the inside vlan connected the same switch sitting on noc vlan again on the insdie zone to the firewall

should i enable giant/jumbo frames on my 6513

or should i increase the mtu on my ethernet of 7613 and 6500 l2 switch ports

i have seen some cisco docs recommending the mtu to increase 9216

your suggestion awaited

Community Member

Re: MPLS MTU ISSUES - urgent


What I understood from your message is that the packets coming to the NOC 7613 router is passing through firewall and 6513 and then reaches the 7613.

If thats the case then 6500 L2 switch port ASCIIs don't accept the jumbo frame without the MTU command in the switchport. So try increasing the MTU to higher value by "MTU" command under the switchport connects to firewall and the 7613. Also you will need to configure "system jumbo mtu " global config command. Do the same stuff on the 7613 remote branch facing ethernet interface.

If it doesn't work please post the logical topology with the firewall and 6513 and 7613 connections.

Community Member

Re: MPLS MTU ISSUES - urgent

I've been here before. We had to increase the MTU from 1500 to 1536 to allow for the frame to hold the lable part of mpLs.

Community Member

Re: MPLS MTU ISSUES - urgent

Hi Scott

Thanks for your response

I will try this today.

But as one of our friend was suggeting to make the switch support the jumbo frames, I will try this also.

But I wonder as none of my ethernet interface is enabled with MPLS why should we alter the mtu.

A packet entering the backbone (mpls domain) at the LER 7200 at branch office the label is pushed on to the packet and it is poped at 7613 at noc.(next hop) and vice versa for packet leaving my noc towards branch location.

Iam enbabling ldp on serial interfaces only between these routers .

Further all my branch office which has 7200 has two links terminated on core router 1 and 2 links terminated on core router 2 .

Iam running hsrp on the ethernet side between the core routers on the noc side

Iam noticing a log on the branch router 7200

stating could not bind label for x.y.x.y (hsrp ip) as already bound for ldp neighbour y.y.y.y. Why is this log message appearing on the branch router

Community Member

Re: MPLS MTU ISSUES - urgent


IS your 7613 and 7200 branch office router are connected directly through serial interface?. I understood from your previous message that there is a firewall before 7613 and both are interconnected by 6513 L2 switch ports (in WAN vlan).

Community Member

Re: MPLS MTU ISSUES - urgent

yes you are right

both are directly connected on seria, that is branch office 7200 connects on 2 e1 links to core 1 (7613) at noc and 2 e1 links to core 2 (7613).

There is a intercore link whichi is a portchannel between these two core routers

iam enabling only on the serial links not on ethernet anywhere

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