The Journey of a Network Engineer

Oct 17 2011   12:32AM GMT

Loopback address and MPLS-VPN!

Sulaiman Syed Profile: Sulaiman Syed

In this article, i will not go deeply into the issue of configuring MPLS-VPN. The steps were mentioned very clearly in an earlier post. MPLS-VPN Tutorial has all the required details.  I would like to mention a mis-step that i did while doing another MPLS-VPN configuration. what resulted in routing updates to work properly. But no traffic was going from one end to another end. After countless of hours, i found the mistake. Before proceeding, the image below shows the sample network. Download the configurations. They can be used to simulate the network using GNS3.


While i was configuring the BGP VPN section i got the following error.

R6(config-router)# neighbor remote-as 100

R6(config-router)# neighbor update-source Loopback0

R6(config-router)# address-family vpnv4

R6(config-router-af)#  neighbor activate

R6(config-router-af)#  neighbor send-community extended

R6(config-router-af)# exit-address-family

*Mar  1 02:08:59.455: %BGP-5-ADJCHANGE: neighbor Up

*Mar  1 02:08:59.463: %BGP-4-VPNV4NH_MASK: Nexthop may not be reachable from neigbor – not /32 mask

Then, i did not mind the error (highlighted in bold) and carried on with configurations. At the end, i had a full working network with proper routing updates in the MPLS-VPN plan. But no traffic is going. I had to troubleshoot many things. Till the end, i decided to re-configure the routers all over. Then i noticed the error. decided to fix it. Changed the loopback address from /24 to /32. The moment i did that, the traffic started passing.

What i learned, is that “Don’t ignore any messages the IOS gives you while configuring”

1  Comment on this Post

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  • ElshazlyMogahed
     Recommended Action: Configure the loopback used as the next hop with a
    > 32-bit
    > IP host mask (/32),  or configure the ip ospf network point-to-point
    > command on
    > this loopback.
    >>>>>> Example Loop back IP address ( 4 255 which mean /32 ) 
    > BR,
    0 pointsBadges:

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