Thursday, October 29, 2009


Yesterday, I was troubleshooting one of the customer call and checking the forwarding table on each and every hop. surprisingly, one of the router was showing "Tag switching is not operational", "CEF or TAG switching has not been enabled.

Figure 1

I had checked all the configuration and seem to be fined as usual. After that enabled ip cef and was able to see the forwardng table entries. According to the perception, it may be a cef hang issue but the srange thing is that after 5 minutes I rolled back to the previous condition. No cef and tag switching was working but MPLS LDP peering was up with the adjacent neighbors.

The next output astonished me after seeing the ping received from other PE routers of the local vrf created on the router. All the vpnv4 labels were properly advertising to the MPLS cloud. I thought may be I was not seeing anything but everything working fine in the background. So I checked the PHP behaviour of loopback to the adjacent routers.

Figure 2

The adjacent routers showing untagged instead of pop tag. The ios was spservicesk9-mz.123-14.YT1.bin.

Below depicted is the output of vrf and ping from the other PE router.

Figure 3

Figure 4

End to end MPLS was working fine without cef,ldp and rsvp. But we cannot live with this situtaion so need to change the IOS.

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