MPLS questions

Hi,

I need something cleared up. We have two networks at work, using 2 different providers. One is Sprint with MPLS. The other is Qwest with MPLS.

If I tracert some of our networks through Sprint's MPLS network, I can see the PE and P routers all along the way.

If I do this in Qwest's MPLS network, I never see any P routers. Its usually just 3 hops.

Are there different kinds of MPLS? I assumed Qwest's MPLS was MPLS-VPN since I do not see the provider routers. If yes, then what is Sprint's?

thanks

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Comments

networker050184

They are most likely using the "no mpls ip propagate-ttl" feature which causes the router not to copy the TTL to the label. That way only the non labeled hops send back the hops for the traceroute.

Sett

Ops, didn't see that networker050184 already replayed.

Probably Qwest's routers have TTL Propagation disabled - this means that the TTL of the original IP packed is not copied to the MPLS Label's TTL field. The Labels TTL is set to 255. In result the traceroute doesn't receive TTL expired message until it reaches the edge of the MPLS Network where the label is stripped and IP Packet's TTL is used again. This "no mpls ip propagate-ttl " command causes the whole MPLS network to be seen as a single hop by the end user. Pretty cool.

wera711

Great, thanks!
Do you guys know if a 2621xm maxed out on memory can run MPLS ok?

networker050184

Check the feature navigator for your IOS.

wera711

Last question......Are the PE routers generally also the LERs? I'm assuming that is where the labeling occurs?

networker050184

Yes the LER is basically the same thing as a PE router. They both serve as the entry/exit to the MPLS cloud.