After working with MPLS Traffic Engineering lab for a few days and interpreting IP addresses from various traceroute outputs, I finally had enough and wrote a simple Perl script (below) that parses router configurations and produces ip host configuration commands for every interface IP address it encounters. When you paste the ip host commands into the configuration of the edge router from which you do the tests, the meaningless numbers finally make sense.

I have always intuitively assumed that the interface bandwidth on MLPPP bundles is the sum of interface bandwidths of individual interfaces that are part of the bundle. Recently I’ve tested my assumption and it works as expected.

An anonymous commenter to my implicit NULL/PHP post made a very valid point:

Most Cisco documentation states that you must enable LDP before doing MPLS-TE, which is a complete fallacy.

If you're using MPLS TE simply to shift IP traffic around your network, he's absolutely right: there is no need to run LDP if you have an IP-only network. If you're running MPLS VPN or BGP on edges/MPLS in the core, the answer becomes “it depends.”

One of my readers has noted an interesting load-balancing behavior: when he was running traceroute tests from various routers in a topology similar to the one displayed below, the traceroute outputs indicated per-packet load balancing (both paths were used) when they were initiated from R2 or R3, but used a single path when initiated from R1 or R4.

 

I've always believed that you need to teach your students (more so if they are engineers) how things work, so they'll be able to understand why they do things they way they do them. It seems to me, though, that the training courses I'm seeing veer ever more toward overviews and recipes ... but there are a few things you can do on your own.

To display bandwidths of all interfaces configured on the router use show interface | include protocol|BW command.

End-to-end troubleshooting of MPLS VPN solutions is one of the more complex network troubleshooting tasks. On top of several sophisticated technologies and protocols used in MPLS VPN solutions, we have to deal with customer-to-provider interaction on the IP routing protocol level, which makes the troubleshooting efforts even more convoluted.

To minimize the impact of your customers on your troubleshooting efforts, you might want to start with the PE-to-PE troubleshooting. When used as the first step in your troubleshooting process, the PE-PE tests will bypass customer errors, intra-site customer routing problems, PE-CE interactions, and route redistribution issues.

In one of the MPLS-related posts, I’ve described the role of implicit NULL in penultimate hop popping (PHP). To make the distinction between implicit and explicit NULL even clearer, I’ve prepared a short explanation with corresponding diagrams.

To display IP addresses assigned to router’s interfaces (excluding interfaces with no IP address) use show ip interface brief | exclude unassigned command.

The RFC 3101 (OSPF NSSA Option) states:

In addition, an NSSA border router should originate a default LSA (IP network is 0.0.0.0/0) into the NSSA. Default routes are necessary because NSSAs do not receive full routing information and must have a default route in order to route to AS-external destinations.

I am pretty sure IOS inserted the type-7 default route into an NSSA area when the NSSA feature was introduced.