Archive for February, 2012

BGP over GRE

February 21, 2012 10 comments

While I was doing some BGP labs I came across an interesting topic which is BGP over GRE. Using automatic tunneling techniques along with BGP is the core of MPLS VPNs and I think it is worth seeing the effect of using manual tunnels along with BGP. Let´s have a look at the following topology which illustrates my example:

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EIGRP STUB challenge

February 7, 2012 4 comments

I would like to invite all my readers to participate to this EIGRP challenge which highlight the EIGRP STUB feature. As you may already know EIGRP use the query/reply process to find an alternate path for a route that it has lost if no feasible successor are present in the topology table. This process can slow down EIGRP convergence. To improve this behavior which is inherent to EIGRP (distance vector protocol, only knows routes being advertised by neighbors, unlike OSPF which has a full overview over the topology) there are 2 solutions. The goal of these 2 solutions is to reduce the query domain.

  • The first solution is to use summarization. When a router lose a route it sends a query to all its connected neighbors to ask for an alternate path. If one of the neighbor has a summary route containing the more specific prefix lost by the neighbor, the query process will stop and the neighbor will send a reply back to the querier router telling him that it has no route for this prefix. The query will not go further. If summarization was not configured the query will have propagated through the network until it eventually reach a last hop router.
  • The second solution is to use the EIGRP STUB feature. When a router is configured as a EIGRP STUB it will annonce itself as so to its connected neighbors. Then these neighbors will know that he is the last hop router and hence will not send query to him.

In this challenge I would use the EIGRP STUB feature to demonstrate what could be the consequence of using this feature in the wrong place in the network. Please consider the following topology:

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