Basic E-TREE
Load basic.vpls.ldp.init.cfg
The CE routers CE1-3 are preconfigured with EIGRP. Configure VPLS using LDP for signaling and manual discovery.
CE1 should be the root of the E-TREE. CE2 and CE3 should be spokes. In E-TREE, spokes do not have direct connectivity. Verify that CE2 and CE3 form EIGRP neighborships with CE1 but not with each other.
Answer
Explanation
E-TREE is a subset of an E-LAN, in which some devices are roots and others are spokes. Roots can communicate with other roots and spokes. Spokes can only communicate with roots.
We can enforce the E-TREE simply with the way we form pseudowires. If PEs with spokes do not form PWs with other PEs with spokes, we can achieve the E-TREE. However, this only works in simple cases where a PE does not have a mix of root and spoke CEs that are directly connected.
In this lab we use tLDP to form the PWs. R1 forms a full mesh of PWs since it has a root node. R3 and R5 only form a PW with R1 since only spokes connect to them.
Verification
CE2 and CE3 only have CE1 as an EIGRP neighbor:
R3 and R5 only have a pseudowire with R1:
Workaround for spoke to spoke connectivity
An interesting way to enable spoke to spoke connectivity is to have CE1 answer ARPs for other spokes on their behalf, but with CE1’s own MAC address. So CE1 answers proxy ARPs with its local MAC. This causes traffic to hairpin at layer 2!
This won’t allow EIGRP to work because that is still multicast, and each spoke PE is only delivering the multicast to R1.
However, we can implement a trick to get this to work as well. We can disable split horizon on the PWs on R1. This won’t form a loop because R3 and R5 do not have a PW to each other.
Split horizon is now disabled for the PWs to R3 and R5:
CE2 and CE3 form an EIGRP neighborship. This is because R1 receives the multicast from R3/R5 and floods it out the other PW because split horizon is turned off.
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