You are correct about a link connecting 2 sites together as a "backdoor". In the link you posted, the diagram shows customer site 4 & 5 having this backdoor link. This link could be a p2p link, LTE, or broadband, etc., using IPSec to provide redundancy for the site.(it could also be a legacy connection of some kind) It's not often I've seen this in production, but I have actually seen it. Typically you use domain-id's when dealing with backdoor links.
Sent: 09-25-2021 06:20
From: Unknown User
Subject: Question on OSPF backdoor link.
Here is Junos Documentation on an OSPF Sham link.
I understand what a Sham link does.
But the documentation references an OSPF backdoor link between L3VPN customer sites. What exactly is a backdoor link? Is it a physical connection between sites? That would seem unlikely since the sites are geographically distant (hence the use of an L3VPN to interconnect them).
Can anyone describe how a backdoor link is implemented?
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