Improving GRE stability | VPN Tunnels Part 3

Once you’ve built your GRE tunnel, you need to make sure it is stable. One of the potential issues that you may face is called Recursive Routing. This can cause your tunnel to flap repeatedly.

Recursive Routing occurs when underlay routes are incorrectly advertised into the overlay. This can be worse when little attention is paid to LPM (Longest Prefix Match), the route metric, and the administrative distance.

Another concern is the stateless nature of the tunnel. This can result in traffic being blackholed. We can use keepalives (heartbeats), as well as tuning the source and destination interfaces, in order to resolve this issue.

There is a catch though. Keepalives do not work with route-based IPSec encryption. Neither does BFD for that matter! Some valid work arounds include using crypto-maps (policy-based encryption), using routing protocols, or using IP SLA with an EEM script.


Part 1: How GRE Works - See the encapsulation process, as a packet moves from one side of the network to another

Part 2: GRE Encryption with IPSec - GRE is not encrypted by default! See the basics of IPSec, and how we can use it with GRE tunnels

Part 3: Improving GRE Stability - There are a few pitfalls to watch out for, including recursive routing. See some of the best practices that you can apply to make your tunnel stable


For more information, have a look at [ Ссылка ]

Anatomy of GRE Tunnels (by ‘Sarah’): [ Ссылка ]

How to Detect IPSec GRE Tunnel Status: [ Ссылка ]

This video is useful for Cisco #CCNA and #CCNP certifications


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