Course Content
Packet Forwarding
0/2
Network Device Communication
Forwarding Architectures
Spanning Tree
An overview of how switches become aware of other switches and prevent loops.
0/2
Spanning Tree Protocol Fundamentals
Rapid Spanning Tree Protocol
Advanced Spanning Tree Tuning
0/2
Spanning Tree Topology Tuning
Additional Spanning Tree Protection Mechanisms
Multiple Spanning Tree Protocol (MST)
0/1
Multiple Spanning Tree Protocol
VLAN Trunks and EtherChannel Bundles
0/3
VLAN Trunking Protocol (VTP)
Dynamic Trunking Protocol (DTP)
EtherChannel Bundle
IP Routing Essentails
0/4
Routing Protocol Overview
Path Selection
Static Routing
Virtual routing and forwarding (VRF)
EIGRP
0/4
EIGRP Fundamentals
EIGRP Path Metric Calculation
EIGRP Failure Detection and Timers
EIGRP Route Summarization
OSPF
0/4
OSPF Fundamentals
OSPF Configuration
OSPF Default Route Advertisement
Common OSPF Optimizations
Advanced OSPF
The (OSPF) protocol scales well with proper network planning. IP addressing schemes, area segmentation, address summarization, and hardware capabilities for each area should considered when designing a network.
0/6
OSPF Areas
OSPF Link-State Announcements
Discontiguous Networks
OSPF Path Selection
OSPF Routes Summarization
OSPF Route Filtering
OSPFv3
0/3
OSPFv3 Fundamentals
OSPFv3 Configurations
IPv4 Support in OSPFv3
BGP
0/4
BGP Fundamentals
Basic BGP Configuration
BGP Route Summarization
Multiprotocol BGP for IPv6
Advanced BGP
0/6
BGP Multihoming
BGP Conditional Matching
BGP Route Maps
BGP Route Filtering and Manipulation
BGP Communities
BGP Path Selection
Multicast
0/5
Multicast Fundamentals
Multicast Addressing
Internet Group Management Protocol
Protocol Independent Multicast
Rendezvous Points
QoS
0/5
The Need for QoS
QoS Models
Classification and Marking
Policing and Shaping
Congestion Management and Avoidance
IP Services
0/3
Time Synchronization
First Hop Redundancy Protocol
Network Address Translation
Overlay Tunnels
0/4
Generic Routing Encapsulation (GRE) Tunnels
IPsec Fundamentals
Cisco Location/ID Separation Protocol (LISP)
Virtual Extensible Local Area Network (VXLAN)
Wireless Signals & Modulation
0/2
Understanding Basic Wireless Theory
Carrying Data over a Wireless Signal
Wireless Infrastructure
0/3
WLAN Topologies
Leveraging Antennas for Wireless Coverage
Pairing Lightweight APs and WLCs
Understanding Wireless Roam and Location Services
0/3
Roaming Overview
Roaming Between Centralized Controllers
Locating Devices in a Wireless Network
Authenticating Wireless Clients
0/4
Open Authentication
Authenticating with Pre-Shared Key (PSK)
Authenticating with EAP
Authenticating with WebAuth
Troubleshooting Wireless Connectivity
0/2
Troubleshooting Client Connectivity from WLC
Troubleshooting Connectivity Problems at the AP
Enterprise Network Architecture
0/2
Hierarchical LAN Design Model
Enterprise Network Architecture Options
Fabric Technologies
0/2
Software-Defined Access (SD-Access)
Software Defined WAN (SD-WAN)
Network Assurance
0/6
Network Diagnostic Tools
Debugging
NetFlow and Flexible NetFlow
Switched Port Analyzer (SPAN) Technologies
IP SLA
DNA Center Assurance
Secure Network Access Control
0/3
Network Security Design for Threat Defense
Network Access Control (NAC)
Next-Generation Endpoint Security
Network Device Access Control and Infrastructure Security
0/5
Access Control Lists (ACLs)
Terminal Lines and Password Protection
Authentication, Authorization, and Accounting (AAA)
Zone-Based Firewall (ZBFW)
Control Plane Policing (CoPP)
Virtualization
0/2
Server Virtualization
Network Functions Virtualization
Foundational Network Programmability Concepts
0/6
CLI
Application Programming Interface (API)
Data Models and Supporting Protocols
DevNet
GitHub
Basic Python Components and Scripts
Introduction to Automation Tools  
To provide a high-level overview of some of the most common configuration management and automation tools that are available.
0/3
Embedded Event Manager (EEM)
Agent-Based Automation Tools
Agentless Automation Tools
ENCOR Course
About Lesson

Generic Routing Encapsulation (GRE) Tunnels

explains GRE and how to configure and verify GRE tunnels.

  • tunneling protocol that provides connectivity to a wide variety of networklayer protocols by encapsulating and forwarding packets over an IP-based network.
  • GRE can be used to tunnel traffic through a firewall or an ACL or to connect discontiguous networks.
  • The most important application of GRE tunnels is that they can be used to create VPNs.

GRE Packet Headers

  • When a router encapsulates a packet for a GRE tunnel, it adds new header information (known as encapsulation) to the packet. This new header contains the remote endpoint IP address as the destination.
  • The new IP header information enables the packet to be routed between the two tunnel endpoints without inspection of the packet’s payload.
  • When the packet reaches the remote tunnel endpoint, the GRE headers removed (known as de-encapsulation) and the original packet is forwarded out of the router.

GRE Tunnel Configuration

R1 and R2 are using their respective ISP routers as their default gateways to reach the internet. Figure 16-2 GRE Tunnel Topology   Example 16-1 R1’s Routing Table Without GRE Tunnel

GRE Tunnel Configuration (Cont.)

  • Step 1. Create the tunnel interface by interface tunnel tunnel-number.
  • Step 2. Identify the local source of the tunnel by tunnel source {ip-address | interface-id}. The tunnel source can be a physical interface or a loopback interface.
  • Step 3. Identify the remote destination IP address by tunnel destination ip-address.
  • Step 4. Allocate an IP address to the tunnel interface ip address ip-address subnetmask.
  • Step 5. (Optional) Define the tunnel bandwidth for use by QoS or for routing protocol metrics. Bandwidth is defined with the interface parameter command bandwidth [1-10000000], which is measured in kilobits per second.
  • Step 6. (Optional) Specify a GRE tunnel keepalive with the interface parameter command keepalive [seconds [retries]]. The default timer is 10 seconds, with three retries. Tunnel keepalives ensure that bidirectional communication exists between tunnel endpoints to keep the line protocol up.
  • Step 7. (Optional) Define the IP maximum transmission unit (MTU) for the tunnel interface. Specifying the IP MTU on the tunnel interface has the router perform the fragmentation in advance of the host having to detect and specify the packet MTU.

IP MTU is configured with the interface parameter command ip mtu mtu.

 

GRE Tunnel Configuration

Example 16-2 provides a GRE tunnel configuration for R1 and R2, following the steps for GRE configuration listed earlier. With this configuration, R1 and R2 become direct OSPF neighbors over the GRE tunnel and learn each other’s routes.

GRE Tunnel Verification

The state of the GRE tunnel can be verified with the command show interface tunnel number.

GRE Tunnel Verification

Additional commands to verify the status of a GRE tunnel include show ip route and traceroute.

Problems with Overlay Networks

Recursive routing and outbound interface selection are two common problems with tunnel or overlay networks.

  • Recursive routing can occur when the transport network is advertised into the same routing protocol that runs on the overlay network.
  • Routers detect recursive route and generate syslog messages.
  • Recursive routing problems are remediated by preventing the tunnel endpoint address from being advertised across the tunnel network.

 

Other useful information:

Join the conversation
Previous
Next