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EVN & VRF Questions

July 21st, 2017 in ROUTE 300-101 Go to comments

Quick review:

Easy Virtual Network (EVN) is an IP-based network virtualization solution that helps enable network administrators to provide traffic separation and path isolation on a shared network infrastructure. EVN uses existing Virtual Route Forwarding (VRF)-Lite technology to:
+ Simplify Layer 3 network virtualization
+ Improve shared services support
+ Enhance management, troubleshooting, and usability

Question 1


All the subinterfaces and associated EVNs have the same IP address assigned. In other words, a trunk interface is identified by the same IP address in different EVN contexts. EVN automatically generates subinterfaces for each EVN. For example, both Blue and Green VPN Routing and Forwarding (VRF) use the same IP address of on their trunk interface:

vrf definition Blue
vnet tag 100
vrf definition Green
vnet tag 200
interface gigabitethernet0/0/0
vnet trunk
ip address

-> A is correct.

In fact answer B & C are not correct because each EVN has separate routing table and forwarding table.

Note: The combination of the VPN IP routing table and the associated VPN IP forwarding table is called a VPN routing and forwarding (VRF) instance.

Question 2


EVN is supported on any interface that supports 802.1q encapsulation, for example, an Ethernet interface. Instead of adding a new field to carry the VNET tag in a packet, the VLAN ID field in 802.1q is repurposed to carry a VNET tag. The VNET tag uses the same position in the packet as a VLAN ID. On a trunk interface, the packet gets re-encapsulated with a VNET tag. Untagged packets carrying the VLAN ID are not EVN packets and could be transported over the same trunk interfaces.

Reference: http://www.cisco.com/c/en/us/products/collateral/ios-nx-os-software/layer-3-vpns-l3vpn/whitepaper_c11-638769.html

Question 3


An example of using “autonomous-system {autonomous-system-number}” command is shown below:

router eigrp 100
address-family ipv4 vrf Cust
autonomous-system 100
no auto-summary

This configuration is performed under the Provide Edge (PE) router to run EIGRP with a Customer Edge (CE) router. The “autonomous-system 100” command indicates that the EIGRP AS100 is running between PE & CE routers.

Question 4

Question 5

Question 6


EVN builds on the existing IP-based virtualization mechanism known as VRF-Lite. EVN provides enhancements in path isolation, simplified configuration and management, and improved shared service support

Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/evn/configuration/xe-3s/evn-xe-3s-book/evn-overview.html

Maybe the “improved shared services support” term here implies about the support of sharing between different VRFs (through route-target, MP-BGP)

Question 7


This question is not clear because we have to configure a static route pointing to the global routing table while it stated that “all interfaces are in the same VRF”. But we should understand both outside and inside interfaces want to ping the loopback interface.

Question 8


EVN supports IPv4, static routes, Open Shortest Path First version 2 (OSPFv2), and Enhanced Interior Gateway Routing Protocol (EIGRP) for unicast routing, and Protocol Independent Multicast (PIM) and Multicast Source Discovery Protocol (MSDP) for IPv4 Multicast routing. EVN also supports Cisco Express Forwarding (CEF) and Simple Network Management Protocol (SNMP).

Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/evn/configuration/xe-3s/evn-xe-3s-book/evn-overview.html

Question 9


Route-target is is tagged to each VPN when it is exported. In other words, when a prefix is exported with a route-target, an extended BGP community is attached to that prefix. If this community is matched with the (import) route-target of the receiving side then the prefix is imported to the receiving VRF.

Question 10


Easy Virtual Network (EVN) is an IP-based virtualization technology that provides end-to-end virtualization of two or more Layer-3 networks. You can use a single IP infrastructure to provide separate virtual networks whose traffic paths remain isolated from each other.

An EVN trunk interface connects VRF-aware routers together and provides the core with a means to transport traffic for multiple EVNs. Trunk interfaces carry tagged traffic. The tag is used to de-multiplex the packet into the corresponding EVN. A trunk interface has one subinterface for each EVN. The vnet trunk command is used to define an interface as an EVN trunk interface.

In other words, EVN trunk interfaces allow multiple VRFs to use the same physical interfaces for transmission but the data of each VRF is treated separately. Without EVN trunk interfaces we need to create many subinterfaces. Therefore virtual network trunk (VNET) decreases the network configuration required.

Note: There is no “Easy Trunk” component or technology.

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  1. Anonymous
    February 26th, 2019

    Which two statements about ICMP unreachable messages are true?(Choose two)
    A. They are sent when a route to the destination is missing from the routing table
    B. They can be enabled and disabled on a device only on a global level
    C. They are sent when a destination address responds to an ARP request
    D. They include the entire packet so that the source can identify the process that generated the message
    E. They include a portion of the original data so that the source can identify the process that generated the message

    Answer: A, C

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