Chapter 3) Implementing Data Center Overlay Protocols

Question 1

Describe the function of MAC routing in regards to OTV.

Answer 1

MAC routing is the process of using a control plane protocol is used to exchange MAC reachability information between network devices.

Question 2

Describe the function of an ED in regards to OTV

Answer 2

An edge device (ED) connects a regular LAN to the WAN/MAN core and is
responsible for performing all the OTV functions. L2 traffic destined for a remote network is encapsulated into an L3 packet before being sent across the OTV transport infrastructure.

Question 3

Describe the function of an OTV join interface.

Answer 3

The join interface is the outside-facing interface on an OTV edge device (ED). This interface is used to discover other EDs and form adjacencies. Join interfaces are also responsible for receiving regular traffic and MAC reachability information.

Question 4

Describe the function of an OTV overlay interface.

Answer 4

OTV overlay interfaces are purely logical multicast-capable interface. This interface is responsible for encapsulating regular L2 into IP packet headers.

Question 5

What is the purpose of the OTV control plane?

Answer 5

The OTV control plane is built between OTV edge devices and is used to advertise MAC reachability information rather than relying on conventional data plane learning.

Question 6

What are the 2 types of ways that devices can form OTV adjacencies?

Answer 6

Edge devices can be made adjacent in two ways, depending on the nature of the transport network interconnecting the various sites:

• Using a specific multicast group to exchange the control protocol messages between the OTV edge devices (Multicast).
• Or by configuring an ED as an adjacency server, which all other EDs in the overlay will register with (Unicast Only).

Question 7

What routing protocol is used to implement the OTV control plane?

Answer 7

The OTV control plan is formed using IS-IS. There is not dependency on any other routing protocols such as OSPF, RIP, BGP, etc…

Question 8

How does data get sent to multiple recipients when using an adjacency server in an OTV overlay?

Answer 8

Since the adjacency server mode only supports unicast, regular broadcast and multicast messages cannot be forwarded onto the overlay. When a multicast or broadcast message is sent to the overlay, multiple copies of the same message are created and individually unicasted to each recipient.

Question 9

What bit is set for all control and data plane packets passing over the OTV overlay?

Answer 9

All OTV control and data plane packets originate from an OTV edge device with the Don’t
Fragment (DF) bit set. In a Layer 2 domain, the assumption is that all intermediate LAN segments support at least the configured interface MTU size of the host.

Question 10

How are ASM and SSM multicast groups used in the OTV overlay?

Answer 10

Any source multicast (ASM) groups are used to transport the OTV control protocol between sites, whereas the source specific multicast (SSM) are used to carry Layer 2 multicast streams.

Question 11

What is the purpose of failure isolation in regards to OTV overlays?

Answer 11

To limit the amount of broadcast traffic that is delivered to sites across the overlay.

Question 12

What 4 main functions are used in OTV to provide failure isolation?

Answer 12

OTV achieves this goal by providing four main functions:

1) Spanning Tree Protocol (STP) isolation
2) Unknown unicast traffic suppression
3) ARP optimization
4) Broadcast policy control.

Question 12

What 4 main functions are used in OTV to provide failure isolation?

Answer 12

OTV achieves this goal by providing four main functions:

1) Spanning Tree Protocol (STP) isolation
2) Unknown unicast traffic suppression
3) ARP optimization
4) Broadcast policy control.

Question 13

What is the purpose of STP Isolation in regards to OTV overlays?

Answer 13

STP Isolation is used to prevent BDPUs from crossing over the overlay. EDs do not forward BDPUs, and as such, each site has its own individual spanning tree configuration that is independent from other sites in the OTV overlay.

Question 14

What is the pur

Question 14

What is the pur

Question 14

What technology is used in OTV overlays to prevent the creation of end-to-end loops?

Answer 14

Multihoming is used to prevent end-to-end loops without sending STP frames across the overlay.

Question 15

What happens when a frame destined for an unknown unicast host is received by an OTV ED?

Answer 15

Just like a regular L2 network, the ED will flood the request out of all internal interfaces on the LAN. However, this request will not be broadcast to the overlay.

Question 16

How does ARP Optimization function in regards to OTV overlays?

Answer 16

When ARP requests are broadcasted across the OTV overlay, the intended host will respond with it’s IP address similarly to a traditional L2 LAN. The OTV ED that sent the initial ARP request will then cache the reply from the responding host.

OTV EDs can snoop ARP mappings in the ARP ND (Neighbor Discovery) table. Effectively syncing the ARP tables across the EDs in the overlay. When another host in the overlay needs to perform a lookup on this same host, the ARP information is cached in the EDs ARP ND table.

Question 17

What are the Cisco Nexus default timers for the OTV ARP & MAC aging timers?

Answer 17

The Cisco NX-OS defaults are as follows:

• OTV ARP aging timer: 480 seconds / 8 minutes
• MAC aging timer: 1800 seconds / 30 minutes

Question 18

What is the purpose of an AED election in regard to OTV overlays?

Answer 18

Authoritative edge devices (AEDs) are used when a LAN has a connect to 2 or more EDs. The AED role is negotiated on a per-VLAN basis and is responsible for the following:

• Forwarding layer 2 traffic onto the overlay.
• Advertising MAC reachability information to other remote EDs.

Question 19

Define the terms site adjacency and overlay adjacency in regards to OTV edge devices (EDs).

Answer 19

The two OTV adjacency types are used to prevent the creation of an end-to-end loop in the overlay:

• Site Adjacency: each OTV ED maintains an adjacency with all other EDs at the same site. These EDs use the site VLAN for discovering and establishing adjacencies with one another.
• Overlay Adjacency: the join interfaces across the overlay are used to create an adjacency. Site identifiers are used to identify which EDs belong to each site. EDs at the same site will have the same site identifier.

Question 20

What is the purpose of an OTV AED VLAN Split?

Answer 20

At a DC site that has more than 1 edge device (ED), and authoritative ED will be elected (AED). However, to enable load balancing between 2 devices, an AED VLAN split can be configured to allow one ED to forward traffic on even VLANs, while another ED is responsible for forwarding traffic on odd VLANs.

Question 21

What is the purpose of FHRP isolation in regards to OTV overlays?

Answer 21

Since different sites can have the same IP & VLAN scheme , FHRP messages should be filtered across the overlay. This will prevent the creation of suboptimal routes when traffic passes across the overlay.

Question 22

What transport protocol is used with A VXLAN?

Answer 22

The transport protocol over the physical data center network is IP plus UDP. VXLAN defines a MAC-in-UDP encapsulation scheme where the original Layer 2 frame has a VXLAN header added and is then placed in a UDP-IP packet.

Question 23

What is the purpose of a VNID in regards to VXLANs? How many bits are in a VNID and how many LANs are supported?

Answer 23

A VXLAN VNID (VXLAN Network Identifier) is used to identify Layer 2 segments and provide layer 2 isolation between segments. A VNID is a 24-bit value that can support up to 16 million LAN segments.

Question 24

What is the purpose of a VXLAN VTEP?

Answer 24

A VXLAN tunnel endpoint (VTEP) is used to map tenants end devices to VXLAN segments and to perform VXLAN encapsulation and decapsulation .

Question 25

What is the purpose of an infrastructure VLAN IP address in regards to VXLANs?

Answer 25

The interface IP address faces toward the outside of the transport-IP network and performs the following functions:

• Identifies the VTEP device on the overlay
• Encapsulating regular L2 frames and transmitting them onto the transport network
• Discovering remote VTEPs
• Learning MAC Address to VTEP mappings

Question 26

What is the purpose of a VNI in regards to VXLANs?

Answer 26

A virtual network identifier (VNI) is used to identify a specific virtual network in the data plan.

Question 27

What are the 2 VNI scopes used in VXLANs?

Answer 27

Network-wide scoped VNIs: used to identify the specific Layer 3 virtual network across all network edge devices.
Locally assigned VNIs: the identifier has local significance to the network edge device that advertises the route.

Question 28

What are the two widely adopted VXLAN Control Planes?

Answer 28

Two widely adopted control planes are used with VXLAN:

• VXLAN Flood and Learn Multicast-Based Control Plane.
• VXLAN MPBGP EVPN Control Plane.

Question 29

What protocol is used with VTEPs to join the multicast group?

Answer 29

VTEPs use Internet Group Management Protocol (IGMP) messages to join the transport overlay network by sending protocol independent multicast (PIM) messages.

Question 30

How is broadcast, unknown unicast, and multicast traffic sent to limit layer 2 flooding in the VXLAN overlay?

Answer 30

Multicast groups are used to limit Layer 2 flooding to devices that have end systems participating in the same VXLAN segment.

Question 31

What node role (as described in the BGP MLPS EVPN model) is equivalent to the VTEP/NVE device?

Answer 31

The PE (Provider Edge) node role.

Question 32

What BGP device is used to reduce the amount of full mesh BGP sessions between VTEPs?

Answer 32

A BGP route reflector (RR) is used to create a single session between each VTEP and the RR.

Question 33

What are the 2 types of VXLAN gateways used to connect to classic ethernet segments?

Answer 33

Layer 2 Gateway: A Layer 2 VXLAN gateway is a device that encapsulates a classical Ethernet (CE) frame into a VXLAN frame and decapsulates a VXLAN frame into a CE frame.

Layer 3 Gateway: a VXLAN router is required for communication between devices that are in different VXLAN segments. The VXLAN router translates frames from one VNI to another.