NAT and Multicast

Question 1

Static NAT

Answer 1

Translates one inside local and one inside global address

R2(config)# ip nat inside source static 10.0.12.1 203.0.113.1

Question 2

Dynamic NAT

Answer 2

Inside global is dynamically allocated from a pool of global addresses. Sometimes called pooled NAT.

R2(config)# ip nat pool R2dynamicNAT 2.0.0.1 2.0.0.3 netmask 255.255.255.248

R2(config)# access-list 1 permit 7.0.0.0 0.0.0.255

R2(config)# ip nat inside source list 1 pool R2dynamicNAT

Question 3

NAT overload or PAT

Answer 3

Translates multiple inside local source addresses to a single global address. Also translates the original source port to a different source port

R2(config)# access-list 1 permit 7.0.0.0 0.0.0.255

R2(config)# ip nat inside source list 1 interface gi0/0 overload

Question 4

PAT with a Global Address Pool

Answer 4

Identical to dynamic NAT, but with the addition of the overload keyword

R2(config)# ip nat inside source list 1 pool R2dynamicNAT overload

Question 5

Outside Static NAT

Answer 5

Translate an inside local destination ip address of a packet to an outside global address. The add-route keyword creates a static route

R2(config)# ip nat outside source static 10.0.12.1 1.1.1.1 add-route

Question 6

Describe Multicast

Answer 6

Refers to any form of one-to-many communication.

A multicast IP packet always has a multicast destination address (that’s what makes it a multicast packet), and it may have a TTL of 1. It will never contain a multicast source address.

Question 7

IP Multicast

Answer 7

IP packet destined to a special multicast ip address. The multicast IP address range is 224.0.0.0/4 which covers 224.0.0.0 through 239.255.255.255

Question 8

Multicast Address Categories

Answer 8

Local Network Control (224.0.0.0/24). They don’t leave the broadcast domain. OSPF 224.0.0.5, 224.0.0.6 and EIGRP 224.0.0.10 are examples

Internetwork Control Block (224.0.1.0/24). These may be routed over the public internet. Commonly used for stock ticker feeds and streaming video

Administratively Scoped Block (239.0.0.0/8). Used on non-publicly routable networks.

Question 9

LAN Multicast

Answer 9

A multicast frame is forwarded to many, but not necessarily all hosts in a subnet.

Reserved Mac address range

ipv4 0100.5e00.0000 through 0100.5e7f.ffff
ipv6 3333.0000.0000 through 3333.ffff.ffff

Question 10

Converting IPv4 Multicast address to a MAC address

Answer 10

Convert last 3 octets to binary
Change the leftmost bit to 0 if it’s not already a 0
Convert each nibble into hex
Prepend 0100.5e

Question 11

IGMP

Answer 11

Used by routers to coordinate with each other to ensure that routers forward multicast packets only to those hosts that need them.

IGMP membership query is sent every 60 seconds and is addressed to 224.0.0.1 and has a TTL of 1

IGMPv2 is the most common version and used by default

All IGMP versions use IP protocol number 2

When a receiver joins a multicast group, it sends an IGMP Membership Report to the group address.

Question 12

IGMP Snooping

Answer 12

A switch will sniff IGMP membership reports to determine which ports multicast traffic should be forwarded out of. Enabled on Cisco switches by default

The exception is traffic sent to 224.0.0.0/24 is forwarded to all ports

Question 13

Multicast Routing

Answer 13

Senders and receivers may not be in the same subnet. Each router must build a multicast IP routing table that links incoming interfaces (source) with outgoing interfaces (receivers)

Question 14

Protocol Independent Multicast (PIM)

Answer 14

Builds each router’s multicast table. 2 modes are Dense mode and Sparse Mode

PIM routers running in sparse mode use PIM Joins to request traffic for a group, whereas those running in dense mode use Grafts

Question 15

PIM -DM (Dense Mode)

Answer 15

The router initially floods every multicast packet out of every interface except the interface it was received on. Specified in RFC 3973

Question 16

PIM- SM (Sparse Mode)

Answer 16

A router doesn’t forward any multicast packets unless they are explicitly requested. specified in RDC 7761

Question 17

Configuring PIM

Answer 17

With PIMv2 routers form adjacencies by sending Hello messages every 30 seconds. PIM hello messages are sent to 224.0.0.13 over IP protocol 103.

PIM elects a DR that’s responsible for forwarding multicast traffic.

Question 18

Sparse-Dense Mode

Answer 18

The routers will operate in dense mode unless they detect an RP

Question 19

Configuring Multicast Routing

Answer 19

R2(config)# ip multicast-routing

R2(config-if)# int range gi0/0, gi0/1, gi0/2
ip pim sparse-dense mode

R2#sh ip m route

(*,239.7.7.7)

• = source, 239.7.7.7 = group

Question 20

Reverse Path Forwarding Check

Answer 20

The router performs RPF check to avoid loops by ensuring that the multicast path will overlap/align with the unicast path, which assumed to be loop-free

Use mtrace command to find the full reverse path

Question 21

Multicast Receiver

Answer 21

R4(config)# int gi0/0

R4(config-if)# ip igmp join-group 239.7.7.7

Question 22

Shortest Path Tree (SPT)

Answer 22

The source is the root of the tree, the receivers are the leaves, and the intermediate routers are the branches

Question 23

RP

Answer 23

A rendezvous point (RP) is required only in networks running Protocol Independent Multicast sparse mode (PIM-SM). By default, the RP is needed only to start new sessions with sources and receivers.