Chap 13 - Multicast (part 1) Flashcards
What 2 components does Multicast rely on?
- IGMP
- PIM
What layer does IGMP operate at?
Layer 2
What layer does PIM operate at?
Layer 3
What is the MDT?
Multicast Distribution Tree
What address range has been assigned for Multicast?
224.0.0.0/4 to 239.255.255.255
What are the first 4 bits of the multicast range?
1110
What addresses does the Local Network Control Block contain?
224.0.0.0/24
What addresses does the Local Internetwork Control Block contain?
224.0.1.0/24
What is the address range of the Local Network Control Block, what is it used for, and what are 3 examples?
- 224.0.0.0/24
- Used for protocol control traffic that is not forwarded outside of a broadcast domain
- All hosts use 224.0.0.1
- All routers use 224.0.0.2
- All PIM routers use 224.0.0.13
What is the range of the Internetwork Control Block, what is it used for, and what are 3 examples?
- 224.0.1.0/24
- Used for protocol control traffic that may be forwarded through the Internet
- NTP uses 224.0.1.1
- Cisco-RP-Announce uses 224.0.1.39
- Cisco-RP-Discovery uses 224.0.1.40
What does SSM stand for and what is it?
- Source Specific Multicast
- PIM extension
- Forwards traffic to receivers from only those sources that receivers have expressed interest
What address range is used by Source Specific Multicast Block?
232.0.0.0/8
What address range is used by the GLOP Block?
233.0.0.0 to 233.251.255.255
What is the scope of the GLOP Block, what address range is it, what does it provide for, and who can use it?
- Global in scope
- 233.0.0.0 to 233.251.255.255
- Provides for statically assigned addresses by mapping an organizations ASN number into the 2nd and 3rd octets of a multicast address.
- For domains with 16-bit ASN numbers
How are ASNs mapped into the GLOP Block?
- Maps a domains 16-bit ASN number into the middle two octets
- 233.x.y.0/24 where x and y contain 8 bits each of the 16-bit ASN.
What 2 alternatives does an organization who wants an address from the GLOP Block but their ASN is 32-bits?
- Take an assignment from the Ad hoc range
- Consider IPV6 Multicast Addresses
What address range has been assigned for the Administratively Scoped Block?
239.0.0.0/8
What address range is the Administratively Scoped Block, what is its scope, what can it be compared with, who can use it, and what is it typically used for?
- 239.0.0.0/8
- Scope is private
- Similar to private IP addressing (RFC1918)
- Network administrators are free to use this address range but only in their domain
- Typically used by SSM even though SSM has its own block
What does the first 24 bits of a Multicast MAC address start with?
01:00:5E
What are the components of a Multicast MAC address?
- bits 1 to 24 - 01:00:5E
- bit 25 is multicast bit always set to 0
- The last 23 bits are copied directly from the multicast IP address
- This leaves the possibility of 32 overlapping addresses
Where in a Multicast MAC address is the Multicast/Broadcast bit and what should it be set at?
- Low order bit of the first 8 bits
- Set to 1 for multicast (or broadcast)
What does a receiver do when a receiver wants to receive a specific multicast feed?
- Sends an Unsolicited Membership Report (aka IGMP Join) using the multicast IP address of that group.
- Reprograms its NIC to accept the multicast MAC group address that correlates to the group IP address
What protocol do receivers use when they want to join multicast groups and receive traffic from that group?
IGMP
What is IGMP, where is it enabled and how many versions are there?
- Internet Group Management Protocol
- Needs to be enabled on receiver and router
- There are 3 versions
What is the TTL for an IGMP packet?
1
What are the 5 different IGMP message along with their types numbers?
- Ver 2 Membership Report - 0x16
- Ver 1 Membership Report - 0x12
- Ver 2 Leave Group - 0x17
- General Membership Query - 0x11
- Group Specific Query - 0x11
What are the 4 fields in an IGMP message?
- Type
- Max Response Time
- Checksum
- Group Address
What is the IGMP Type for a Ver 2 Membership Report, and when are 2 times when it is used?
- 0x16
- aka IGMP Join message
- used by receivers to join a Multicast group
- Also used by receivers to respond to local router’s Membership Query
What is the IGMP Type for the Ver 1 Membership Report, and what does this message type allow for?
- Type 0x12
- Provides backward compatibility with IGMP V1
What is the IGMP Type for a Ver 2 Leave Group message and when is this sent?
- Type 0x17
- Used by receivers to indicate they want to leave a group they previously joined
What is the IGMP Type for a General Membership Query, what is the destination address what is the group address?
- Type 0x11
- Sent to the all-hosts Group Address (224.0.0.1)
- Used to check if there are any Receivers in the attached subnet
- Sets Group Address to 0.0.0.0
What is the IGMP Type for Group Specific Query, when is it sent, what is its destination set to?
- Type 0x11
- Sent in response to a Leave Group message
- Sent to the Group Address that the Receiver wants to leave
- Destination IP address of this is the Group IP address and also in the Group Address Field
What is the IGMP Field Max Response Time, when is it used, and what is it’s unit of measurement?
- Specifies maximum amount of time a receiver has before it must reply with a Membership Report message
- Used only in General Membership Query and Group Specific Query
- In any other type of message value is set to 0x00 and ignored by Receivers
- Units of one tenth of 1 second
How many bits in the IGMP Field Checksum and how is the checksum calculated?
- 16 bits
- Set to 1’s complement of the 1’s complement sum of the message
- Standard checksum method used by TCP/IP
What is the IGMP Field Group Address used for in a General Query message, Group Specific Query, Membership Report and Leave Group messages?
- In General Query messages is set to 0.0.0.0
- In Group Specific messages is set to the Group Address
- Membership and Leave messages have the Group Address being joined or left
What 4 things does a router do when it receives a Membership report?
- Router forwards the request in a PIM Join Message
- When router starts receiving the stream it forwards it to the Subnet where the Join message came from
- Router then starts sending periodic General Membership Queries to the subnet using 224.0.0.1 to see if there are any members in the attached subnet
- These general query messages have Max Response Time set to 10 seconds by default
What is the default Max Response Time in a General Membership Query?
10 seconds
What does a Receiver do when it receives a General Membership Query?
- Sets an internal random timer between 0 and 10 seconds
- When timer expires Receiver sends Membership Reports for each group it belongs to
- If Receiver sees a Membership Report from another Receiver it stops its timer for the specified group
- This is meant to suppress duplicate Reports
What does a Receiver do when it wants to leave a group?
- If the Receiver was the last to respond to the latest General Membership Query it sends a Leave Group message to 224.0.0.2 (AllRouters)
- If it wasn’t the last to respond it can leave the group quietly because there are other Receivers on the subnet
How does a router know if there are any Receivers left on the subnet after having received a Leave message?
- Router sends a Group Specific Query to see if there are any Receivers left who are interested in the group
- If none then router removes the IGMP state for that group
What happens if there is more than one router on a subnet?
- IGMP Querier Election takes place
- Routers send General Membership queries to 224.0.0.1
- Routers check each other’s source IP addresses
- Router with lowest interface address on the LAN wins
What 3 things happen after the IGMP Querier election?
- Non-querier routers set timers that reset each time they receive a General Membership Query Report from the Querier
- If Querier stops sending Membership Query Reports the router waits 120 seconds (twice the default 60 second query interval)
- If still no Membership Query Reports the router will trigger another Querier Election
In IGMPv2 can a Receiver specify the source it would like to use when it sends the IGMP Join message?
No, but IGMPv3 allows for that.
What are 2 things IGMPv3 does and 2 things that it added that are different than IGMPv2?
- IGMPv3 allows Receivers to specify the Multicast source
- V3 is backward compatible
- V3 added new fields to the IGMP Membership Query
- V3 added a new message type called Ver 3 Membership Report to support source filtering
In IGMPv3 what are the 2 modes a Receiver can use when signaling they want to join a multicast group?
- Include mode - Receiver sends join message that includes a list of sources it wants to receive Multicast from
- Exclude mode - Receiver sends join message listing sources it does NOT want multicast from
In IGMPv3 what does the Receiver do if it doesn’t care which source it uses for multicast?
Receiver uses Exclude mode and leaves the Exclude list empty
What 2 methods does Cisco use to prevent switches from flooding multicast frames out all ports?
- IGMP Snooping
- Static MAC address entries
How does IGMP Snooping work?
- Switch watches for group-specific IGMP Joins and adds that information to the MAC address table
- When the switch sees multicast traffic for a specific group it forwards it out only on interfaces where it saw the IGMP join for that group
Does IGMP Snooping prevent all flooding on switches?
No, some multicast addresses are still flooded
