Multicast - Implementing Multicast (MCAST V1.0) - Multicast at Layer 2 (3)

IP Multicasting at Layer 2


 multicast IP mapped to MAC address


L3 addressing


    * IP-Class-D - 224.0.0.0 to 239.255.255.255
    * high-order bits "1110" (224.0.0.0/4) 
    * special group with TTL=1 (link local)


          o 224.0.0.1 - all systems
          o 224.0.0.2 - all routers
          o 224.0.0.4 - DVMRP routers
          o 224.0.0.5 - all OSPF Routers
          o 224.0.0.6 - all OPSF DRs
          o 224.0.0.13 - PIMv2 router

L2 addressing

                <---------32bits-IP------------>
                <-5bits->
<-24bits-OUI-01-00-5e-->0<-23bits-low-order-IP->
<-----------------48bit-Multicast-MAC---------->       

transmission of 24-01-00-5e-bits + 0 (25th) + 23 low-order-IP-bits = 48 bit Multicast MAC 

See Dr. Deering-Story 

Be-Aware 32:1 overlap

224.10.0.1

11100000.00001010.00000000.00000001

  - 226.10.0.1

11100010.00001010.00000000.00000001

  - 236.138.0.1

11100110.10001010.00000000.00000001

= 01-00-53-0a-00-01

L2 Multicast Frame Switching 

typical switches treat multicast as unknown or broadcast -> flood on every port

static entries which ports receive which ports

dynamic configuration

Multicast Layer-2 Switching Solutions

• CGMP ( Cisco Group Message Protocol) simple,proprietary, routers and switches
• IGMP snooping (Internet Group Message Protocol Snooping) - complex, standard, proprietary implement, switches only, L3-aware switches
• GMRP (GARP Mulicast Registration Protocol) - standarized, not widely available, swithces and hosts, L2 protocol
• RGMP (Router port Group Management Protocol) - simple, proprietary, routers and switches

CGMP (Cisco Group Message Protocol) 
CGMP packets send by router
0100.0cdd.dddd - CGMP MAC
Router = CGMP Server / Switch = CGMP Client
Ver 4bits, Type 4bits (0000 join - 0001 leave), Reserved 2bits (00), Count (1 byte - Anzahl GDA/USA), GDA 6byte / 6byte USA
GDA = Group destination address
USA = unicast source address

IGMP snooping
switches become IGMP aware, examines the IGMP payload
because routers do not send IGMP reports flooding on router-only-switched network

GMRP (GARP Mulicast Registration Protoco)
runs on hosts and switches
hosts protocol stack must support it


RGMP (Router Group Management Protocol) - RFC3488
allows per port forwarding in router-only-switched networks
Routers send RGMP hellos on (224.0.0.25)
Switch learns about multicast routers 
no traffic to directly connected receives -> restricted to IGMP snooping
RGMP routers send (*,G) joins for a group
RGMP switch forwards only groups router needs