AI, ML, Development + Cisco Learning Blog Learning about Machine Learning, Artificial Intelligence, related devlopment topics and formerly Routing and Switching, Datacenter, Security and other topics, CCIE #23664, Frank Wagner

23. April 2008

ip igmp snooping for to

Filed under: Bridging + Switching,Multicast — ocsic @ 12:32

I have read about this subnet range, it`s not possible to disable the snooping feature on a switch. So routing protocols (like OSPF or EIGRP) are always forwarded to other ports regardless the snooping feature. Is that true? Someone with more informations on that.

In general, addresses from to are reserved and used by various protocols (standard or proprietary, such as Hot Standby Router Protocol (HSRP)). Cisco recommends that you not use these for GDA in a multicast network. CGMP and IGMP snooping do not work with this reserved address range.


21. Dezember 2006

A definition of different PIM modes

Filed under: Multicast — ocsic @ 12:44

There are three different PIM (Protocol Independent Multicast) modes.

  • dense mode – havving multiple clients, which are tightly spaced together, it’s implicit for all devices in the PIM domain that they are joined to the multicast domain, if they don’t want to receive the multicast stream, they have to send a „prune“ message. It’s called the „flood and prune“ behavior
  • sparse mode – devices have to send a join message, without they will not receive the multicast stream. It’s designed for networks which have clients that are few and far between. It’s called an „explicit join“ mechanism. It depends on a central RP (rendezvous point“, which is organizing forwarding in the multicast domain
  • sparse-dense mode – is a combination of both. if there is an RP configured it will use sparse mode. But if the RP fails or is not reachable any more, the router will fall back to dense mode. If you want to prohibit this, there is an „no ip pim dense fallback“

3. Oktober 2006

Convert from a multicast address to a HW address

Filed under: Bridging + Switching,Multicast — ocsic @ 10:35

Sometime i may be neccessary to block a Multicast address on a vlan.

This could be acomplished by blocking the hardware address of this vlan. It’s possible to staticly map certain ports for a mac address. So that only these ports are recievers of the address.

So first it’s neccessary to convert the multicast address to a hardwareaddress.

The first fields 0100.5e are reserved by the IANA for such demands.

The range is 0100.5e00.0000 through 0100.5e7f.ffff.

Starting with these first digits, continue to add the last ones, by converting the numbers to hexadecimal numbers.

For example, the multicast address of EIGRP is

This gives:

0 => 00

0 => 00

10 => 0a

So the corresponding mac address for the EIGRP Mmulticast address is 0100.5e 00 00 0a

The switch does snooping for packets on vlans, to determin the recieving ports. First we have do disable ip igmp snooping for te specific vlan:

no ip igmp snooping vlan 10

Then it’s possible to map the certain multicast mac address with certain receiver switchports.

mac-address-table static 0100.5e00.000a vlan 10 int f0/1 f0/2

So now only port f0/1 and f0/2 recieve this multicast address. This is only possible by turning of snooping functionality by the switch.


11. September 2006

important terms to know about Multicast

Filed under: Multicast — ocsic @ 13:43
  • IGMP – is used to track the multicast group members between hosts and routers on a lan
  • CGMP – Cisco Group Management Protokoll, for routers connected to catalyst switches
  • PIM – Protocol independet Multicast is for routers to know which multicast packets to forward and which not
  • DVMRP – Multicast used on the internet MBONE
  • Sparse Mode – unless there is an explicit request for traffic a router assumes that others routers do not want to forward traffic.When a host joins a multicast group, the connected routers send a PIM join message to the RP (rendezvous point). The RP keeps track of group mappings
  • Dense Mode – the router assumes that all other routers want to forward the multicast stream for a group, if a router receives a multicast and has no pim neighbor and no receiving host, it sends back a prune message to the source


Important commands:

ip pim send-rp-announce Configures a router to be the RP

ip pim send-rp-discovery Assigns the RP mapping agent

ip igmp join-group the router will accept multicast packets and also forward them

ip igmp static-group the router will not accept and only forward a multicast stream

RP (rendezvouz point)

  • A RP is acting as the central sender and receiver for data. Sources must send their data to the RP over a shared distribution tree. The RP is then just the initiator for new sessions between receivers and senders.
  • static versus Auto-RP
    • for Auto-RP a router must be designated as the RP mapping agent. This agent sends the group-to-RP mappings.

Good programm for generating multicast traffic is iperf. See the link provided below.


23. August 2006

Multicast networks

Filed under: Multicast — ocsic @ 09:14

IP Multicast is defined in RFC 1112/2236(IGMPv2).

Ip Multicast can send traffic to a single address and does not care about the receiving hosts. Only one destination address is needed and all hosts listening to the stream or host that have registred can receive the datastream. It does support dynamic meberships to groups. So clients can themselfes register for the stream.

One multicast address can carry multiple streams. It uses UDP as transport protocol.

Address range:

Class D –

The range from to is reserved for routing protocols and other types of protocols.

Static Addresses (local scope):

  • all hosts on a subnet
  • all routers on a subnet
  • Distance Vector Multicast Routing Protocols (DVMRP)
  • OSPF routers
  • OSPF designated routers
  • RIP Version 2 routers
  • Protocol independent Multicast (PIM)

Global scope addresses: –

Administratively scoped: –

Internet Multicast address range reserved by IANA (lower 23 bits):

0100.5e00.000 – 0100.5e7f.ffffAll Multicast addresses use the 01.00.5e in their multicast address.


Traffic types

Filed under: Multicast — ocsic @ 08:51
  • Unicast – individual packets are send to every requesting address, so the number of clients is fixed to the bandwidth the streaming applications needs
  • Broadcast – traffic is send to one broadcast address and then forwarded to the entire broadcast domain, you can ping for example a broadcast address and see, that all hosts receive this ping. But broadcast are limitted to layer 3 boarders
  • Multicast – sending a packet to a predefined multicast address, receiving hosts, can register for receiving streams in groups

11. August 2006

How to setup Multicast on a cisco router

Filed under: Multicast — ocsic @ 11:21
  1. enable ip multicast routing with „ip multicast-routing“
  2. enable PIM on an interface
  3. configure a rendezvous point

Here for example you use sparse-mode
# ip multicast-routing

# int e0/0

# ip pim sparse-mode

# ip igmp join-group

Dense Mode – Sparse Mode

Filed under: Multicast — ocsic @ 11:10
Dense Mode
You should use Dense Mode for dense population of hosts and plentiful bandwidth
  • DVMRP Distance Vector Multicast Routing Protocol
    • Used frequently on the MBONE
    • Uses reverse path flooding
    • Periodic floodings to offer groups to new hosts
    • Unicast path based on hop counts
    • Cisco routers use PIM to unteract with DVMRP routers, but do not directly implement DVMRP
Sparse Mode
  • Use for a sparse distribution of multicast members
  • assumes specific requests are used to join
  • used when crossing WANs

Multicast, RFC 112, then RFC 2236

Filed under: Multicast — ocsic @ 09:49
  • sending ip packets to a group of hosts in the network, RFC 2236 Internet Group Management Protocol v2
  • delivery of IP datagramms to multiple hosts using a single destination address
  • dynamic membership to groups, clients can dynamicly join groups
  • supports all size of groups

single group address for registred applications, that means multiple applications can stream to a single group address


Address range:

Multicast uses Class D –

Well-known Addresses:

  • All hosts on a subnet
  • All routers on a subnet
  • All distance vector multicast routing protocol routers
  • All OSPF routers
  • All OSPF designated routers
  • All RIP designated routers
  • IGRP router
  • All Protocl independent Multicast (PIM) routers

To configure a router supporting PIM, the cisco way of supporting multicast, you have to enable ip multicast-routing

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