why we need to type the word (broadcast) at the end of fram relay command?

[hanyawad]

Hi,

when i configure the router with the frame relay one of the commands we type is frame-relay map ip 172.16.124.2 102 broadcast

my question is: why we need to type the broadcast keyword at the end of that command

thanks

Labib

[lgijssel]

Check the command reference output below:

frame-relay map

To define the mapping between a destination protocol address and the data-link connection identifier (DLCI) or Frame Relay permanent virtual circuit (PVC) bundle that connects to the destination address, use the frame-relay map command in interface configuration mode. To delete the map entry, use the no form of this command.

frame-relay map protocol protocol-address {dlci | vc-bundle vc-bundle-name}[broadcast] [ietf | cisco] [payload-compression {packet-by-packet | frf9 stac [one-way-negotiation] [ratio level] [skip-zero-sync] [software | hardware-options] | data-stream stac [one-way-negotiation] [ratio level] [software | hardware-options]}]

no frame-relay map protocol protocol-address

Syntax Description

protocol	One of the following values: appletalk, decnet, dlsw, ip, ipx, llc2, and rsrb.
protocol-address	Destination protocol address.
dlci	DLCI number used to connect to the specified protocol address on the interface. Acceptable numbers are integers from 16 through 1007, inclusive.
vc-bundle vc-bundle-name	A specific Frame Relay PVC bundle configured on the interface.
broadcast	(Optional) Forwards broadcasts to this address when multicast is not enabled (see the frame-relay multicast-dlci command for more information about multicasts). This keyword also simplifies the configuration of Open Shortest Path First (OSPF) (see the "Usage Guidelines" section for more detail).
ietf	(Optional) Internet Engineering Task Force (IETF) form of Frame Relay encapsulation, based on RFC 1490 and RFC 2427. Used when the router or access server is connected to another vendor's equipment across a Frame Relay network.
cisco	(Optional) Cisco-proprietary encapsulation method consisting of a four-byte header, with two bytes to identify the DLCI and two bytes to identify the packet type.
payload-compression	(Optional) Enables payload compression.
packet-by-packet	(Optional)Packet-by-packet payload compression using the Stacker method.
frf9 stac	(Optional)Enables FRF.9 compression using the Stacker method. •If the router contains a CSA1 , compression is performed in the CSA hardware (hardware compression). •If the CSA is not available, compression is performed in the software installed on the VIP22 (distributed compression). •If the VIP2 is not available, compression is performed in the main processor of the router (software compression).
one-way-negotiation	(Optional) Enables one-way negotiation. Use this keyword if your router will be negotiating compression with another device that is running Cisco IOS Release 12.1(9) or earlier releases. Later Cisco IOS releases use a two-way handshake by default to negotiate compression.
ratio level	(Optional) Sets throughput versus compression ratio. This option is available only with hardware compression. Possible values for the level argument are as follows: high—high compression versus low throughput medium—medium compression versus medium throughput low—low compression versus high throughput (default)
software	(Optional) Specifies that compression is implemented in the Cisco IOS software installed in the main processor of the router.
hardware-options	(Optional) Choose one of the following hardware options: caim element-number—Enables the CAIM3 to perform compression. distributed—Specifies that compression is implemented in the software that is installed in a VIP2. If the VIP2 is not available, compression is performed in the main processor of the router (software compression). This option applies only to the Cisco 7500 series routers. This option is not supported with data-stream compression. csa csa_number—Specifies the CSA to use for a particular interface. This option applies only to Cisco 7200 series routers.
skip-zero-sync	(Optional) Causes compression frames to be numbered starting from 1 rather than 0. Use this keyword if your router will be interoperating with a device conforming to IBM partner conventions.
data-stream stac	(Optional) Enables data-stream compression using the Stacker method. •If the router contains a CSA, compression is performed in the CSA hardware (hardware compression). •If the CSA is not available, compression is performed in the main processor of the router (software compression).

regards,

Leo

[ameya_oke]

Hi Labibmaker,

By default Frame Relay is a NBMA network i.e non broadcast multi access(please google for more details on NBMA).

NBMA networks do not support multicast or broadcast  traffic by defualt.

Mostly this keyword is used when we need to send broad cast packets (OSPF neighborship packets) over frame relay.

Ameya

[Peter Paluch · Accepted solution]

Labib,

To add to other friends' answers here, NBMA network like Frame Relay are not capable of replicating broadcasts and multicasts by themselves, i.e. they do not support creating multiple copies of a single multicast or broadcast packet and sending these copies to all other stations. The main reason is that they are based on point-to-point virtual circuits, and a frame traversing one VC can not jump into another nor be copied into it by the network itself - it would break the logical isolation between VCs.

This is in strong contrast to BMA networks like Ethernet where the replication of multicast and broadcast frames is handled by the switches themselves.

So if a router is connected to a NBMA network and needs to send a multicast packet over it, it needs to replicate the multicast packet itself. In other words, the router itself must send multiple copies of the multicast packets over proper VCs to neighbors.

This is where the broadcast keyword comes in. It flags a particular IP/DLCI mapping as eligible to receive a copy of a multicast/broadcast packet when sending it towards the NBMA network. A multicast/broadcast packet sent over an NBMA interface will be replicated by the router itself over all mappings that have the broadcast flag set. VCs without the broadcast flag will not receive a copy of that packet. As simple as that

Best regards,

Peter

[ameya_oke]

Hi Peter,

Respect!!!!

Simply Amazing!!

Ameya

[JohnTylerPearce]

I'd like to add a final note to Peter's great explanation. Make sure that you only add the 'broadcast' keyword

once per DLCI or you will get multiple copies instead of just one. For instance, if you have multicast frame map

statements like such. Will this setup you could get several copies of the broadcast instead of one.

frame map ip 1.1.1.1 205 broadcast

frame map ip 2.2.2.2 205 broadcast

frame map ip 3.3.3.3 205 broadcast

The more efficent way would be as follows.

frame map ip 1.1.1.1 205 broadcast

frame map ip 2.2.2.2 205

frame map ip 3.3.3.3 205

[cadet alain]

Hi,

if we assume the 3 ip addresses are reachable through 3 different VCs then we would have to issue the broadcast keyword 3 times.

Regards.

Alain.

[JohnTylerPearce]

correct-da-mondo.

[hanyawad]

thank you very much for all guys who tried to help me

Best Regards

Labib