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TOPIC: frame-relay and subinterfaces

frame-relay and subinterfaces 14 years 5 months ago #12391

  • gleeb
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ok its alittle complicated but go with me....
when you config an interface to work in frame-relay you can assighen only 1 DLCI number to it... right... ok and if its a multiaccess enviorment you need to define subinterfaces. now hwo the hell it is possible to assighn more then one DLCI number to 1 subinterface.
and i quote from testout.
LAX(config)#int s0
LAX(config-if)#encap frame
LAX(config-if)#no frame inverse-arp
LAX(config-if)#int s0.114 mult
LAX(config-subif)#ip addr
LAX(config-subif)#frame map ip 115
LAX(config-subif)#frame map ip 116

how does this work.
i really dont understand... as i see it you need to have 2 subinterfaces 1 to DLCI 115 and the other to 116...
please can someone explane whats going on here.[/quote]

frame-relay and subinterfaces 14 years 5 months ago #12463

Hi Gleeb

I have just covered this topic while studying for my CCNA, so lets see if I can explain it to you.

It's because you are using multipoint subinterfaces, ie you have a full mesh between a set of routers.

Multipoint means that there is more than one VC, so you can send and receive to and from more than one VC on the subinterface.

Re: frame-relay and subinterfaces 14 years 5 months ago #12464

Check the link below. See if it helps you understand better. Below is the excerpts from the link.

Frame Relay subinterfaces provide a mechanism for supporting partially meshed Frame Relay networks. Most protocols assume transitivity on a logical network; that is, if station A can talk to station B, and station B can talk to station C, then station A should be able to talk to station C directly. Transitivity is true on LANs, but not on Frame Relay networks unless A is directly connected to C.

Additionally, certain protocols, such as AppleTalk and transparent bridging, cannot be supported on partially meshed networks because they require "split horizon" in which a packet received on an interface cannot be transmitted out the same interface even if the packet is received and transmitted on different virtual circuits.

Configuring Frame Relay subinterfaces ensures that a single physical interface is treated as multiple virtual interfaces. This capability allows us to overcome split horizon rules. Packets received on one virtual interface can now be forwarded out another virtual interface, even if they are configured on the same physical interface.

Subinterfaces address the limitations of Frame Relay networks by providing a way to subdivide a partially meshed Frame Relay network into a number of smaller, fully meshed (or point-to-point) subnetworks. Each subnetwork is assigned its own network number and appears to the protocols as if it is reachable through a separate interface. (Note that point-to-point subinterfaces can be unnumbered for use with IP, reducing the addressing burden that might otherwise result).
-There Is A Foolish Corner In The Brain Of The Wisest Man- Aristotle

Re: frame-relay and subinterfaces 14 years 5 months ago #12470

I think the issue is: mapping more than one DLCI to the same subintfc? You are only mapping the IP to the far-end dlci. It is just as easy to create multiple subintfcs. one reason for frame mapping is to allow the same ACLs, bridge groups, ASes, or protocols (in ACLs) to use different statements (permit, deny) for different IPs to permit or restrict traffic flows.

:arrow: IMHO
A scapegoat is often as welcome as a solution...never memorize what you can look up.
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