Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

Frame-Relay multi-point mapping question

I'm currently learning the basics about frame-relay, and everything makes perfect sense so far. There is one thing I just don't get though.

Lets say we have three routers going through a frame-relay switch. R1 is connected to two other routers, R2 and R3. We are using multi-point, split-horizon is turned off on the WAN interface on R1, and EIGRP is running on all routers.

At this point all three router's routing tables are good to go, but if R2 pings R3 it will fail and vice versa. This is, of course, because both routers (R2,R3) have R1's IP address mapped to their local DLCI. They don't have the router's IP that is not directly connected mapped to the DLCI, so when they recieve the packet it is dropped because it doesn't know how to respond to the source IP address,even if it's on the same subnet. The solution is to map that IP address to the appropriate DLCI. Now everything works fine, no problems.

Thats the part that makes sense to me. The part that doesn't is why does mapping that IP address to the DLCI work for ALL networks after that? If there is a second network beyond our first one, it doesn't have to same problem our first network did.

Basically,

- R2 recieves a packet from a router in Network 1. It doesn't have to source IP mapped to a DLCI, so it is dropped.

- We map that IP to the DLCI, and everything is fine.

- R2 recieves a packet from a router on Network 2. It doesn't have the source IP mapped to a DLCI, but it knows to send the response to the appropriate DLCI anyway. Why is that?

Is this an encapsulation issue?

Thanks in advance for the help.

  • WAN Routing and Switching
1 ACCEPTED SOLUTION

Accepted Solutions
Purple

Frame-Relay multi-point mapping question

Hi,

If all your advertised networks on the spoke have the same next-hop in the routing table of the other spoke and if you have a corresponding mapping for that next-hop then your spoke will know which DLCI to use to send the packet so indeed it is an encapsulation problem that has been solved  with a static frame-relay mapping.

Regards

Alain

Don't forget to rate helpful posts.

Don't forget to rate helpful posts.
1 REPLY
Purple

Frame-Relay multi-point mapping question

Hi,

If all your advertised networks on the spoke have the same next-hop in the routing table of the other spoke and if you have a corresponding mapping for that next-hop then your spoke will know which DLCI to use to send the packet so indeed it is an encapsulation problem that has been solved  with a static frame-relay mapping.

Regards

Alain

Don't forget to rate helpful posts.

Don't forget to rate helpful posts.
57
Views
0
Helpful
1
Replies
This widget could not be displayed.