Frame Relay - hubs can not ping eachother

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Re:Frame Relay - hubs can not ping eachother

by Guest » Fri Dec 03, 2010 9:24 pm

Hello, As I indicated in my previous post, the problem here is caused by the selection of the source IP address by the spoke router. Without modifying it explicitly, it selects the IP address of the outgoing interface as the source for its ping packets. The ensuing reachability problem is natural and is typical for partially-meshed multiaccess networks: the IP thinks that just because two machines are in the same IP subnet they must be able to communicate directly - and that does not generally hold in partially-meshed networks like yours. The OSPF in point-to-multipoint network mode works this around by adding the individual routers IP addresses with the /32 mask into the routing table and treating them just like any other reachable subnet, resulting in all other spokes IP addresses being reachable, according to the routing table, via the hub router. The OSPF thus helps to solve this situation without actually having to modify the IP/DLCI mappings. With EIGRP, I do not know of any such additional support, and you have to add the mappings manually. Certainly, in a large hub&spoke topologies, this can be tedious to do. I am thinking whether the NHRP (Next Hop Resolution Protocol) - a generalized version of ARP - could be used for this, as it allows dynamic spoke address registration and resolution, but I have not tried that yet. I may do that in a few days. Best regards,Peter

Re:Frame Relay - hubs can not ping eachother

by Guest » Fri Dec 03, 2010 7:51 pm

Peter, Thank you very much.  That worked.  I just assumed with the routing table showing that subnet on S0/0.1 it would route any traffic out that interface, but I guess you must manually assign the map to all other spokes.  This sounds tedious with more routers!

Re:Frame Relay - hubs can not ping eachother

by Guest » Fri Dec 03, 2010 6:35 pm

Hello, Oh yes, now its clear! Why has this eluded me so long?!  The problem is the missing IP/DLCI mapping - note that when you pinged the 10.3.1.1 from R2, your ping was sourced from the IP address 172.16.124.2 on R2. From the R3s viewpoint, this IP address is in the directly connected network on the Frame Relay subinterface, so instead of routing it through any particular neighboring router, it simply tries to directly send the reply back (which is a perfectly correct thing to do under IP). Because you did not create the IP/DLCI mapping on R3 that maps the 172.16.124.2 to the DLCI 301, the R3 is unable to send the reply back, resulting in failed pings. Note that if you pinged the 10.3.1.1 using the source IP address 10.2.1.1, all would work because the ping would be sourced from a different IP address that is not considered to be directly reachable by the R3, and would instead be correctly routed via R1. This problem can be remedied easily: On R2, add this command to the Frame Relay subinterface: frame-relay map ip 172.16.124.3 201 and on R3: frame-relay map ip 172.16.124.2 301 Let me know if this worked. Best regards,Peter

Re:Frame Relay - hubs can not ping eachother

by Guest » Fri Dec 03, 2010 5:15 pm

Peter, Thanks for working with me.  Pinging the serial interface IPs was one of the first things I tried when setting this up and R2 can not ping R3 (172.16.124.3) or the other way around. After running the debugs it looks like this is happening (both ways):R2 sends ping to R3R3 receives pingR3 sends ping replyR2 never receives reply This is the output from R3 when I had R2 pinging 10.3.1.1*Mar  1 02:22:17.923: ICMP: echo reply sent, src 10.3.1.1, dst 172.16.124.2R3#*Mar  1 02:22:19.875: ICMP: echo reply sent, src 10.3.1.1, dst 172.16.124.2R3#*Mar  1 02:22:21.839: ICMP: echo reply sent, src 10.3.1.1, dst 172.16.124.2R3#*Mar  1 02:22:23.795: ICMP: echo reply sent, src 10.3.1.1, dst 172.16.124.2R3#*Mar  1 02:22:25.851: ICMP: echo reply sent, src 10.3.1.1, dst 172.16.124.2 When pinging R3 (172.16.124.3) from R2 then none of the routers show any debugging output.R1 does not show any debugging output regardless of what you ping from R2/R3.

Re:Frame Relay - hubs can not ping eachother

by Guest » Fri Dec 03, 2010 3:38 pm

Hello, Can you please use the debug ip icmp on all three routers and try to ping the R3 from R2? Make note of the debugging messages on all three routers if there is anything interesting going on. Also, on R2, does it work if you ping the 172.16.124.3 (the R3s Frame Relay IP address)? Best regards,Peter

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