Abdel, There are no issues using /31 links on FrameRelay physical interfaces. Here's the quick configuration I've don between R2 and R4 in ProctorLabs using 24.24.24.0/31 subnet, where R2 is using 24.24.24.0 and R4 24.24.24.1 address:
R2: ------------------------- interface Serial0/1/0 no ip address encapsulation frame-relay no frame-relay inverse-arp ! interface Serial0/1/0.204 point-to-point ip address 24.24.24.0 255.255.255.254 frame-relay interface-dlci 204 ! R4: ------------------------- interface Serial0/0/0 ip address 24.24.24.1 255.255.255.254 encapsulation frame-relay frame-relay map ip 24.24.24.0 402 no frame-relay inverse-arp ! Here are the ping tests: R2#ping 24.24.24.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 24.24.24.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 12/12/16 ms R4#ping 24.24.24.0 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 24.24.24.0, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 12/13/16 ms Now, it would be good to see your configuration and analyze it to see if you made some kind of a mistake. Also, did you use real equipment or some kind of an emulation? -- Marko Milivojevic - CCIE #18427 Senior Technical Instructor - IPexpert FREE CCIE training: http://bit.ly/vLecture Mailto: [email protected] Telephone: +1.810.326.1444 Web: http://www.ipexpert.com/ On Mon, Aug 16, 2010 at 15:59, abdel el anazi <[email protected]> wrote: > > Hi All, > > During VL 2 lab 5 task 2.0 I got confused by the concept of /31 mask. > according the requirement I need to setup the FR between R2 and R4 with /31 > mask. > but when i try to ping from R2 to R4 the ping fails. > > *Aug 16 14:47:27.099: FIBipv4-packet-proc: route packet from (local) src > 0.0.0.0 dst 172.16.24.4 > *Aug 16 14:47:27.103: FIBfwd-proc: Default:0.0.0.0/0 proces level forwarding > *Aug 16 14:47:27.103: FIBfwd-proc: depth 0 first_idx 0 paths 1 long 0(0) > *Aug 16 14:47:27.103: FIBfwd-proc: try path 0 (of 1) v4-sp first short ext > 0(-1) > *Aug 16 14:47:27.103: FIBfwd-proc: v4-sp valid > *Aug 16 14:47:27.103: FIBfwd-proc: no nh type 8 - deag > *Aug 16 14:47:27.103: FIBfwd-proc: ip_pak_table 0 ip_nh_table 65535 if none > nh none deag 1 chg_if 0 via fib 0 path type special prefix > *Aug 16 14:47:27.103: FIBfwd-proc: Default:0.0.0.0/0 not enough info to > forward via fib (none none) > *Aug 16 14:47:27.103: FIBipv4-packet-proc: packet routing failed > *Aug 16 14:47:27.103: IP: s=0.0.0.0 (local), d=172.16.24.4, len 100, > unroutable > *Aug 16 14:47:27.103: ICMP type=8, code=0. > > no physical problems when I change the mask on both sides to /29 ping is > successfully working. my understanding of the /31 is little bit rusty can > someone explain what the reason of the task if you can't send anything over > the link? > > This is what i found on Cisco side: > Using 31-Bit Prefixes on IPv4 Point-to-Point Links > RFC 3021 leavingcisco.com describes using 31-bit prefixes for point-to-point > links. This leaves 1 bit for the host-id portion of the IP address. Normally > a host-id of all zeros is used to represent the network or subnet, and a > host-id of all ones is used to represent a directed broadcast. Using 31-Bit > prefixes, the host-id of 0 represents one host, and a host-id of 1 > represents the other host of a point-to-point link. > Local link (limited) broadcasts (255.255.255.255) can still be used with > 31-bit prefixes. But directed broadcasts are not possible to a 31-bit > prefix. This is not really a problem because most routing protocols use > multicast, limited broadcasts, or unicasts. > > > > According to the solution guide R2 use frame relay P-to-P and R4 use the > physical interface. but if i change both sides to point-to-point sub-int it > still doesn't ping so what is the purpose of having /31 mask _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com
