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
Regards,
A. el Anazi
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