NFS挂载卡在这个问题上

配置信息：
OpenJTAG> set bootargs noinitrd root=/dev/nfs nfsroot=172.20.72.126:/work/nfs_root/tmp/fs_mini_mdev ip=172.20.75.120:172.20.75.126:172.20.74.1:255.255.254.0::eth0:off init=/linuxrc console=ttySAC0
OpenJTAG> save
Saving Environment to NAND...
Erasing Nand...Wri ting to Nand... done
OpenJTAG> reboot

重启打印信息：
IP-Config: Complete:
device=eth0, addr=172.20.75.120, mask=255.255.254.0, gw=172.20.74.1,
host=172.20.75.120, domain=, nis-domain=(none),
bootserver=172.20.75.126, rootserver=172.20.72.126, rootpath=
Looking up port of RPC 100003/2 on 172.20.72.126
r PCBind: server 172.20.72.126 not responding, timed out
Root-NFS: Unable to get nfsd port number from server, using default
Looking up port of RPC 100005/1 on 172.20.72.126
rpcbind: server 172.20.72.126 not responding, timed out
Root-NFS: Unable to get mountd port number from server, using default
mount: server 172.20.72.126 not responding, timed out
Root-NFS: Server returned error -5 while mounting /work/nfs_root/tmp/fs_mini_mdev
VFS: Unable to mount root fs via NFS, trying floppy.
VFS: Cannot open root device "nfs" or unknown-block(2,0)
Please append a correct "root=" boot option; here are the available partitions:
1f00 256 mtdblock0 (driver?)
1f01 128 mtdblock1 (driver?)
1f02 2048 mtdblock2 (driver?)
1f03 259712 mtdblock3 (driver?)
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(2,0)