431
Figure 134 uRPF work flow
1.
uRPF checks source address validity:
{
uRPF discards packets with a broadcast source address.
{
uRPF discards packets with an all-zero source address but a non-broadcast destination
address. (A packet with source address 0.0.0.0 and destination address 255.255.255.255
might be a DHCP or BOOTP packet and cannot be discarded.)
{
uRPF proceeds to step 2 for other packets.
2.
uRPF checks whether the source address matches a FIB entry:
Checks the received packet
Broadcast source
address?
All-zero source address?
Matching FIB entry
found?
Broadcast destination
address?
Default route found?
Loose uRPF?
Allows the packet to pass
Discards the packet
Yes
Yes
Yes
Yes
No
Yes
Yes
No
No
Receiving
interface matches the output
interface of the default
route
？
No
No
No
No
Loose uRPF?
Yes
No
No
Yes
Yes
Receiving
interface matches the
output interface of the
matching FIB entry
？
Matching route is a direct
route?
No
Source IP address
matches an ARP entry?
Yes
No
Yes
