398
Figure 126 URPF work flow
1. URPF checks the source address validity:
{ Discards packets with a limited source broadcast address.
{ 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 is not discarded.)
Check the source
address of the
received packet
A broadcast
source address?
An all-zone
source address?
Does the FIB
entry match the
source address?
A broadcast
destination
addres?
Is there a default
route?
Loose URPF?
Check passed
Discard
Yes
Yes
Yes
Yes
No
Yes
Yes
No
Does
the output interface of
the default route match
the receiving
interface?
No
No
NoNo
Loose URPF?
Yes
No
No
Yes
Yes
Does
the receiving
interface match the
output interface of the
matching FIB entry?
Is the matching
route a direct
route?
No
Does
the source IP
address match an
ARP entry?
No
No
Yes
No
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