R2511-HP MSR Router Series Security Configuration Guide(V5)

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3. URPF checks whether the matching route is a default route:
{ If yes, URPF checks whether the allow-default-route keyword is configured to allow the default
route: if yes, proceeds to step 4, if not, proceeds to step 5.
{ If not, proceeds to step 4.
4. URPF checks whether the receiving interface matches the output interface of the matching FIB entry:
{ If yes, the packet is forwarded.
{ If not, URPF checks whether the check mode is loose: if yes, the packet is forwarded, if not,
proceeds to step 5.
5. URPF checks whether the packet is permitted by the ACL:
{ If yes, the packet is forwarded (such a packet is displayed in the URPF information as a
"suppressed drop").
{ If not, the packet is discarded.
Network application
Figure 146 Network diagram
• Configure strict URPF check between an ISP network and a customer network, and loose URPF
check between ISPs.
• Configure ACLs for special packets or users.
Configuring URPF
You can configure URPF on a specific interface. URPF configured on an interface takes effect on the
interface only.
URPF checks only packets arriving at an enabled interface.