Specifications

IP Switching Commands
ip verify unicast reverse-path
ISW-65
Cisco IOS IP Switching Command Reference
May 2008
For example, routers at the edge of the network of an ISP are more likely to have symmetrical reverse
paths than routers that are in the core of the ISP network. Routers that are in the core of the ISP network
have no guarantee that the best forwarding path out of the router will be the path selected for packets
returning to the router. In this scenario, you should use the new form of the command, ip verify unicast
source reachable-via, if there is a chance of asymmetrical routing.
Examples The following example shows that the Unicast Reverse Path Forwarding feature has been enabled on a
serial interface:
ip cef
! or "ip cef distributed" for RSP+VIP based routers
!
interface serial 5/0/0
ip verify unicast reverse-path
The following example uses a very simple single-homed ISP to demonstrate the concepts of ingress and
egress filters used in conjunction with Unicast RPF. The example illustrates an ISP-allocated classless
interdomain routing (CIDR) block 192.168.202.128/28 that has both inbound and outbound filters on the
upstream interface. Be aware that ISPs are usually not single-homed. Hence, provisions for
asymmetrical flows (when outbound traffic goes out one link and returns via a different link) need to be
designed into the filters on the border routers of the ISP.
ip cef distributed
!
interface Serial 5/0/0
description Connection to Upstream ISP
ip address 192.168.200.225 255.255.255.255
no ip redirects
no ip directed-broadcast
no ip proxy-arp
ip verify unicast reverse-path
ip access-group 111 in
ip access-group 110 out
!
access-list 110 permit ip 192.168.202.128 10.0.0.31 any
access-list 110 deny ip any any log
access-list 111 deny ip host 10.0.0.0 any log
access-list 111 deny ip 172.16.0.0 255.255.255.255 any log
access-list 111 deny ip 10.0.0.0 255.255.255.255 any log
access-list 111 deny ip 172.16.0.0 255.255.255.255 any log
access-list 111 deny ip 192.168.0.0 255.255.255.255 any log
access-list 111 deny ip 209.165.202.129 10.0.0.31 any log
access-list 111 permit ip any any
The following example demonstrates the use of ACLs and logging with Unicast RPF. In this example,
extended ACL 197 provides entries that deny or permit network traffic for specific address ranges.
Unicast RPF is configured on interface Ethernet 0 to check packets arriving at that interface.
For example, packets with a source address of 192.168.201.10 arriving at interface Ethernet 0 are
dropped because of the deny statement in ACL 197. In this case, the ACL information is logged (the
logging option is turned on for the ACL entry) and dropped packets are counted per-interface and
globally. Packets with a source address of 192.168.201.100 arriving at interface Ethernet 0 are forwarded
because of the permit statement in ACL 197. ACL information about dropped or suppressed packets is
logged (the logging option is turned on for the ACL entry) to the log server.
ip cef distributed
!
int eth0/1/1
ip address 192.168.200.1 255.255.255.255