05-21-2012 11:02 PM - edited 03-08-2019 06:45 PM
Starting with ASA version 8.4(3), the ASA will not respond to ARP requests received on an interface, for IP addresses that are not a part of that interface's IP subnet. Prior to version 8.4(3), the ASA would respond to ARP requests that were not in the IP subnet of the ASA's interface.
This change might manifest itself immediately after upgrading the ASA to version 8.4(3). In some cases, internet users might be unable to connect to the global address of a translated server through the ASA.
The following message will be displayed if this situation is encountered, and 'debug arp' is enabled on the ASA's CLI:
arp-in: Arp packet received from 192.168.10.1 which is in different subnet than the connected interface 192.168.11.1/255.255.255.0
The root cause of this issue is not a bug; see the information below to learn more about potential causes and solutions to the issue.
To encounter this situation, the ASA must receive an ARP request for an IP address that matches a global address in a configured NAT translation; the global IP address must reside in an IP subnet that is different from the IP subnet configured on the ASA's interface.
To understand the full ramifications of this issue, it is important to get a complete understanding of how this issue could appear, and what is the best way to mitigate the problem.
Below are some situations where this situation could be encountered:
This is probably the most common cause of this situation, and it is due to a non-optimal configuration of an upstream device. It is preferred to configure IP routes such that the next hop of the IP route is an IP address in the same subnet as that interface's address, like this:
ip route 10.1.2.0 255.255.255.0 192.168.1.2
However, sometimes network administrators configure an interface, instead of an IP address, as the next hop, like this:
ip route 10.1.2.0 255.255.255.0 FastEthernet0/1
This causes the router to route traffic destined to the 10.1.2.0/24 network to the FastEthernet0/1 interface, and then send an ARP request for the destination IP address in the IP packet. It is assumed that some device will respond to the ARP request, and the router then forwards the packet to the MAC address that was resolved due to the ARP process. The benefits of this type of configuration is that it is very easy to configure and administer; the administrator doesn't have to explicitly configure a next hop IP address for the route, and they assume that an adjacent device will have proxy-ARP enabled and will respond to the ARP request if it is capable of routing the packets to the destination IP address.
However, there are serious problems with this type of ip route config:
Therefore, it is absolutely a best practice to configure all routes with explicit IP next-hop addresses, and not use routes that have an interface name by itself to identify the outgoing interface. If the interface is needed to tie the route to the egress interface for failover enter both the egress interface name and the next hop in the static route.
Mismatched IP subnet masks configured on the ASA's interface and the adjacent device's interface could cause a similar situation. If the adjacent device had a subnet mask that was a supernet (255.255.240.0) of the ASA's interface IP subnet mask (255.255.255.0), the adjacent device would ARP for IP addresses that are not in the ASAs interface IP subnet. Ensure that the subnet masks are correct.
Another side effect of this change is the inability to learn MAC addresses from non-directly-connected subnets in Transparent mode. This will break communication in the following scenarios:
1. The ASA does not have a management IP address configured or the configuration is incorrect.
2. The ASA is using secondary subnets on the same segment.
There is no workaround for this issue in Transparent mode aside from the downgrade.
The solution to this problem (in the case that the IP address in question is not in the same layer-3 subnet as the ASA's interface IP) is to make the changes necessary to ensure that devices adjacent to the ASA route traffic directly to the ASA's interface IP address as the next hop device, instead of relying on a device to proxy-ARP on behalf of the IP address.
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Author
The author of this document are Aaron S Mcquaid (amcquaid) and Derek Lynch (delynch).
Does anyone know if this also affects 8.6 onwards as i have this an issue with secondary outside subnets on a pair of 5515-X's
This behavior change is version 8.4.3 onwards, and it does affect 8.6 as well.
We have a problem with this as many seemed to have, since we have 3 IP subnets assigned by our provider how are we supposed to make this work, we want to use the 3 subnets on one outside interface and use static NAT to use the IP addresses of the other subnets that don't have and IP assigned to the outside interface. We can't change the routing of the upstream providers router.
What to do? isn't there a workaround?
Hi All
If your ASA image is the 8.6,maybe you can try to upgrade to the 9.0. The Command Reference has the "arp non-connected-subnet" ,it's behavior is just like the before "arp permit-nonconnected"! But this version is published very lately(10/29......).SO...Take care.
See Command Reference---
http://www.cisco.com/en/US/docs/security/asa/asa90/command/reference/a3.html#wp1824414
- See more at: https://supportforums.cisco.com/message/3781445#3781445
arp non-connected-subnet has been added in the versions 8.4(5)+.
Cisco recommends however to set static routes (w/ ip as next hop to the ASA) on the upstream router.
"arp permit-nonconnected"
ASA starts to proxy arp for the non-connected (floating) subnets.
Note that on FTD, the "arp permit-nonconnected" is a non-default behavior (just like on ASA).
To change it, you currently (as of FTD 6.2.3.4) have to use a Flexconnect object.
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