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Address Resolution Protocol - ARP

Address Resolution Protocol (ARP) is a communication protocol used for discovering the link layer address, such as a MAC address, associated with a given network layer address. ARP is defined by RFC 826.

The Cumulus Linux ARP implementation differs from standard Debian Linux ARP behavior in a few ways because Cumulus Linux is an operating system for routers/switches rather than servers. This chapter describes the differences in ARP behavior, why the changes were made, where the changes were implemented, and how to change port-specific values.

Standard Debian ARP Behavior and the Tunable ARP Parameters

Debian has these five tunable ARP parameters:

  • arp_accept
  • arp_announce
  • arp_filter
  • arp_ignore
  • arp_notify

These parameters are described in the Linux documentation, but snippets for each parameter description are included in the table below and are highlighted in italics.

In a standard Debian installation, all of these ARP parameters are set to 0, leaving the router as wide open and unrestricted as possible. These settings are based on the assertion made long ago that Linux IP addresses are a property of the device, not a property of an individual interface. Thus an ARP request or reply could be sent on one interface containing an address residing on a different interface. While this unrestricted behavior makes sense for a server, it is not the normal behavior of a router. Routers expect the MAC/IP address mappings supplied by ARP to match the physical topology, with the IP addresses matching the interfaces on which they reside. With these tunable ARP parameters, Cumulus Linux has been able to specify the behavior to match the expectations of a router.

ARP Tunable Parameter Settings in Cumulus Linux

The ARP tunable parameters are set to the following values by default in Cumulus Linux.

ParameterDefault SettingTypeDescription
arp_accept0BOOLDefines the behavior for gratuitous ARP frames when the IP address is not already in the ARP table:
  • 0: Do not create new entries in the ARP table.
  • 1: Create new entries in the ARP table.

You can set arp_accept on an individual interface which differs from the rest of the switch (see below).
arp_announce2INTDefines different restriction levels for announcing the local source IP address from IP packets in ARP requests that send on an interface:
  • 0: Use any local address configured on any interface.
  • 1: Avoid local addresses that are not in the target subnet for this interface. You can use this mode when target hosts reachable through this interface require the source IP address in ARP requests to be part of their logical network configured on the receiving interface. When Cumulus Linux generates the request, it checks all subnets that include the target IP address and preserves the source address if it is from such a subnet. If there is no such subnet, Cumulus Linux selects the source address according to the rules for level 2.
  • 2: Always use the best local address for this target. In this mode, Cumulus Linux ignores the source address in the IP packet and tries to select the local address preferred for talks with the target host. To select the local address, Cumulus Linux looks for primary IP addresses on all the subnets on the outgoing interface that include the target IP address. If there is no suitable local address, Cumulus Linux selects the first local address on the outgoing interface or on all other interfaces, so that it receives a reply for the request regardless of the announced source IP address.
The default Debian behavior (arp_announce is 0) sends gratuitous ARPs or ARP requests using any local source IP address and does not limit the IP source of the ARP packet to an address residing on the interface that sends the packet.

Routers expect a different relationship between the IP address and the physical network. Adjoining routers look for MAC and IP addresses to reach a next hop residing on a connecting interface for transiting traffic. By setting the arp_announce parameter to 2, Cumulus Linux uses the best local address for each ARP request, preferring the primary addresses on the interface that sends the ARP.
  • 0: The kernel can respond to ARP requests with addresses from other interfaces to increase the chance of successful communication. The complete host on Linux (not specific interfaces) owns the IP addresses. For more complex configurations, such as load balancing, this behavior can cause problems.
  • 1: Allows you to have multiple network interfaces on the same subnet and to answer the ARPs for each interface based on whether the kernel routes a packet from the ARPd IP address out of that interface (you must use source based routing).
arp_filter for the interface is on if at least one of conf/{all,interface}/arp_filter is TRUE, it is off otherwise.

Cumulus Linux uses the default Debian Linux arp_filter setting of 0.
The switch uses arp_filter when multiple interfaces reside in the same subnet and allows certain interfaces to respond to ARP requests. For OSPF with IP unnumbered interfaces, multiple interfaces appear in the same subnet and contain the same address. If you use multiple interfaces between a pair of routers and set arp_filter to 1, forwarding can fail.

The arp_filter parameter allows a response on any interface in the subnet, where the arp_ignore setting (below) limits cross-interface ARP behavior.
arp_ignore1INTDefines different modes for sending replies in response to received ARP requests that resolve local target IP addresses:
  • 0: Reply for any local target IP address on any interface.
  • 1: Reply only if the target IP address is the local address on the incoming interface.
  • 2: Reply only if the target IP address is the local address on the incoming interface and the sender IP address is part of same subnet on this interface.
  • 3: Do not reply for local addresses with scope host; the switch replies only for global and link addresses.
  • 4-7: Reserved.
  • 8: Do not reply for all local addresses.
The switch uses the maximum value from conf/{all,interface}/arp_ignore when the {interface} receives the ARP request.

The default arp_ignore setting of 1 allows the device to reply to an ARP request for any IP address on any interface. While this matches the expectation that an IP address belongs to the device, not an interface, it can cause some unexpected behavior on a router.

For example, if arp_ignore is 0 and the switch receives an ARP request on one interface for the IP address residing on a different interface, the switch responds with an ARP reply even if the interface of the target address is down. This can cause traffic loss because the switch does not know if it can reach the next hops and results in troubleshooting challenges for failure conditions.

If you set arp_ignore to 2, the switch only replies to ARP requests if the target IP address is a local address and both the sender and target IP addresses are part of the same subnet on the incoming interface. The router does not create stale neighbor entries when a peer device sends an ARP request from a source IP address that is not on the connected subnet. Eventually, the switch sends ARP requests to the host to try to keep the entry fresh. If the host responds, the switch now has reachable neighbor entries for hosts that are not on the connected subnet.
arp_notify1BOOLDefines the mode to notify address and device changes.
  • 0: Do nothing.
  • 1: Generate gratuitous ARP requests when the device comes up or the hardware address changes.
The default Debian arp_notify setting is to remain silent when an interface comes up or the hardware address changes. Because Cumulus Linux often acts as a next hop for several end hosts, it notifies attached devices when an interface comes up or the address changes, which speeds up new information convergence and provides the most rapid support for changes.

Change Tunable ARP Parameters

You can change the ARP parameter settings in several places, including:

  • /proc/sys/net/ipv4/conf/all/arp* (all interfaces)
  • /proc/sys/net/ipv4/conf/default/arp* (default for future interfaces)
  • /proc/sys/net/ipv4/conf/swp*/arp* (individual interfaces)

The ARP parameter changes in Cumulus Linux use the default file locations.

The all and default locations sound similar, with the exception of which interfaces are impacted, but they operate in significantly different ways. The all location can potentially change the value for all interfaces running IP, both now and in the future. The reason for this uncertainty is that the all value is applied to each parameter using either MAX or OR logic between the all and any port-specific settings, as the following table shows:

ARP ParameterCondition

For example, if the /proc/sys/net/conf/all/arp_ignore value is set to 1 and the /proc/sys/net/conf/swp1/arp_ignore value is set to 0, to try to disable it on a per-port basis, interface swp1 still uses the value of 1 in its operation. While it may appear that the port-specific setting should override the global all setting, it does not actually work that way. Instead, the MAX value between the all value and port-specific value defines the actual behavior.

The default location /proc/sys/net/ipv4/conf/default/arp* defines the values for all future IP interfaces. Changing the default setting of an ARP parameter does not impact interfaces that already contain an IP address. If changes are being made to a running system that already has IP addresses assigned to it, port-specific settings should be used instead.

The way the default setting is implemented in Linux, the value of the default parameter is copied to every port-specific location, excluding those that already have an IP address assigned, as previously mentioned. Therefore there is not any complicated logic between the default setting and the port-specific setting like there is when using the all location. This makes the application of particular port-specific policies much simpler and more deterministic.

To determine the current ARP parameter settings for each of the the locations, use the following mechanism; other methods are available but this one is quite simple:

cumulus@switch:~$ sudo grep . /proc/sys/net/ipv4/conf/all/arp*

cumulus@switch:~$ sudo grep . /proc/sys/net/ipv4/conf/default/arp*

cumulus@switch:~$ sudo grep . /proc/sys/net/ipv4/conf/swp1/arp*

Note that Cumulus Linux implements this change at boot time using the arp.conf file at the following location:

cumulus@switch:~$ cat /etc/sysctl.d/arp.conf
net.ipv4.conf.default.arp_announce = 2
net.ipv4.conf.default.arp_notify = 1

Change Port-specific ARP Parameters

The simplest way to configure port-specific ARP parameters in a running device is with the following command:

cumulus@switch:~$ sudo sh -c "echo 0 > /proc/sys/net/ipv4/conf/swp1/arp_ignore"
cumulus@switch:~$ sudo grep . /proc/sys/net/ipv4/conf/swp1/arp*

To make the change persist through reboots, edit the /etc/sysctl.d/arp.conf file and add your port-specific ARP setting.

Configure Proxy ARP

The proxy ARP setting is a kernel setting that you can manipulate using sysctl or sysfs. Proxy ARP works with IPv4 only, since ARP is an IPv4-only protocol.

You need to set /proc/sys/net/ipv4/conf/<INTERFACE>/proxy_arp to 1:

cumulus@switch:~$ net add interface swp1 post-up "echo 1 > /proc/sys/net/ipv4/conf/swp1/proxy_arp"
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

These commands create the following snippet in the /etc/network/interfaces file:

auto swp1
iface swp1
    post-up echo 1 > /proc/sys/net/ipv4/conf/swp1/proxy_arp

If you are running two interfaces in the same broadcast domain (typically seen when using VRR, which creates a -v0 interface in the same broadcast domain), set /proc/sys/net/ipv4/conf/<INTERFACE>/medium_id to 2 on both the base SVI interface and the -v0 interface so that only one of the two interfaces replies when getting an ARP request. This prevents the v0 interface from proxy replying on behalf of the SVI (and the SVI from proxy replying on behalf of the v0 interface). You can only prevent duplicate replies when the ARP request is for the SVI or the v0 interface directly.

cumulus@switch:~$ net add interface swp1 post-up "echo 2 > /proc/sys/net/ipv4/conf/swp1/medium_id"
cumulus@switch:~$ net add interface swp1-v0 post-up "echo 1 > /proc/sys/net/ipv4/conf/swp1-v0/proxy_arp"
cumulus@switch:~$ net add interface swp1-v0 post-up "echo 2 > /proc/sys/net/ipv4/conf/swp1-v0/medium_id"
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

These commands create the following snippet in the /etc/network/interfaces file:

auto swp1
iface swp1
    post-up echo 1 > /proc/sys/net/ipv4/conf/swp1/proxy_arp
    post-up echo 2 > /proc/sys/net/ipv4/conf/swp1/medium_id

auto swp1-v0
iface swp1-v0
    post-up echo 1 > /proc/sys/net/ipv4/conf/swp1-v0/proxy_arp
    post-up echo 2 > /proc/sys/net/ipv4/conf/swp1-v0/medium_id

If you are running proxy ARP on a VRR interface, add a post-up line to the VRR interface stanza similar to the following. For example, if vlan100 is the VRR interface for the configuration above:

cumulus@switch:~$ net add vlan 100 post-up "echo 1 > /proc/sys/net/ipv4/conf/swp1/proxy_arp && echo 1 > /proc/sys/net/ipv4/conf/swp1-v0/proxy_arp && echo 2 > /proc/sys/net/ipv4/conf/swp1/medium_id && echo 2 > /proc/sys/net/ipv4/conf/swp1-v0/medium_id"
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

Duplicate Address Detection (Windows Hosts)

In centralized VXLAN environments, where ARP/ND suppression is enabled and SVIs exist on the leaf switches but are not assigned an address within the subnet, problems with the Duplicate Address Detection process on Microsoft Windows hosts can occur. For example, in a pure layer 2 scenario or with SVIs that have the ip-forward option set to off, the IP address is not assigned to the SVI. The neighmgrd service selects a source IP address for an ARP probe based on the subnet match on the neighbor IP address. Because the SVI on which this neighbor is learned does not contain an IP address, the subnet match fails. This results in neighmgrd using UNSPEC ( for IPv4) as the source IP address in the ARP probe.

To work around this issue, run the neighmgrctl setsrcipv4 <ipaddress> command to specify a non- address for the source; for example:

cumulus@switch:~$ neighmgrctl setsrcipv4

The configuration above takes effect immediately but does not persist if you reboot the switch. To make the changes apply persistently:

  1. Create a new file called /etc/cumulus/neighmgr.conf and add the setsrcipv4 <ipaddress> option; for example:

    cumulus@switch:~$  sudo nano /etc/cumulus/neighmgr.conf
  2. Restart the neighmgrd service:

    cumulus@switch:~$ sudo systemctl restart neighmgrd

Configure ARP Timers

Cumulus Linux does not interact directly with end systems as much as end systems interact with each another. Therefore, after ARP places a neighbor into a reachable state, if Cumulus Linux does not interact with the client again for a long enough period of time, the neighbor can move into a stale state. To keep neighbors in the reachable state, Cumulus Linux includes a background process (/usr/bin/neighmgrd). The background process can track neighbors that move into a stale, delay, or probe state, and attempt to refresh their state before removing them from the Linux kernel and from hardware forwarding. If you want the neighmgrd process to add a neighbor if the sender IP address in the ARP packet is in one of the SVI’s subnets, create the /etc/cumulus/neighmgr.conf file and add the subnet_checks=1 parameter under the [snooper] header. By default, the subnet_checks option is set to 0 (disabled) so that neighmgrd allows out-of-network neighbors to be processed from SVIs.

The ARP refresh timer defaults to 1080 seconds (18 minutes).

cumulus@leaf02:mgmt:~$ sudo nano /etc/cumulus/neighmgr.conf