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Redistribute Neighbor

Redistribute neighbor provides a mechanism for IP subnets to span racks without forcing the end hosts to run a routing protocol.

The fundamental premise behind redistribute neighbor is to announce individual host /32 routes in the routed fabric. Other hosts on the fabric can then use this new path to access the hosts in the fabric. If multiple equal-cost paths (ECMP) are available, traffic can load balance across the available paths natively.

The challenge is to accurately compile and update this list of reachable hosts or neighbors. Luckily, existing commonly-deployed protocols are available to solve this problem. Hosts use ARP to resolve MAC addresses when sending to an IPv4 address. A host then builds an ARP cache table of known MAC addresses: IPv4 tuples as they receive or respond to ARP requests.

In the case of a leaf switch, where the default gateway is deployed for hosts within the rack, the ARP cache table contains a list of all hosts that have ARP’d for their default gateway. In many scenarios, this table contains all the layer 3 information that’s needed. This is where redistribute neighbor comes in, as it is a mechanism of formatting and syncing this table into the routing protocol.


Redistribute neighbor is distributed as python-rdnbrd.

Target Use Cases and Best Practices

Redistribute neighbor was created with these use cases in mind:

  • Virtualized clusters
  • Hosts with service IP addresses that migrate between racks
  • Hosts that are dual connected to two leaf nodes without using proprietary protocols such as MLAG
  • Anycast services needing dynamic advertisement from multiple hosts

Follow these guidelines with redistribute neighbor:

  • A host can connect to one or more leafs. Each leaf advertises the /32 it sees in its neighbor table.
  • A host-bound bridge/VLAN should be local to each switch only.
  • Leaf switches with redistribute neighbor enabled should be directly connected to the hosts.
  • IP addressing must be non-overlapping, as the host IPs are directly advertised into the routed fabric.
  • Run redistribute neighbor on Linux-based hosts primarily; other host operating systems may work, but NVIDIA has not actively tested any at this stage.

How It Works

Redistribute neighbor works as follows:

  1. The leaf/ToR switches learn about connected hosts when the host sends an ARP request or ARP reply.
  2. An entry for the host is added to the kernel neighbor table of each leaf switch.
  3. The redistribute neighbor daemon, rdnbrd, monitors the kernel neighbor table and creates a /32 route for each neighbor entry. This /32 route is created in kernel table 10.
  4. FRRouting is configured to import routes from kernel table 10.
  5. A route-map is used to control which routes from table 10 are imported.
  6. In FRRouting these routes are imported as table routes.
  7. BGP, OSPF and so forth are then configured to redistribute the table 10 routes.

Example Configuration

The following example configuration is based on the reference topology. Other configurations are possible, based on the use cases outlined above. Here is a diagram of the topology:

Configure the Leaf(s)

The following steps demonstrate how to configure leaf01, but the same steps can be applied to any of the leafs.

  1. Configure the host facing ports, using the same IP address on both host-facing interfaces as well as a /32 prefix. In this case, swp1 and swp2 are configured as they are the ports facing server01 and server02:

    cumulus@leaf01:~$ net add loopback lo ip address
    cumulus@leaf01:~$ net add interface swp1-2 ip address
    cumulus@leaf01:~$ net pending
    cumulus@leaf01:~$ net commit

    The commands produce the following configuration in the /etc/network/interfaces file:

    auto lo
    iface lo inet loopback
    auto swp1
    iface swp1
    auto swp2
    iface swp2
  2. Enable the daemon so it starts at bootup:

    cumulus@leaf01:~$ sudo systemctl enable rdnbrd.service
  3. Start the daemon:

    cumulus@leaf01:~$ sudo systemctl restart rdnbrd.service
  4. Configure routing:

    1. Define a route-map that matches on the host-facing interfaces:

      cumulus@leaf01:~$ net add routing route-map REDIST_NEIGHBOR permit 10 match interface swp1
      cumulus@leaf01:~$ net add routing route-map REDIST_NEIGHBOR permit 20 match interface swp2
    2. Import routing table 10 and apply the route-map:

      cumulus@leaf01:~$ net add routing import-table 10 route-map REDIST_NEIGHBOR
    3. Redistribute the imported table routes in into the appropriate routing protocol.

      cumulus@leaf01:~$ net add bgp autonomous-system 65001
      cumulus@leaf01:~$ net add bgp ipv4 unicast redistribute table 10


      cumulus@leaf01:~$ net add ospf redistribute table 10
    4. Save the configuration by committing your changes.

      cumulus@leaf01:~$ net pending
      cumulus@leaf01:~$ net commit
Click here to expand the contents of /etc/frr/frr.conf

This configuration uses OSPF as the routing protocol.

cumulus@leaf01$ cat /etc/frr/frr.conf
frr version 3.1+cl3u1
frr defaults datacenter
ip import-table 10 route-map REDIST_NEIGHBOR
username cumulus nopassword
service integrated-vtysh-config
log syslog informational
router bgp 65001
  address-family ipv4 unicast
  redistribute table 10
route-map REDIST_NEIGHBOR permit 10
  match interface swp1
route-map REDIST_NEIGHBOR permit 20
  match interface swp2
router ospf
  redistribute table 10
line vty

Configure the Host(s)

There are a few possible host configurations that range in complexity. This document only covers the basic use case: dual-connected Linux hosts with static IP addresses assigned.

Additional host configurations will be covered in future separate knowledge base articles.

Configure a Dual-connected Host

Configure a host with the same /32 IP address on its loopback (lo) and uplinks (in this example, eth1 and eth2). This is done so both leaf switches advertise the same /32 regardless of the interface. Cumulus Linux relies on ECMP to load balance across the interfaces southbound, and an equal cost static route (see the configuration below) for load balancing northbound.

The loopback hosts the primary service IP address(es) and to which you can bind services.

Configure the loopback and physical interfaces. Referring back to the topology diagram, server01 is connected to leaf01 via eth1 and to leaf02 via eth2. You should note:

  • The loopback IP is assigned to lo, eth1 and eth2.
  • The post-up ARPing is used to force the host to ARP as soon as its interface comes up. This allows the leaf to learn about the host as soon as possible.
  • The post-up ip route replace is used to install a default route via one or both leaf nodes if both swp1 and swp2 are up.
user@server01:$ cat /etc/network/interfaces
# The loopback network interface
auto lo
iface lo inet loopback

auto lo:1
iface lo:1

auto eth1
iface eth1
  post-up for i in {1..3}; do arping -q -c 1 -w 0 -i eth1; sleep 1; done
  post-up ip route add nexthop via dev eth1 onlink nexthop via dev eth2 onlink || true

auto eth2
iface eth2
  post-up for i in {1..3}; do arping -q -c 1 -w 0 -i eth2; sleep 1; done
  post-up ip route add nexthop via dev eth1 onlink nexthop via dev eth2 onlink || true

Install ifplugd

Additionally, install and use ifplugd. ifplugd modifies the behavior of the Linux routing table when an interface undergoes a link transition (carrier up/down). The Linux kernel by default leaves routes up even when the physical interface is unavailable (NO-CARRIER).

After you install ifplugd, edit /etc/default/ifplugd as follows, where eth1 and eth2 are the interface names that your host uses to connect to the leaves.

user@server01:$ cat /etc/default/ifplugd
INTERFACES="eth1 eth2"
ARGS="-q -f -u10 -d10 -w -I"

For full instructions on installing ifplugd on Ubuntu, follow this guide.

Known Limitations

TCAM Route Scale

This feature adds each ARP entry as a /32 host route into the routing table of all switches within a summarization domain. Take care to keep the number of hosts minus fabric routes under the TCAM size of the switch.

Possible Uneven Traffic Distribution

Linux uses source L3 addresses only to do load balancing on most older distributions.

Silent Hosts Never Receive Traffic

Freshly provisioned hosts that have never sent traffic may not ARP for their default gateways. The post-up ARPing in /etc/network/interfaces on the host should take care of this. If the host does not ARP, then rdnbrd on the leaf cannot learn about the host.

Support for IPv4 Only

This release of redistribute neighbor supports IPv4 only.

VRFs Are not Supported

This release of redistribute neighbor does not support VRFs.

Only 1024 Interfaces Supported

Redistribute neighbor does not work with more than 1024 interfaces. Doing so can cause the rdnbrd service to crash.

Unsupported with EVPN

Redistribute neighbor is unsupported when the BGP EVPN Address Family is enabled. Enabling both redistribute neighbor and EVPN will lead to unreachable IPv4 ARP and IPv6 neighbor entries.


How do I determine if rdnbrd (the redistribute neighbor daemon) is running?

Use systemd to check:

cumulus@leaf01$ systemctl status rdnbrd.service
* rdnbrd.service - Cumulus Linux Redistribute Neighbor Service
  Loaded: loaded (/lib/systemd/system/rdnbrd.service; enabled)
  Active: active (running) since Wed 2016-05-04 18:29:03 UTC; 1h 13min ago
  Main PID: 1501 (python)
  CGroup: /system.slice/rdnbrd.service
  `-1501 /usr/bin/python /usr/sbin/rdnbrd -d

How do I change rdnbrd’s default configuration?

Editing the /etc/rdnbrd.conf file, then run systemctl restart rdnbrd.service:

cumulus@leaf01$ cat /etc/rdnbrd.conf
# syslog logging level CRITICAL, ERROR, WARNING, INFO, or DEBUG
loglevel = INFO

# TX an ARP request to known hosts every keepalive seconds
keepalive = 1

# If a host does not send an ARP reply for holdtime consider the host down
holdtime = 3

# Install /32 routes for each host into this table
route_table = 10

# Uncomment to enable ARP debugs on specific interfaces.
# Note that ARP debugs can be very chatty.
# debug_arp = swp1 swp2 swp3 br1
# If we already know the MAC for a host, unicast the ARP request. This is
# unusual for ARP (why ARP if you know the destination MAC) but we will be
# using ARP as a keepalive mechanism and do not want to broadcast so many ARPs
# if we do not have to. If a host cannot handle a unicasted ARP request, set
# the following option to False.
# Unicasting ARP requests is common practice (in some scenarios) for other
# networking operating systems so it is unlikely that you will need to set
# this to False.
unicast_arp_requests = True
cumulus@leaf01:~$ sudo systemctl restart rdnbrd.service

What is table 10? Why was table 10 chosen?

The Linux kernel supports multiple routing tables and can utilize 0 through 255 as table IDs. However, tables 0, 253 254 and 255 are reserved, and 1 is usually the first one utilized. Therefore, rdnbrd only allows you to specify 2-252. The number 10 was chosen for no particular reason. Feel free to set it to any value between 2-252. You can see all the tables specified here:

cumulus@switch$ cat /etc/iproute2/rt_tables
# reserved values
255 local
254 main
253 default
0 unspec
# local
#1  inr.ruhep

Read more information on Linux route tables, or you can read the Ubuntu man pages for ip route.

How do I determine that the /32 redistribute neighbor routes are being advertised to my neighbor?

For BGP, check the advertised routes to the neighbor.

cumulus@leaf01:~$ sudo vtysh
Hello, this is FRRouting (version
Copyright 1996-2005 Kunihiro Ishiguro, et al.
leaf01# show ip bgp neighbor swp51 advertised-routes
BGP table version is 5, local router ID is
Status codes: s suppressed, d damped, h history, * valid, > best, = multipath,
              i internal, r RIB-failure, S Stale, R Removed
Origin codes: i - IGP, e - EGP, ? - incomplete

    Network          Next Hop            Metric LocPrf Weight Path
*>                  0         32768 i
*>     ::                                     0 65020 65012 i
*>     ::                                     0 65020 i
*>     ::                                     0 65020 i

Total number of prefixes 4

How do I verify that the kernel routing table is being correctly populated?

Use the following workflow to verify that the kernel routing table is being populated correctly and that routes are being correctly imported/advertised:

  1. Verify that ARP neighbor entries are being populated into the Kernel routing table 10.

    cumulus@switch:~$ ip route show table 10 dev swp1 scope link

    If these routes are not being generated, verify that that the rdnbrd daemon is running. Then, check /etc/rdnbrd.conf to verify the correct table number is used.

  2. Verify that routes are being imported into FRRouting from the kernel routing table 10.

    cumulus@switch:~$ sudo vtysh
    Hello, this is FRRouting (version
    Copyright 1996-2005 Kunihiro Ishiguro, et al.
    switch# show ip route table
    Codes: K - kernel route, C - connected, S - static, R - RIP,
           O - OSPF, I - IS-IS, B - BGP, A - Babel, T - Table,
           > - selected route, * - FIB route
      T[10]>* [19/0] is directly connected, swp1, 01:25:29

    Both the > and * should be present so that table 10 routes are installed as preferred into the routing table. If the routes are not being installed, verify that the imported distance of the locally imported kernel routes using the ip import 10 distance X command, where X is not less than the adminstrative distance of the routing protocol. If the distance is too low, routes learned from the protocol may overwrite the locally imported routes. Also verify that the routes are in the kernel routing table.

  3. Confirm that routes are in the BGP/OSPF database and are being advertised.

    switch# show ip bgp