Redistribute neighbor provides a way for IP subnets to span racks without forcing the end hosts to run a routing protocol by announcing individual host /32 routes in the routed fabric. Other hosts on the fabric can use this new path to access the hosts in the fabric. If ECMP is available, traffic can load balance across the available paths natively.
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.
For a leaf switch, where hosts within the rack use the default gateway, the ARP cache table contains a list of all hosts that ARP for their default gateway. In most cases, this table contains all the layer 3 information necessary. Redistribute neighbor formats and synchronizes this table into the routing protocol.
The current implementation of redistribute neighbor:
Target Use Cases and Best Practices
You use redistribute neighbor in these configurations:
- 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 that need dynamic advertisement from multiple hosts
Follow these guidelines:
- You can connect a host to one or more leafs. Each leaf advertises the /32 it sees in its neighbor table.
- Make sure that a host-bound bridge or VLAN is local to each switch.
- Connect the leafs with redistribute neighbor directly to the hosts.
- Make sure that IP addresses do not overlap, as the host IP addresses are directly advertised into the routed fabric.
- Run redistribute neighbor on Linux-based hosts. NVIDIA does not test other host operating systems.
How Does Redistribute Neighbor Work?
Redistribute neighbor works as follows:
- The leaf or ToR switch learns about connected hosts when the host sends an ARP request or ARP reply.
- The kernel neighbor table adds an entry for the host of each leaf.
- The redistribute neighbor daemon (
rdnbrd) monitors the kernel neighbor table and creates a /32 route for each neighbor entry. This /32 route is in kernel table 10.
- FRR imports routes from kernel table 10.
- A route map controls which routes to import from table 10.
- FRR imports these routes as table routes.
- You configure BGP or OSPF to redistribute the table 10 routes.
The following example configuration uses the following topology.
Configure the Leafs
/etc/network/interfacesfile to configure the same IP address with a /32 prefix on both interfaces that face the host. In this example, swp1 and swp2 face server01 and server02:
cumulus@leaf01:~$ sudo nano /etc/network/interfaces auto lo iface lo inet loopback address 10.0.0.1/32 auto swp1 iface swp1 address 10.0.0.1/32 auto swp2 iface swp2 address 10.0.0.1/32 ...
Enable the daemon to start at boot up, then start the daemon:
cumulus@leaf01:~$ sudo systemctl enable rdnbrd.service cumulus@leaf01:~$ sudo systemctl restart rdnbrd.service
Add the table as routes into the local routing table:
cumulus@leaf01:~$ sudo vtysh leaf01# configure terminal leaf01(config)# ip import-table 10
Define a route map that matches on the host-facing interface:
leaf01(config)# route-map REDIST_NEIGHBOR permit 10 leaf01(config-route-map)# match interface swp1 leaf01(config-route-map)# route-map REDIST_NEIGHBOR permit 20 leaf01(config-route-map)# match interface swp2
Apply that route map to routes imported into table:
leaf01(config)# ip import-table 10 route-map REDIST_NEIGHBOR
To set the administrative distance to use for the routes, add the
distanceoption before the route map name:
leaf01(config)# ip import-table 10 distance 20 route-map REDIST_NEIGHBOR
Redistribute the imported table routes into the appropriate routing protocol.
leaf01(config)# router bgp 65001 leaf01(config-router)# address-family ipv4 unicast leaf01(config-router-af)# redistribute table 10 leaf01(config-router-af)# exit leaf01(config-router)# exit leaf01(config)# exit leaf01# write memory leaf01# exit cumulus@leaf01:~$
leaf01(config)# router ospf leaf01(config-router)# redistribute table 10 leaf01(config-router)# exit leaf01(config)# exit leaf01# write memory leaf01# exit cumulus@leaf01:~$
The commands save the configuration in the
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 exit-address-family ! 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 Hosts
This document describes dual-connected Linux hosts with static IP addresses.
Configure a host with the same /32 IP address on its loopback and uplinks so that both leafs 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) to load balance northbound.
The loopback hosts the primary service IP address to which you can bind services.
Configure the loopback and physical interfaces. In the example topology above:
server01 connects to leaf01 through eth1 and to leaf02 through eth2.
lo, eth1, and eth2 use the loopback IP address.
post-up arpingcommand forces the host to ARP as soon as its interface comes up. This allows the leaf to learn about the host as soon as possible.
post-up ip routecommands install a default route through one or both leafs if both swp1 and swp2 are up.▼ Configuration
Install and use
ifplugd, which modifies the behavior of the Linux routing table when an interface undergoes a link transition (carrier up/down). By default, the Linux kernel keeps routes up even when the physical interface is unavailable (NO-CARRIER).
After you install
/etc/default/ifplugd as follows, where eth1 and eth2 are the interface names that your host uses to connect to the leafs.
user@server01:$ sudo nano /etc/default/ifplugd INTERFACES="eth1 eth2" HOTPLUG_INTERFACES="" ARGS="-q -f -u10 -d10 -w -I" SUSPEND_ACTION="stop"
For complete instructions to install
ifplugd on Ubuntu, follow this guide.
Check if rdnbrd is Running
rdnbrd is the redistribute neighbor daemon. To check if the daemon is running, run the
systemctl status rdnbrd.service command:
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
Change rdnbrd Configuration
To change the default configuration of
rdnbrd, edit the
/etc/rdnbrd.conf file, then run
systemctl restart rdnbrd.service:
cumulus@leaf01:~$ sudo nano /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 # # 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
Set the Routing Table ID
The Linux kernel supports multiple routing tables and can use 0 through 255 table IDs; however it reserves tables 0, 253, 254 and 255, and uses 1 first. Therefore,
rdnbrd only allows you to specify between 2 and 252. Cumulus Linux uses table ID 10, however you can set the ID to any value between 2 and 252. You can see all the tables specified here:
cumulus@leaf01:~$ cat /etc/iproute2/rt_tables # # reserved values # 255 local 254 main 253 default 0 unspec # # local # #1 inr.ruhep
Check /32 Redistribute Neighbor Advertised Routes
For BGP, run the vtysh
show ip bgp neighbor <interface> advertised-routes command. For example:
cumulus@leaf01:~$ show ip bgp neighbor swp51 advertise-routes BGP table version is 5, local router ID is 10.0.0.11 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 *> 10.0.0.11/32 0.0.0.0 0 32768 i *> 10.0.0.12/32 :: 0 65020 65012 i *> 10.0.0.21/32 :: 0 65020 i *> 10.0.0.22/32 :: 0 65020 i Total number of prefixes 4
Verify the Kernel Routing Table
Use the following workflow to verify that the kernel routing table populates correctly and that routes import and advertise correctly:
Verify that ARP neighbor entries populate into the Kernel routing table 10.
cumulus@leaf01:~$ ip route show table 10 10.0.1.101 dev swp1 scope link
If these routes do not generate, verify that the
rdnbrddaemon is running and check that the
/etc/rdnbrd.conffile includes the correct table number.
Verify that routes import into FRR from the kernel routing table 10.
cumulus@leaf01:~$ sudo vtysh leaf01# 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>* 10.0.1.101/32 [19/0] is directly connected, swp1, 01:25:29
Both the > and * must be present so that table 10 routes install as preferred into the routing table. If the routes do not install, verify the imported distance of the locally imported kernel routes with the
ip import 10 distance Xcommand (where X is not less than the administrative distance of the routing protocol). If the distance is too low, routes learned from the protocol can overwrite the locally imported routes. Also, verify that the routes are in the kernel routing table.
Confirm that routes are in the BGP/OSPF database and that they advertise.
leaf01# show ip bgp
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. Make sure the number of hosts minus fabric routes is under the TCAM size of the switch. Review the Cumulus Networks datasheets for up to date scalability limits of your hardware platform. If in doubt, contact your support representative.
Uneven Traffic Distribution
Linux uses source layer 3 addresses only to load balance on most older distributions.
Silent Hosts Never Receive Traffic
Sometimes, freshly provisioned hosts that have yet to send traffic do not ARP for their default gateways. The
post-up arping command in the
/etc/network/interfaces file on the host takes care of this. If the host does not ARP, then
rdnbrd on the leaf does not learn about the host.