Virtual eXtensible Local Area Network (VXLAN) is a protocol to overlay a virtualized L2 network over an existing IP network with little setup. It is currently described in an Internet-Draft. It adds the following perks to VLANs while still providing isolation:
- It uses a 24-bit VXLAN Network Identifier (VNI) which should be enough to address any scale-based concerns of multitenancy.
- It wraps L2 frames into UDP datagrams. This allows one to rely on some interesting properties of IP networks like availability and scalability. A VXLAN segment can be extended far beyond the typical reach of today VLANs.
The VXLAN Tunnel End Point (VTEP) originates and terminates VXLAN tunnels. Thanks to a serie of patches from Stephen Hemminger, Linux can now act as a VTEP. Let’s see how this works.
When possible, I try to use IPv6 for my labs. This is not the case here for several reasons:
- IP multicast is required and PIM-SM implementations for IPv6 are not widespread yet. However, they exist. This explains why I use XORP for this lab: it supports PIM-SM for both IPv4 and IPv6.
- VXLAN Internet-Draft specifically addresses only IPv4. This seems a bit odd for a protocol running on top of UDP and I hope this will be fixed soon. This is not a major stopper since some VXLAN implementations support IPv6.
- However, the current implementation for Linux does not support IPv6. IPv6 support will be added later.
Once IPv6 support is available, the lab should be easy to adapt.
UPDATED: The latest draft addresses IPv6 support. It is available in Linux 3.11.
So, here is the lab used.
R3 will act as VTEPs. They
do not make use of PIM-SM. Instead, they have a generic multicast route
E3 are edge routers while
C3 are core routers. The proposed lab is not resilient but
convenient to explain how things work. It is built on top of QEMU
hosts. Have a look at my previous article for more details on this.
The lab is hosted on GitHub. I have made the lab easier to try by
including the kernel I have used for my tests. XORP comes
preconfigured, you just have to configure the VXLAN part. For this,
you need a recent version of
$ sudo apt-get install screen vde2 qemu-system-x86 iproute xorp git $ git clone git://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git $ cd iproute2 $ ./configure && make You get `ip' as `ip/ip' and `bridge' as `bridge/bridge'. $ cd .. $ git clone git://github.com/vincentbernat/network-lab.git $ cd network-lab/lab-vxlan $ ./setup
The first step is to setup unicast routing. OSPF is used for this
purpose. The chosen routing daemon is XORP. With
xorpsh, we can
check if OSPF is working as expected:
root@c1# xorpsh root@c1$ show ospf4 neighbor Address Interface State ID Pri Dead 192.168.11.11 eth0/eth0 Full 220.127.116.11 128 36 192.168.12.22 eth1/eth1 Full 18.104.22.168 128 33 192.168.101.133 eth2/eth2 Full 22.214.171.124 128 36 192.168.102.122 eth3/eth3 Full 126.96.36.199 128 38 root@c1$ show route table ipv4 unicast ospf 192.168.1.0/24 [ospf(110)/2] > to 192.168.11.11 via eth0/eth0 192.168.2.0/24 [ospf(110)/2] > to 192.168.12.22 via eth1/eth1 192.168.3.0/24 [ospf(110)/3] > to 192.168.102.122 via eth3/eth3 192.168.13.0/24 [ospf(110)/2] > to 192.168.102.122 via eth3/eth3 192.168.21.0/24 [ospf(110)/2] > to 192.168.101.133 via eth2/eth2 192.168.22.0/24 [ospf(110)/2] > to 192.168.12.22 via eth1/eth1 192.168.23.0/24 [ospf(110)/2] > to 192.168.101.133 via eth2/eth2 192.168.103.0/24 [ospf(110)/2] > to 192.168.102.122 via eth3/eth3
Once unicast routing is up and running, we need to setup multicast routing. There are two protocols for this: IGMP and PIM-SM. The former one allows routers to forward multicast datagrams while the first one allows hosts to subscribe to a multicast group.
IGMP is used by hosts and adjacent routers to establish multicast
group membership. In our case, it will be used by
R2 to let
know it subscribed to
188.8.131.52 (a multicast group).
Configuring XORP to support IGMP is simple. Let’s test with
have a multicast listener on
root@r2# iperf -u -s -l 1000 -i 1 -B 184.108.40.206 ------------------------------------------------------------ Server listening on UDP port 5001 Binding to local address 220.127.116.11 Joining multicast group 18.104.22.168 Receiving 1000 byte datagrams UDP buffer size: 208 KByte (default) ------------------------------------------------------------
E2, we can now check that
R2 is properly registered for
root@e2$ show igmp group Interface Group Source LastReported Timeout V State eth0 22.214.171.124 0.0.0.0 192.168.2.2 248 2 E
XORP documentation contains a good overview of IGMP.
PIM-SM is far more complex. It does not have its own topology discovery protocol and relies on routing information from other protocols, OSPF in our case.
I will describe here a simplified view on how PIM-SM works. XORP documentation contains more details about PIM-SM.
The first step for all PIM-SM routers is to elect a rendez-vous
point (RP). In our lab, only
C3 have been
configured to be elected as a RP. Moreover, we give better priority to
C3 to ensure it wins.
root@e1$ show pim rps RP Type Pri Holdtime Timeout ActiveGroups GroupPrefix 192.168.101.133 bootstrap 100 150 135 0 126.96.36.199/8
Let’s suppose we start
iperf on both
R3. Using IGMP, they
subscribe to multicast group
E3 send a join message (also known as a
(*,G) join) to the RP (
C3) for that multicast group. Using the
unicast path from
E3 to the RP, the routers along the
paths build the RP tree (RPT), rooted at
C3. Each router in the
tree knows how to send multicast packets to
188.8.131.52: it will send
them to the leaves.
root@e3$ show pim join Group Source RP Flags 184.108.40.206 0.0.0.0 192.168.101.133 WC Upstream interface (RP): eth2 Upstream MRIB next hop (RP): 192.168.23.133 Upstream RPF'(*,G): 192.168.23.133 Upstream state: Joined Join timer: 5 Local receiver include WC: O... Joins RP: .... Joins WC: .... Join state: .... Prune state: .... Prune pending state: .... I am assert winner state: .... I am assert loser state: .... Assert winner WC: .... Assert lost WC: .... Assert tracking WC: O.O. Could assert WC: O... I am DR: O..O Immediate olist RP: .... Immediate olist WC: O... Inherited olist SG: O... Inherited olist SG_RPT: O... PIM include WC: O...
Let’s suppose that
R1 wants to send multicast packets to
220.127.116.11. It sends them to
R1 which does not have any
information on how to contact all the members of the multicast group
because it is not the RP. Therefore, it encapsulates the multicast
packets into PIM Register packets and sends them to the RP. The RP
decapsulates them and sends them natively. The multicast packets are
routed from the RP to
R3 using the reverse path formed by
the join messages.
root@r1# iperf -c 18.104.22.168 -u -b 10k -t 30 -T 10 ------------------------------------------------------------ Client connecting to 22.214.171.124, UDP port 5001 Sending 1470 byte datagrams Setting multicast TTL to 10 UDP buffer size: 208 KByte (default) ------------------------------------------------------------ root@e1# tcpdump -pni eth0 10:58:23.424860 IP 192.168.1.1.35277 > 126.96.36.199.5001: UDP, length 1470 root@c3# tcpdump -pni eth0 10:58:23.552903 IP 192.168.11.11 > 192.168.101.133: PIMv2, Register, length 1480 root@e2# tcpdump -pni eth0 10:58:23.896171 IP 192.168.1.1.35277 > 188.8.131.52.5001: UDP, length 1470 root@e3# tcpdump -pni eth0 10:58:23.824647 IP 192.168.1.1.35277 > 184.108.40.206.5001: UDP, length 1470
As presented here, the routing is not optimal: packets from
R2 could avoid the RP. Moreover, encapsulating multicast packets
into unicast packets is not efficient either. At some point, the RP
will decide to switch to native multicast1. Rooted at
the shortest-path tree (SPT) for the multicast group will be built
using source-specific join messages (also known as a (S,G) join).
From here, each router in the tree knows how to handle multicast
R1 to the group without involving the RP. For example,
E1 knows it must duplicate the packet and sends one through the
C3 and the other one through the interface to
root@e1$ show pim join Group Source RP Flags 220.127.116.11 192.168.1.1 192.168.101.133 SG SPT DirectlyConnectedS Upstream interface (S): eth0 Upstream interface (RP): eth1 Upstream MRIB next hop (RP): 192.168.11.111 Upstream MRIB next hop (S): UNKNOWN Upstream RPF'(S,G): UNKNOWN Upstream state: Joined Register state: RegisterPrune RegisterCouldRegister Join timer: 7 KAT(S,G) running: true Local receiver include WC: .... Local receiver include SG: .... Local receiver exclude SG: .... Joins RP: .... Joins WC: .... Joins SG: .OO. Join state: .OO. Prune state: .... Prune pending state: .... I am assert winner state: .... I am assert loser state: .... Assert winner WC: .... Assert winner SG: .... Assert lost WC: .... Assert lost SG: .... Assert lost SG_RPT: .... Assert tracking SG: OOO. Could assert WC: .... Could assert SG: .OO. I am DR: O..O Immediate olist RP: .... Immediate olist WC: .... Immediate olist SG: .OO. Inherited olist SG: .OO. Inherited olist SG_RPT: .... PIM include WC: .... PIM include SG: .... PIM exclude SG: .... root@e1$ show pim mfc Group Source RP 18.104.22.168 192.168.1.1 192.168.101.133 Incoming interface : eth0 Outgoing interfaces: .OO. root@e1$ exit [Connection to XORP closed] root@e1# ip mroute (192.168.1.1, 22.214.171.124) Iif: eth0 Oifs: eth1 eth2
Setting up VXLAN§
Once IP multicast is running, setting up VXLAN is quite easy. Here are the software requirements:
- A recent kernel. Pick at least 3.7-rc3. You need to enable
CONFIG_VXLANoption. You also currently need a patch on top of it to be able to specify a TTL greater than 1 for multicast packets.
- A recent version of
ip. Currently, you need the version from git.
R3, we create a
vxlan42 interface with the following commands:
root@rX# ./ip link add vxlan42 type vxlan id 42 \ > group 126.96.36.199 \ > ttl 10 dev eth0 root@rX# ip link set up dev vxlan42 root@rX# ./ip -d link show vxlan42 10: vxlan42: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1460 qdisc noqueue state UNKNOWN mode DEFAULT link/ether 3e:09:1c:e1:09:2e brd ff:ff:ff:ff:ff:ff vxlan id 42 group 188.8.131.52 dev eth0 port 32768 61000 ttl 10 ageing 300
Let’s assign an IP in
192.168.99.0/24 for each router and check they can ping each other:
root@r1# ip addr add 192.168.99.1/24 dev vxlan42 root@r2# ip addr add 192.168.99.2/24 dev vxlan42 root@r3# ip addr add 192.168.99.3/24 dev vxlan42 root@r1# ping 192.168.99.2 PING 192.168.99.2 (192.168.99.2) 56(84) bytes of data. 64 bytes from 192.168.99.2: icmp_req=1 ttl=64 time=3.90 ms 64 bytes from 192.168.99.2: icmp_req=2 ttl=64 time=1.38 ms 64 bytes from 192.168.99.2: icmp_req=3 ttl=64 time=1.82 ms --- 192.168.99.2 ping statistics --- 3 packets transmitted, 3 received, 0% packet loss, time 2003ms rtt min/avg/max/mdev = 1.389/2.375/3.907/1.098 ms
We can check the packets are encapsulated:
root@r1# tcpdump -pni eth0 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes 11:30:36.561185 IP 192.168.1.1.43349 > 192.168.2.2.8472: UDP, length 106 11:30:36.563179 IP 192.168.2.2.33894 > 192.168.1.1.8472: UDP, length 106 11:30:37.562677 IP 192.168.1.1.43349 > 192.168.2.2.8472: UDP, length 106 11:30:37.564316 IP 192.168.2.2.33894 > 192.168.1.1.8472: UDP, length 106
Moreover, if we send broadcast packets (with
ping -b or ARP
requests), they are encapsulated into multicast packets:
root@r1# tcpdump -pni eth0 11:31:27.464198 IP 192.168.1.1.41958 > 184.108.40.206.8472: UDP, length 106 11:31:28.463584 IP 192.168.1.1.41958 > 220.127.116.11.8472: UDP, length 106
Recent versions of
iproute also comes with
bridge, an utility
allowing one to inspect the FDB of bridge-like interfaces:
root@r1# ../bridge/bridge fdb show vxlan42 3e:09:1c:e1:09:2e dev vxlan42 dst 192.168.2.2 self 0e:98:40:c6:58:10 dev vxlan42 dst 192.168.3.3 self
For a demo, have a look at the following video (it is also available as an Ogg Theora video).
The decision is usually done when the bandwidth used by the follow reachs some threshold. With XORP, this can be controlled with
switch-to-spt-threshold. However, I was unable to make this works as expected. XORP never sends the appropriate PIM packets to make the switch. Therefore, for this lab, it has been configured to switch to native multicast at the first received packet. ↩