ng_one2many v.s. AFT (NIC Fault Tolerance/Fail Over/Redundancy
Revisited)
Brian A. Seklecki
lavalamp at spiritual-machines.org
Sat Oct 15 16:25:16 PDT 2005
Re:
http://lists.freebsd.org/pipermail/freebsd-questions/2005-October/100623.html
First: This is all very preliminary from some testing over the weekend.
Dell's reponse was that Intel's AFT/ALB was entirely software based.
That left me with few options:
1) Try userland layer 3 failover (ugly)
2) Use ng_one2many
However, ng_one2many only permits for two algorithms:
NG_ONE2MANY_XMIT_ROUNDROBIN and NG_ONE2MANY_XMIT_ALL.
However, none of these meet the need:
- Round-Robin results in 50% packet loss if a hook/interface is lost (not
acceptable in any mission critical environment).
- Xmit-All causes twice as much load on to be placed on the switch /fabric
and switch CPU.
What ng_one2many needs is a "Active-Standy" XMIT algorithm (STP BOFH's
will think BLOCKING/FORWARDING). It could even be used on top of other
NetGraph nodes like ng_fec or possibly (hopefully) ng_802.3ad >:}
Essentially, a single layer 3 IP address needs to be visible in a "switch
fault tolerant" or "adapter fault tolerant" configuration. A
userland-level daemon could be scripted, and it has been done before:
http://lists.freebsd.org/pipermail/freebsd-isp/2003-November/001314.html
So when a fail-over occurs, the layer IP 3 address moves from one layer 2
MAC address to another layer 2 MAC address on the same machine (and same
subnet, same ethernet segment, just a different interface). TCP sockets
should not be affected due to layer abstraction.
This got me thinking about HSRP/VRRP. That protocol is designed strictly
to move a layer 3 address between two different hosts. Excellent
applications are Router/Firewall and VPN concentrator, as OpenBSD's
carp(4) has implemented with the help of pfsync. I was experimenting with
the OpenBSD variant and I realized that client hosts weren't seeing the
usual warnings about MAC address changes.
As of 3.7, OpenBSD's CARP shares a virtual MAC address between the hosts,
Cisco's HSRP does not.
Then I was thinking about the OpenBSD/NetBSD bridge(4) interface. If the
host acting as the bridge wishes too, it can participate in the bridged
networks by assigning a layer 3 address. The address isn't ifconfig(8)'d
do the "bridge0" interface. Instead, it's assigned to the first interface
included in the "bridge[0-9]", say fxp0.
Further more, regardless of what network segment/port a host participating
in a bridge(4)'d network resides, the ARP'd IP address of the
OpenBSD/NetBSD host is persistently the MAC first physical interface
ifconfig(8)'d with the IP.
Plus OpenBSD/NetBSD bridge(4) supports 802.1d spanning tree >:}
This is important. Spanning Tree as an alogirth could provide Intel AFT
"Fault Tolerance" intelligence if the persistent layer2 address of a host
was unchanged with the NIC interface change. The function of STP is to
provide a loop free path to every layer2 MAC in a segment. But a STP
enabled bridge(4) with an IP address assigned has a persistent MAC address
associated with a layer 3 address!
Therefore, the solution has been there all along. The attached diagram
explains in greater detail.
http://digitalfreaks.org/~lavalamp/OpenBSD_Bridge_AFT.png
In this diagram, switch 0 is configured manually as the spanning tree root
and switch 1 is the backup spanning tree root. By default, rl0 will be in
BLOCKING and rl1 will being FORWARDING. However, as tcpdump(8)
illustrates, regardless of which interface is the root port, ARP replys
will always return the MAC if the bridge(4) member interface ifconfig(8)'d
with the IP.
rl0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
address: 00:50:fc:9d:24:d6
media: Ethernet autoselect (100baseTX full-duplex)
status: active
inet 192.168.100.1 netmask 0xffffff00 broadcast 192.168.100.255
rl1: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
address: 00:50:fc:9d:08:cd
media: Ethernet autoselect (100baseTX full-duplex)
status: active
---
bridge0: flags=41<UP,RUNNING>
Configuration:
priority 32768 hellotime 2 fwddelay 15 maxage 20
Interfaces:
rl1 flags=b<LEARNING,DISCOVER,STP>
port 2 ifpriority 128 ifcost 55 forwarding
rl0 flags=b<LEARNING,DISCOVER,STP>
port 1 ifpriority 128 ifcost 55 blocking
Addresses (max cache: 100, timeout: 240):
00:01:63:bb:f7:c9 rl1 1 flags=0<>
00:0f:1f:c1:f2:b7 rl1 1 flags=0<>
-----
# tcpdump -i rl1 -n arp
12:38:17.806885 arp who-has 192.168.100.1 tell 192.168.100.254
12:38:17.806951 arp reply 192.168.100.1 is-at 0:50:fc:9d:24:d6
12:38:17.806966 arp reply 192.168.100.1 is-at 0:50:fc:9d:24:d6
bs0#sh spanning-tree vlan 11 interface fa0/9
Spanning tree 11 is executing the IEEE compatible Spanning Tree protocol
Bridge Identifier has priority 100, address 0001.63bb.f7c2
Configured hello time 2, max age 20, forward delay 15
We are the root of the spanning tree
Topology change flag not set, detected flag not set, changes 54
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 0, topology change 0, notification 0
Interface Fa0/9 (port 22) in Spanning tree 11 is FORWARDING
Port path cost 19, Port priority 128
Designated root has priority 100, address 0001.63bb.f7c2
Designated bridge has priority 100, address 0001.63bb.f7c2
Designated port is 22, path cost 0
Timers: message age 0, forward delay 0, hold 0
BPDU: sent 10592, received 30
bs1#sh spanning-tree vlan 11 interface fa0/9
Spanning tree 11 is executing the IEEE compatible Spanning Tree protocol
Bridge Identifier has priority 32768, address 0002.fd0e.f382
Configured hello time 2, max age 20, forward delay 15
Current root has priority 100, address 0001.63bb.f7c2
Root port is 38, cost of root path is 19
Topology change flag not set, detected flag not set, changes 54
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 0, topology change 0, notification 0
Interface Fa0/9 (port 22) in Spanning tree 11 is FORWARDING
Port path cost 19, Port priority 128
Designated root has priority 100, address 0001.63bb.f7c2
Designated bridge has priority 32768, address 0002.fd0e.f382
Designated port is 22, path cost 19
Timers: message age 0, forward delay 0, hold 0
BPDU: sent 45454, received 1196
bs0#sh mac-address-table | include 24d6
0050.fc9d.24d6 Dynamic 11 FastEthernet0/9
bs1#sh mac-address-table | include 24d6
0050.fc9d.24d6 Dynamic 11 FastEthernet0/24
The behavior is similar in FreeBSD using ng_bridge(4) (I haven't tried
FreeBSD bridge(4)). However, both of these claim "a privative loop
prevention algorithm"); ... 'debug stp events' shows no STP traffic from a
FreeBSD host, though.
Also, FreeBSD differs in behavior in that the MAC address ARP'd is that of
which ever NG node bridge member is assigned the IP.
The disadvantage is that without FreeBSD speaking 802.1d, it can't know to
fail an interface on any event other than a media state change. i.e., the
currently active port could be connected a switch that looses it's uplink.
Of course, neither the FreeBSD or NetBSD/OpenBSD implementation features a
"heartbeat" algorithm to add intelligence, as Intel AFT/ALB might, but
that wasn't the design principal goal.
Also, my initial tests are with managed switches using PVST. Behavior may
differ with unmanaged switches where no STP debugging is possible or
possibly a uni-stp is used.
More on this on Monday...
http://www.cisco.com/application/pdf/en/us/guest/netsol/ns304/c649/cdccont_0900aecd800ea162.pdf
~BAS
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