Download Spanning-Tree Direct VS Indirect Link Failures

<Network redundancy of IP traffic of mobile network>
J.T. Jansson*
* Oulu University of Applied Sciences, School of Engineering, Oulu, Finland
[email protected]
Abstract
The availability must be considered when
building a network. Business runs all day, every day
and even in off hours and so reliable network
availability is required all the time. This is referred
to five nines (99.999) uptime where the small
percentage of downtime is accounted for unforeseen
incidents, or ‘scheduled maintenance’. Fast
convergence around link or component failures is a
must with growing technologies such as Voice over
IP (VoIP) and Video over IP. In this paper the
purpose and implementation of network redundancy
of IP traffic in the mobile network is discussed.
Keywords: network redundancy, IP traffic, link failure,
network device failure, STP, VRRP
1.
Introduction
The mobile network must be operable during the
whole day regardless of failures and thus some
techniques must be taken to make the network available
all the time. The possible failures that might occur
during the network operation are categorized into two
types: link failure and network device failures. Some
methods how to overcome these failures are studied.
One of the ways to increase availability is to provide
redundancy for critical components when building
networks. This usually involves duplicating routers,
switches and links to ensure continuity of service across
failures. Routing protocols are used to keep the network
running despite the network problems. [3]
2.
Possible network failures and methods to
overcome them
The solution in link failures is to provide multiple
links between devices such that when a link is down,
other link takes its role. This can lead to problems
because when there are more than one link connecting
the devices the data will find multiple links and the
switch will forward the data to multiple links and the
receiving end will receive the data more than one time.
Also looping can occur because the data will be
forwarded across the links forever. This will occur
because there is more than one path to the destination
and because the switch or the hub forwards the data to
allports. [6]
The solution to looping is to use some protocols
like spanning tree protocol (STP) that block some ports
on the switch and open others so that only one path is
existed to the destination. The concept of looping and
STP are applied only to devices connected through a
LAN and not to routers. [6]
Device failure occurs when a network device fails
and thus is unable to forward the packets. The device
failure can have a big impact on the network if it occurs
in the core layer of the network which connects the
whole network together. Therefore the solution is
suggested to be applied at the core layer where two
network devices (2N) are connected to the network to
forward the data. One network device is the primary and
the other is the secondary. If primary device fails, the
secondary becomes available. [6] [3]
The above technique is configured on the device.
Some protocols like Virtual Router Redundancy
Protocol (VRRP) are developed to accomplish this
function. When implementing it, one must connect the
device at the core which must be redundant to the
proper ports of other devices and connect similar device
to the rest of the network in the same manner the
primary device is connected. If the primary fails, this
secondary device takes it’s role. [6] [3]
3.
Spanning tree protocol (STP)
Spanning tree protocol (STP) is Data Link Layer
protocol. STP implements the 802.1D IEEE algorithm
by exchanging BPDU messages with other switches to
detect loops, and then removes the loop by shutting
down selected bridge interfaces. This algorithm
guarantees that there is one and only one active path
between two network devices. [1]
3.1 STP algorithm

Forward-Delay — This determines how long
a switch will spend in each of the listening and
learning states of STP. The default is 15
seconds, which means that out of the box we
spend 15 seconds in listening and 15 seconds
in learning. [2]
3.3 STP states
The different states of STP are as follows:

Picture 1 L2 switching diagram
There are four steps in STP algorithm:





First there is root election. In this case Cat2 is
the root bridge because we have manually
given it the lowest priority. By receiving
superior BPDUs from other switches they all
eventually agree on who is the root bridge.
The root bridge is the bridge with the lowest
BID. A BID is a priority appended to a MAC
address (See picture 1). [2]
Each Non-Root bridge elects a root-port (RP)
which is the port on that switch with the
lowest cost path to the root bridge. In the event
of a tie, they will go with lowest sending BID,
and finally lowest port-priority. [2]
On each segment, a designated port (DP) is
elected. The DP is the port on that particular
segment with the lowest cost path to the root
bridge. The DP has the responsibility of
sending BPDUs on to the segment. [2]
At the end of all this, if a port is not a RP or a
DP, it is put into the blocking state. [2]
3.2 STP timers


Hello Timer – This is how often the root
bridge will send out BPDUs. These BPDUs
get relayed down the spanning-tree to all the
other switches. The default is 2 seconds. [2]
Max Age Timer – This is how often a bridge
will actually save the BPDU information it
receives from other switches. Think of it as
sort of a hold timer. The default is 20 seconds,
and it helps prevent against loops in the event
of indirect link failures. [2]



Blocking — In the blocking state the port is
essentially shut down. The switch discards
frames received on the interface. It will
receive BPDUs from the DP on the segment
but will not pass them along to other switches.
A switch will go through the blocking state
when it is first initialized (boots up) and it will
place ports that could cause L2 loops into
blocking when necessary. The blocking state
is typically only seen during indirect link
failures.[2]
Listening — In listening state the port is
starting to transition into doing something. In
this state, the switch will actually process the
BPDUs it receives on the port although we are
still discarding frames at this point. Note that
per the RFC Listening and Learning MUST be
the same amount of time. [2]
Learning — In the learning state the port
continues it’s transition by learning MAC
addresses on the port, continuing to receive
and process BPDUs, and transmitting BPDUs
on to neighboring switches.[2]
Forwarding — In the forwarding state the
port is up and running. At this point the port
actually forwards frames and continues to
monitor BPDUs. [2]
Disabled — This isn’t really a state of STP.
This means STP is essentially turned off. [2]
3.4 Example of Direct and Indirect link failure
In picture 1 can be that Fa0/23 on Cat3 goes
into the blocking state to prevent an L2 loop from
occurring. In addition the link between Cat1 and Cat2 is
shut down. This will be an example of an indirect link
failure from the perspective of Cat3 and a direct link
failure from the perspective of Cat1. [2]
Cat1:

Cat1 lost it’s root-port and has no idea who the
root bridge is. Therefore, Cat1 advertises itself
as the root bridge out fa0/21 towards Cat3
immediately. [2]

After max-age expires over on Cat3, Cat3
transitions Fa0/23 into listening mode which
means it now forwards BPDUs from the path
Cat2 –> Cat4 –> Cat3 over to Cat1. Cat1
realizes it is not the real root bridge and
submits to Cat2 being the real root. [2]


Cat3:





4.
When the Cat1/Cat2 link goes down Cat3 starts
receiving BPDUs from Cat1 who is now
claiming to be the root bridge. Cat3 will ignore
these claims completely until the max-age
timer expires. [2]
Cat3 transitions Fa0/23 from blocking into
listening after max-age expires. It learns it’s
new root-port is via Fa0/23 and awaits to move
it into learning and finally forwarding. [2]
15 seconds after going into listening Cat3`s
Fa0/23 goes into learning. [2]
15 seconds after going into learning Cat3`s
Fa0/23 goes into forwarding. [2]
The total convergence time for the whole
network here is 50 seconds. [2]
Virtual Router Redundancy Protocol (VRRP)
Virtual Router Redundancy Protocol (VRRP) is a
non-proprietary redundancy protocol described in RFC
3768 designed to increase the availability of the default
gateway servicing hosts on the same subnet. VRRP
introduces the concept of a “virtual router” that is
addressed by IP clients requiring gateway service. The
actual routing service is provided by physical routers
running the VRRP protocol. An example of this is
shown in Picture 2. Two or more physical routers are
then configured to stand for the virtual router, with only
one doing the actual routing at any given time. If the
current physical router that is routing the data on behalf
of the virtual router fails, an arrangement is made for
another physical router to automatically replace it. In a
VRRP configuration, one router is elected as the virtual
router master, with the other routers acting as backups
in case the virtual router master fails. [3] [4]


Router rC does not have VRRP function, but
uses the VIP for VRID 3 to reach the
ClientLAN subnet. [4]
Router rD is the master of VRID 2. Router rF
is the master of VRID 5. Router rE is the
backup for both of these VRIDs. If rD or rF
fails, rE will become the master for that VRID.
In fact, both rD and rF could fail at the same
time; the fact that a VRRP router is a master
for one VRID does not preclude it from being
master for another. [4]
Router rG is the WAN gateway for the
Backbone LAN. All of the routers attached to
the backbone are sharing routing information
with the routers on the WAN using a dynamic
routing protocol such as OSPF. VRRP is not
involved in this, although Router rC will
advertise that the path to the Client LAN
subnet is via the VIP of VRID 3. [4]
Router rH is the master of VRID 10 and
backup for VRID 11. Router rJ is the master
for VRID 11 and the backup for VRID 10. This
is a VRRP load-sharing configuration and it
illustrates that multiple VRIDs can exist on a
single router interface. [4]
VRRP can be used as part of a network design that
provides almost total routing redundancy for all systems
in the network. [4]
4.1 VRRP concepts
VRRP concepts are showed in Picture 2.
 Router rA is the master of virtual router VRID
1, and the backup for VRID 3. At this time, it
handles the routing of packets addressed to the
VIP for VRID1, and is ready to take on the
routing role for VRID 3. [4]
 Router rB is the master of virtual router VRID
3, and the backup for VRID 1. At this time, it
handles the routing of packets addressed to the
VIP for VRID3, and is ready to take on the
routing role for VRID 1. [4]
Picture 2 VRRP concepts
4.2 Simple environment with VRRP
requirements. With growing technologies such as Voice
over IP (VoIP) and Video over IP, fast convergence
around link or component failures is a must. Because
things break and unforeseen events do take place, there
is the need for creating an architecture that is ‘highly
available’.
7.
References
[1 ]
Cisco, spanning tree protocol introduction
http://www.cisco.com/en/US/tech/tk389/tk621/tsd_tech
nology_support_protocol_home.html
Date of data acquisition 1 May 2011
Picture 3 Routing with VRRP
By configurating VRRP into environment, the
redundancy can be provided for outgoing traffic.
In picture 3, Virtual IP (VIP) is configured as the
default gateway to linux guests (sA, sB, sC and sD).
Now VRRP will provide a continuous router service
across the two routers. [4]
5.
Results and Discussion
One of the ways to increase availability is to
provide redundancy for critical components when
building networks. This usually involves duplicating
links, switches and routers to ensure continuity of
service across failures. [4]
The 802.1D Spanning Tree Protocol (STP) standard
was designed at a time when the recovery of
connectivity after an outage within a minute or so was
considered adequate performance. Rapid Spanning Tree
Protocol (RSTP; IEEE 802.1w) can be seen as an
evolution of the 802.1D standard. [5]
Dynamic routing protocols are used to keep the
network running, routing traffic around network
problems. It can be difficult to provide this level of
redundancy at the endpoints of the network. Due to
prohibitive cost and duplication of horizontal cabling it
is impractical to provide multiple network connections
for end-stations. Running dynamic routing protocols on
end-stations, to allow them to take advantage of
multiple network paths and/or multiple gateways, is not
feasible due to the network overhead and resulting
complexity of the routing environment. VRRP gives
network designers a way to provide reliable, redundant
gateway service for IP end-stations. [4]
6.
Conclusions
Today’s businesses require reliable network
connectivity. Switched networks must fulfill stringent
robustness,
resiliency,
and
high-availability
[2 ]
Joe Astorino,
Spanning-Tree Direct VS Indirect Link Failures
Available on the Internet
http://blog.ipexpert.com/2010/03/22/spanning-treedirect-vs-indirect-link-failures/
Date of data acquisition 1 May 2011
[3 ]
Cisco IOS and NX-OS Software,
Configurating VRRP
Available on the Internet
http://www.cisco.com/en/US/docs/ios/ipapp/configurati
on/guide/ipapp_vrrp.html
Date of data acquisition 11 May 2011
[4 ]
Linux on IBM zSeries and S/390,
Virtual Router Redundancy Protocol on
VM Guest LANs
Available on the Internet
http://www.redbooks.ibm.com/redpapers/pdfs/redp3657
.pdf
Date of data acquisition 12 May 2011
[5]
Cisco, Understanding Rapid Spanning Tree Protocol
(802.1w)
Available on the Internet
http://www.cisco.com/en/US/tech/tk389/tk621/technolo
gies_white_paper09186a0080094cfa.shtml#conclusion
Date of data acquisition 14 May 2011
[6]
WindowsNetworking.com,The importance of network
redundancy
Available on the Internet
http://www.windowsnetworking.com/articles_tutorials/I
mportance-Network-Redundancy.html
Date of data acquisition 1 May 2011