Building Redundant Internet Connections using Dial Backup
Why Internet Dial Backup?
Corporate networks have long implemented network link redundancy to
mitigate the business losses associated with outage of crucial WAN
circuits. With businesses and other organizations increasingly dependent
on the Internet for electronic commerce, email, and marketing via the World
Wide Web, there is a corresponding need to consider implementing network
redundancy on dedicated, high speed synchronous Internet connections. Dial
backup is an easily implemented method for providing redundancy to
dedicated Internet connections. The lower costs of high speed analog and
digital modems combined with the increased availability of ISDN and
Switched 56 digital circuits make dial backup redundancy an affordable
option for today's Internet connected businesses.
The Components Needed for Automatic Internet Dial Backup
Dial Backup is performed by an Internet access router that connects the
internal corporate network to the Internet via a high speed synchronous
leased line or synchronous Frame Relay service at speeds up to T1/E1. To
perform dial backup, the access router must have a secondary asynchronous
routing port that can connect to a modem, ISDN Terminal Adapter (TA), or
Switched 56 CSU/DSU. In addition, the router must have the software
capability to sense when the primary synchronous link has failed and be
able to automatically dial out using the secondary routing port to
reconnect to the Internet. All Livingston Internet access routers,
including IRX Access Routers and PortMaster Synchronous Office Routers,
provide these hardware and software features and functionality.
Each Livingston Synchronous Office Router includes a single asynchronous
port using an RJ-45 interface that supports speeds up to 115.2Kbps. An
RJ-45 to DB-25 modem cable is used to allow connection to an external
modem, Switched 56 CSU/DSU or ISDN TA.
PortMaster Synchronous Office Router with T1/E1 Wan port and RJ-45 S0
Asynchronous routing port supporting speeds up to 115.2Kbps and dial on
demand backup routing. An RJ-45 to DB25 male cable is required to connect
the S0 port to a modem, Switched 56 CSU/DSU, or ISDN Terminal Adapter.
Each Livingston IRX Access Router includes an integrated asynchronous port
using a DB-25 interface that supports speeds up to 115.2Kbps. A DB-25
cable is used to connect this port to an external modem, Switched 56
CSU/DSU or ISDN Terminal Adapter (TA).
(Diagram)
IRX-114 with DB-25 S0 Asynchronous routing port that supports speeds up to
115.2Kbps and dial on demand backup routing. A standard male to male DB-25
modem cable is sufficient for connecting the S0 port to a modem, Switched
56 CSU/DSU, or ISDN Terminal Adapter. All IRX routers have an S0 port.
Livingston's ComOS router operating system provides all the software
features needed to implement automatic dial backup such as dial backup
triggering, automatic route sensing, and dial on demand routing.
The last component needed is an external analog modem, switched 56 CSU/DSU,
or ISDN Terminal Adapter and the corresponding telephone line to use for
the dial backup connection. A dial-up Internet connection from your
Internet Service Provider that will support your IP network or subnetwork
address space is also needed (i.e. a single user dial-in account will not
work as a dial backup routing account).
How Dial Backup is Triggered:
To perform automatic dial backup of a dedicated Internet connection, the
router must be able to sense if the dedicated WAN connection and its
associated Internet route has failed. The router must then be able to use
this to trigger a dial on demand LAN to LAN connection to resume routing
service.
Livingston routers sense the loss of a dedicated Internet connection in
three separate ways:
1. Carrier Detect Sensing: When the synchronous CSU/DSU supports Carrier
Detect, then the Livingston synchronous router port that is attached to the
CSU/DSU can be programmed to automatically detect if the CSU/DSU's WAN
connection is lost. Detection of loss by Carrier Detect is accomplished
by turning on the modem control option on the synchronous port. Modem
control carrier detect sensing is very efficient and allows for almost
immediate dialback restoral of Internet connectivity, as illustrated below:
(diagram)
2. Frame Relay Sensing: If Frame Relay is the dedicated synchronous WAN
service in use, then the Livingston synchronous router port can detect
whether or not the Frame Relay connection is up using the LMI or Annex-D
link maintenance protocols. LMI or Annex-D expect to receive status
packets at typically 10 second intervals. A threshold of six missed status
packets is used to indicate that the Frame Relay interface is down,
triggering dial backup approximately 60 seconds after the Frame Relay
circuit fails, as illustrated below:
(diagram)
3. Default Route Sensing: If neither of the above two methods are used,
the RIP routing protocol provided in Livingston access routers will sense
if the IP route associated with the synchronous router port has become
unavailable and initiate a dial backup connection. Following is a more
detailed explanation of how routers discover failed links and initiate dial
backup.
Understanding Default Routes and their Application to Dial Backup:
An Internet router is an intelligent switching device that examines all the
TCP/IP packets that enter its local area network (LAN) or wide area network
(WAN) ports. If a packet is destined for a different network than the port
it entered, the router consults its memory where it stores a list of
destination networks, the corresponding route to reach that destination,
and a numeric weighting of that route called a metric (the lower the
metric number, the higher priority that route assumes). This list is
known as a "Routing Table". If the packet's destination network is found
in its memory, the router sends the packet on the route specified by the
table. Since a RIP router cannot contain all possible destinations on the
Internet, it is possible that it will not know the route to the packet's
destination. In this case, the router uses a special, catch-all
destination and route called the default route. All packets whose
destinations aren't specifically in the router's memory are sent to this
destination. Default routes are entered into the router's memory or
"routing table" in one of two ways.
Dynamically Discovered Default Route: When RIP routing software is
configured in a router, it can be configured to "listen" for routes being
broadcast by other routers. The first neighboring router whose broadcast
reaches it is always listed as the default route for all otherwise unknown
destinations. This is known as a dynamic default route.
Statically (Manually) Configured Default Route: A system administrator can
configure a "static", or manual default route by listing a neighboring
router as the default router. In Internet parlance, this default router is
also known as the "default gateway setting".
Static Default Route Precedence: Unless specifically configured to be
otherwise, the static default route will always take precedence over a
dynamic default route. Assigning a higher metric number to the static
default route than the dynamic default route reverses this precedence.
(diagram)
Routing Configuration Required for Dial Backup:
In order to implement dial backup to the Internet with Livingston routers,
the router must be specifically configured so that the dynamic default
takes precedence over the static default route. This is accomplished by
assigning the static default route a high routing "metric" number, which
causes it to assume a lower priority in the routing table, while the
dynamically discovered default route assumes a higher priority since it
will have a lower metric number. Following are three conditions that must
be met to make dial backup default routes work properly:
=85 The Customer's router must listen for route broadcasts coming only from
the Internet Service Provider's router. Typically this condition is true
of stub networks that are only routing to the Internet and not internally.
The result is that the dynamic default route will always be the ISP's
router.
=85 The ISP's router must broadcast routes so that the Customer's router
receives routes from the ISP's router and enters it into its routing table
as the dynamic default router . If it is a Livingston router, it must have
a static default route configured.
=85 The router's manually programmed static default route must point to the
ISP's router, and must have a metric of 4 or higher. This causes the
static default route to be given lower routing priority than the dynamic
default route