Download Case Study: Dial Backup with Two Routers

Case Study: Dial Backup with Two Routers
Dial backup using a single router at the remote site still leaves a single point of
failure—the router at the remote site. The obvious solution is to install two routers at
the remote site, as illustrated in Figure 3-18.
Figure 3-18 Dial Backup with Two Routers
There are two problems with this solution; the first is that hosts on the
172.16.9.0/24 network must set their default gateways to either Router B's or
Router D's Ethernet IP address to reach the rest of the network. No matter which
one is used, if that router fails completely, all connectivity to this segment will
effectively be lost.
The second is Router B must be able to signal Router D that its serial link to Router A
has failed.
The first problem can be resolved using Hot Standby Router Protocol (HSRP). HSRP
allows Router B and Router D to share a virtual IP address between them with only
the active HSRP router accepting (and forwarding) traffic destined to that IP address.
Following is an exa mple of how this would work.
On Router B, the configuration is as follows:
interface e0
ip address 172.16.9.2 255.255.255.0
standby ip 172.16.9.1
standby priority 10
standby preempt
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On Router D, the configuration is as follows:
interface e0
ip address 172.16.9.3 255.255.255.0
standby ip 172.16.9.1
standby priority 20
standby preempt
Router B and Router D are both configured to act as standby routers for 172.16.9.1.
They will also create a virtual physical layer address between them, and the active
router will forward or process traffic transmitted to that physical layer address.
Because Router B needs to be the active route in normal operation, standby
priority and standby preempt have been configured. The hosts on the
172.16.9.0/24 segment will be configured to use 172.16.9.1 as their default
gateway.
When a host on the 172.16.9.0/24 network attempts to transmit a packet to a
destination that is off the local segment, it will ARP for its default gateway's physical
address, and the HSRP active router will answer with the virtual address. The host
will then send all off-segment traffic to the virtual address.
If Router B fails, Router D will take over as the active HSRP router and will begin
forwarding traffic across the ISDN link. This resolves the first problem—how to
configure the host's default gateway—but doesn't resolve the second problem.
How would Router B notify Router D that its serial link has failed? The simplest way
is to configure HSRP to track the state of the serial interface on Router B; if the serial
interface fails, Router D should take over.
On Router B, the configuration is as follows:
interface e0
ip address 172.16.9.2 255.255.255.0
standby ip 172.16.9.1
standby priority 10
standby preempt
standby track serial 0 20
When Router B's serial interface fails, it will increase its standby priority to 30, and
Router D will take over as the HSRP active router. Note this solution still relies on the
physical layer failing on Router B's Serial 0; on some types of links, it is possible to
lose IP connectivity while physical layer connectivity still appears to be good.