Example 4: host routes, Example 4: host routes 92, Figure 4-8 – Nortel Networks WEB OS 212777 User Manual
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Web OS 10.0 Application Guide
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Chapter 4: OSPF
212777-A, February 2002
Example 4: Host Routes
The Web OS 10.0 implementation of OSPF includes host routes. Host routes are used for
advertising network device IP addresses to external networks and allows for Server Load Bal-
ancing (SLB) within OSPF. It also makes ABR load sharing and failover possible.
Consider the example network in
. Both Web switches have access to servers with
identical content and are configured with the same virtual server IP addresses: 10.10.10.1 and
10.10.10.2. Web switch #1 is given a host route with a low cost for virtual server 10.10.10.1
and another host route with a high cost for virtual server 10.10.10.2. Web switch #2 is config-
ured with the same hosts but with the costs reversed; one host route has a high cost for virtual
server 10.10.10.1 and another has a low cost for virtual server 10.10.10.2.
All four host routes are injected into the upstream router and advertised externally. Traffic
comes in for both virtual server IP addresses (10.10.10.1 and 10.10.10.2). The upstream router
sees that both addresses exist on both Web switches and uses the host route with the lowest
cost for each. Traffic for 10.10.10.1 goes to Web switch #1 because its host route has the low-
est cost for that address. Traffic for 10.10.10.2 goes to Web switch #2 because its host route has
the lowest cost. This effectively shares the load among ABRs. Both Web switches then use
standard Server Load Balancing (SLB) to distribute traffic among available real servers.
In addition, if one of the Web switches were to fail, the upstream routing device would forward
the traffic to the ABR whose host route has the next lowest cost. In this example, the remaining
Web switch would assume the entire load for both virtual servers.
Figure 4-8 Configuring OSPF Host Routes
ABR Load Sharing
Standard SLB
Preferred path
(lowest cost)
to Host Route
10.10.10.1
Preferred path
(lowest cost)
to Host Route
10.10.10.2
Virtual Servers/Host Routes:
10.10.10.1, Cost
1
10.10.10.2, Cost
100
Web Switch #1
Web Switch #2
Virtual Servers/Host Routes:
10.10.10.1, Cost
100
10.10.10.2, Cost
1
Area 1
Stub Area
Area 0
Backbone
Real Servers
with Identical
Content