Ospf Design Guidelines; Ospf And Cpu Utilization; Ospf Network Design Examples - Avaya 8800 Planning And Engineering

Layer 3 network design

OSPF design guidelines

Follow these additional OSPF guidelines:
• Use OSPF area summarization to reduce routing table sizes.
• Use OSPF passive interfaces to reduce the number of active neighbor adjacencies.
• Use OSPF active interfaces only on intended route paths.
Configure wiring closet subnets as OSPF passive interfaces unless they form a legitimate
routing path for other routes.
• Minimize the number of OSPF areas per switch to avoid excessive shortest path
calculations.
The switch executes the Djikstra algorithm for each area separately.
• Ensure that the OSPF dead interval is at least four times the OSPF hello interval.

OSPF and CPU utilization

When you create an OSPF area route summary on an area boundary router (ABR), the
summary route can attract traffic to the ABR for which the router does not have a specific
destination route. The enabling of ICMP unreachable message generation on the switch may
result in a high CPU utilization rate.
To avoid high CPU utilization, Avaya recommends that you use a black hole static route
configuration. The black hole static route is a route (equal to the OSPF summary route) with
a next-hop of 255.255.255.255. This ensures that all traffic that does not have a specific next-
hop destination route is dropped.

OSPF network design examples

Three OSPF network design examples are presented in the sections that follow.
Example 1: OSPF on one subnet in one area
Example 1 describes a simple implementation of an OSPF network: enabling OSPF on two
switches (S1 and S2) that are in the same subnet in one OSPF area. See the following figure.
140 Planning and Engineering — Network Design November 2010