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

Layer 3 network design
For example, assume that a switch has a configuration of three areas with a total of 18 adjacencies
and 1000 routes. This includes:
• 3 adjacencies with an LSA_CNT of 500 (Area 1)
• 10 adjacencies with an LSA_CNT of 1000 (Area 2)
• 5 adjacencies with an LSA_CNT of 200 (Area 3)
Calculate the number as follows:
3*500+10*1000+5*200=12.5K < 40K
This configuration ensures that the switch operates within accepted scalability limits.

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.
June 2016 Planning and Engineering — Network Design
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