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Route Summerization

 Manual Summarization with Any Routing Protocol

Manual summarization allows administrators to consolidate routes by summarizing smaller networks into a single summary route, helping to reduce the size of the routing table and speed up convergence. Each routing protocol has its own method of summarization.

EIGRP Summarization

EIGRP automatically performs summarization at classful network boundaries, but it can also perform manual summarization at any interface. EIGRP’s classless behavior allows the advertisement of both a prefix and a mask with each route, and summarization is critical for scalability in larger networks.

  • Technical Tip: Disabling automatic summarization is often preferred as it provides more control over route advertisements, especially in discontiguous networks. Summarization should be done both upstream and downstream for optimized traffic flow.
  • Summarization is configured in interface configuration mode, making it specific to certain neighbors. When summarization is applied, EIGRP generates a discard route (route to null0) to prevent routing loops.

Example:

R2(config)# interface serial1/3

R2(config-if)# ip summary-address eigrp 100 2.2.4.0 255.255.252.0

  • Technical Tip: Summarization triggers EIGRP to bring down and bring back up all neighbors on that interface, forcing them to relearn the network topology.

To create a summary that covers all Class A, B, and C networks:

R1(config)# interface serial1/2

R1(config-if)# ip summary-address eigrp 100 0.0.0.0 0.0.0.0

  • Administrative Distance (AD): When a summary is created, the route to null0 is assigned an AD of 5, helping in loop prevention.

OSPF Summarization

OSPF summarization occurs only on ABRs (Area Border Routers) or ASBRs (Autonomous System Boundary Routers). Summarization is essential to reduce the complexity of routing tables and improve the speed of Shortest Path First (SPF) calculations.

  • Technical Tip: Summarization works only for Type 1 LSAs and helps by limiting the scope of LSAs in the network. A well-summarized OSPF network will improve the efficiency of SPF calculations.

OSPF can summarize routes between areas using the area area-id range command on an ABR, or summarize external routes on an ASBR using summary-address.

Example for ABR summarization:

router ospf 1

area 12 range 172.16.0.0 255.255.0.0 cost 45

  • Technical Tip: When inter-area summarization occurs, the ABR creates a discard route (route to null0) to avoid routing loops if no specific route exists for the summary.
  • Discard Route Metrics: The AD for an OSPF discard route is 110 for internal networks and 254 for external networks. This prevents unwanted traffic looping in areas where summarized routes might not be fully covered.

Default Route Advertisement: OSPF can also advertise a default route into the domain using the default-information originate command. The always keyword ensures the advertisement of a default route even if one does not exist in the local routing table.

Example:

router ospf 1

default-information originate always metric 5 metric-type 1

  • Technical Tip: OSPF assigns a default metric of 1 for default route advertisements, and the metric-type option allows customization of the metric for external routes.

Summary of Technical Tips:

  • EIGRP Summarization: Manual summarization is essential for scaling, but be mindful of how EIGRP handles neighbors during this process. Discard routes are crucial for loop prevention.
  • OSPF Summarization: Only occurs at ABRs or ASBRs, and discard routes are also essential to avoid routing loops. Efficient summarization improves SPF performance significantly.
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