Ccna Ospf Cheat Sheet

Ccna Ospf Cheat Sheet

CCNA: OSPF CHEAT SHEET 1 CCNA: OSPF CHEAT SHEET Key Characteristics Router ID (RID) Type: Link State Algorithm: Dij

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CCNA: OSPF CHEAT SHEET

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CCNA: OSPF CHEAT SHEET Key Characteristics

Router ID (RID)

Type: Link State Algorithm: Dijkstra’s (Shortest Path First) Algorithm Standard: RFC 2328 Administrative Distance: 110 Metric: Cost Protocol/Protocol Number: IP/89 Authentication: Yes (MD5 and Plain Text) Supports VLSM and Route Summarization Support for IPv6 (RFC 2740) Fast Convergence

Router ID must be configured before an OSPF process could be started. Cisco Routers uses the following criteria to select a router ID:

Metric Calculation Cost = 100 Mbps / Link Speed OSPF cost can be modified in three ways: 1. 2. 3.

(config-if)#ip ospf cost (config-if)#bandwidth (config-router)#auto-costreference-bandwidth

Neighbor Process

1. RID configured with “router-id” command 2. If manual RID not configured, select the highest number IP address on any loopback interface in “up/up” state 3. If loopback interfaces not configured, select the highest number IP address on any non-loopback interface in “up/up” state

Router Types Internal Router: whose (all) interfaces resides within the same area Backbone Router: A router that resides in the backbone area Area Border Router: an ABR connect two or more Areas ASBR: Autonomous System Boundary Router or an ASBR connects an external routing domain to an OSPF routing domain

To become neighbors OSPF routers must be agree on the following parameters

Route Types and Preference

1. 2. 3. 4. 5. 6.

Intra-Area Routes: A route to a network in the same area as the router. Denoted by “O” in the routing table.

Area ID Same subnet Authentication (if used) Hello Interval and Dead Interval Area Type (Stub, NSSA) Router IDs must be unique

OSPF Neighbor States Down: Previously known neighbor has failed Init: an interim state in which Hello has been heard from the neighbor but that Hello does not list the local router’s RID Two-way: the neighbor has sent a Hello that lists the local router’s RID in the list of seen routers Full: Both routers complete the database exchange process and have identical LSDB. Fully adjacent

Inter-Area Routes: A route to a network in another area as the router. Denoted by “O IA” in the routing table External Route: A route to network that is external to the OSPF routing domain. Denoted by ‘E1’ or ‘E2’ in the routing table. Routes Preference: 1. Intra-Area (O) > Inter-Area (O IA) 2. Inter-Area (O IA) > External Type-1 (E1) 3. External Type-1 (E1) > External Type-2 (E2)

Areas OSPF runs SPF algorithm and requires a lot of processing power and memory. If the size of network is too large this could cause slower convergence and can lead to following problems: 1. 2. 3. 4.

more memory is required to maintain the link state database more processing power is required to process the link state database the links state database grows exponentially with the size of OSPF domain a single change in network topology (for example: link up/down) would trigger all routers to re-run the SPF (again) to calculate the shortest path To cope with these problems, areas are configured. There are two basic types: 1. Backbone Area or Area 0: All other area must be connected to area 0 2. Non-backbone Area: any other area with area-id other than zero

Timers Hellos are sent to multicast address: 224.0.0.5 (ALLSPFRouters) Broadcast Multi-access = 10 seconds Point-to-Point & NBMA = 30 seconds Dead Timer = Four Times the hello interval Broadcast = 40 seconds NBMA and P2P = 120 seconds To change hello and dead intervals use the command ‘config-if)# ip ospf hello-interval seconds’ and ‘config-if)# ip ospf dead-interval seconds’ 2

CCNA: OSPF CHEAT SHEET Designated Router (DR) / Backup DR (BDR) Election There are two problems with multi-access networks: 1. For “N” routers, it requires “N(N-1)/2” adjacencies 2. Flooding of this excess LSAs would be chaotic itself for the net