So it's that time again for me to renew my Cisco Certifications. As a result, this post is based on my preparation for the CCNP Route Exam (300-101).
In this post I will be focusing on OSPF basics
Open Shortest Path First (OSPF)
- Uses Link-state (LS)
- Database is called Link-state database
- Information held in Link State Database include router IDs, interface IP address, mask and subnet, list of reachable routers
- Each router interface exists in a single area
- Uses Multicast Hello Messages
- Process ID does not need to match for host to become neighbors
- Uses Finite State Machine (FSM)
- Uses 8 states to determine neighbors status
- Reducing hello and dead timers can speed up convergence
Area Border Routers
- Uses Area Border Routers (ABR)
- ABRs are responsible for sharing information between different areas
- ABRs doe not advertise detailed information. They instead advertise subnets prefix and length
- Backbone (Area 0) is connected to all other areas
- All communications between non backbone devices must pass through at lease 1 backbone
- ABR keeps a copy of the Link State Database for each area attached to it
- First network number matching an interface shown in the running config determines the interface area
- Does not forward Type 1 or Type 2 LSA into another area
OSPF Key Terms
- Link-state Database (LSDB) - stores topology database
- Shortest Path First (SPF) - The algorithym used by the protocol
- Link-state update (LSU) - Contains detail topology information
- Link State Advertisement (LSA) - Holds topology information. Gets sent out via LSU
- Area - Contigious group of routers
- Area Border Router (ABR) - Has interfaces connected to two or more areas
- Backbone Router - Connects with other router
- Internal Routers - Connected to only one area
- Designated Router (DR) - Elected to perform special functions
- Backup Designated Router (BDR) - Secondary DR Router
- Transport - Uses TCP/UDP 89
- Metric - cost of all outgoing interface in a route-id
- Hello Interval - notify neighbors (10 seconds default on LAN)
- Dead Interval - detect failed neghbors (40 seconds default on LAN)
- Update destination address - sent to broadcast 224.0.0.5 and 224.0.0.6
- Updates (Full or partial)
- Authentication - MD5 and clear text
- VLSM/classless - masks gets included in each route
- route tags - tag for redistributed routes
- next-hop field - manually specify a next hop router other than the one advertised
- Manual Route Summarization - summary routes
Steps for choosing router id
- router-id command takes precedence
- if no router-id configured, highest IP on loopback interface is used
- if no route-id or loopback interface, it uses the highest IP address on any non-loopback interface
Common OSPF configuration verification command
- show ip ospf interface brief - Shows OSPF enabled interface
- show ip protocols - shows OSPF network configurations
- show ip ospf neighbors - shows known neighbors
- show ip ospf database - LSA for connected areas
- show ip route-id - shows the routing table
Two classes of neighborship
- Neighbors
- Fully Adjacent Neighbors
The following must match for neighborship to be completed
- router id
- stub area flag
- hello interval
- dead interval
- subnet mask
- list of neighbors reachable on the interface
- area id
- router priority
- designated router (DR) IP address
- Backup DR IP address
- Authentication digest
OSPF Network Types
- Broadcast
- Point-to-point
- Normally serial links
- DRs are not typically elected
- Loopback
- Nonbroadcast (NBMA)
- Point-to-multipoint
- Point-to-multipoint nonbroadcast
OSPF Virtual Links
- two ABRs connecting to the same nonbackbone area can form neighborship
- acts like a virtual point-to-point connection between two routers
- ABR sends messages as unicast
- Do Not Age (DNA) Bit is set LSAs - other routers do not expect flooded messages every 30 minutes
- uses area virtual-link command
- There is no interface to use for virtual links
References:
CCNP official Cert Guide - CCNP Routing and Switching - Route 300-101
https://www.ietf.org/rfc/rfc2328.txt
http://www.cisco.com/c/en/us/td/docs/security/asa/asa82/configuration/guide/config/route_ospf.html
http://www.cisco.com/c/dam/en/us/products/collateral/ios-nx-os-software/open-shortest-path-first-ospf/prod_presentation0900aecd80310f6d.pdf
http://www.cisco.com/c/dam/en/us/products/collateral/ios-nx-os-software/open-shortest-path-first-ospf/prod_presentation0900aecd80310f71.pdf
http://www.cisco.com/c/en/us/products/collateral/ios-nx-os-software/open-shortest-path-first-ospf/whitepaper_c11-668030.html
http://en.wikipedia.org/wiki/Open_Shortest_Path_First
http://www.cisco.com/networkers/nw00/pres/2205.pdf
http://www.cisco.com/networkers/nw00/pres/2206_7-5.pdf
http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html
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