Module 8 Notes

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Emma edited this page Dec 1, 2025 · 1 revision

Interior Routing

Routing = building maps and giving directions
Forwarding = moving packets between interfaces using the "directions"
IP Routing
- Path derived from information received from a routing protocol
- Several alternative paths may exist -> best paths stored in forwarding table
- Decisions are updated periodically or as topology changes
- Decisions are based on: topology, policies and metrics (hop count, filtering, delay, bandwidth, etc.)
IP Route lookup
- Based on destination IP address
- "longest match" routing
  - More specific prefix preferred over less specific
IP Forwarding
- Router decides which interface to send a packet to
- Forwarding decisions:
  - destination address
  - class of service (fair queuing, precedence, others)
  - local requirements
- Forwarding usually aided by special hardware
RIB and FIB
- FIB -> Forwarding Table
  - Contains destinations and interfaces to get to those destinations
  - Used by router to send the packet
- RIB -> Routing Table
  - Contains a list of all the destinations and the various next hops used to get to those destinations - and lots of other information too.
  - One destination can have lots of possible next-hops only the best next hop goes into FIB
Routing Protocol
- IGP: processes running on routers in and an organization's network
  - Interior Gateway Protocol
  - OSPF, ISIS
- EGP: processes running between routers bordering directly connected ISP networks.
  - Exterior Gateway Protocol
  - Routing info between organizations
  - Decoupled from the IGP
  - Current widely-use EGP is BGP (Border Gateway Protocol)
Dynamic IGP Types
- Distance Vector
  - Routers broadcast with their routing table (every 30-60)
  - Other routers listen and update tables
  - Based solely on Hops
  - Noisy, slow, doesn't scale well
  - RipV2
- Link-state
  - More popular
  - Routers learn neighbors and send hellos (keep-alive) and link state updates
  - used more advanced route-selection metrics
  - Open Shortest Path First (OSPF) is the most popular
OSPF
- OSPF doesn't use UDP or TCP it is its own layer 4 - IP protocol 89
- Uses "Area" to determine which routers listen and talk to each other
Configuring OSPF
- Easy on Cisco
- Create an OSPF instance: The process ID specific to that router
- #(config) router ospf instance_number (usually just 1)
- Don't confuse instance and area
  - Instance: Router can have multiple OSPF instances routing such as a router connecting two organizations and configuring OSPF for both
  - Area: Must be the same among all routers that need to share info
- Add all of the Directly connected Networks on that router
  - Network address with wildcard mask and area #
  - #(config-router) network 192.168.3.0 0.0.0.255 area 0