IP Routing Advanced Info

Howto by jmke @ 2003-01-23

This guide will be updated with more info now & then! Want to know what makes the internet work? Here you can find some info at least :)
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RIP : Routing Information Protocol
(based on hop count)

- RIP update is sent out every 30 seconds on all RIP configured interfaces. Update contains known networks and the distances (hop count) associated with each one.
- No mask information is exchanged!
- 1 network , 1 subnetmask
- All networks need to be together (you should be able to go anywhere in the network without leaving it)
- If received advertisement results in better path (smaller hopcount) -> update routing table
- RIP handles equal cost paths by using the first one it learns, unless multipath is configured
- RIP 1st timeout: 90 seconds, cost raised to U (infinity)
- RIP 2nd timeout: 180 seconds, removed from routing table

- first entry in RIP-2 update is used for authentication purposes
- subnet mask is included in a RIP-2 update (unless RIP-2 aggregate is configured)
- Next hop address can be specified (RIP-1 next hop address is always derived from the source address from the RIP-1 update)
- Multicast: RIP-2 multicasts packets to Reduces load on routers not running RIP. Can share RIP-2 information independant from RIP-1

Loop Prevention:
- Poisoned Reverse: send out RIP update with hop count of 16 (max = 16) -> packets dies after arrival/not send further on
- Split Horizon: Do not send RIP updates out of the interface where they came from
- Actual: No Loop Prevention.

IP addressing in FrameRelay

- Group Access Mode: All PVC´s form 1 LAN
== Full-meshed: every FR node has a PVC to every other FR node
== Non-Fully meshed: each node has a PVC to a central node (hub/spoke)
Split Horizon/Poison Reverse doesn´t work in Non-Fully meshed mode and will prevent the spokes from communicating with each other. To resolve: set central node to Actual, or define policy no to send out updates & set default routes to the spokes.

- Direct Access Mode: Each PVC is a Point-to-Point link & each PVC has a unique IP address

Unnumbered Interfaces

- Unnumbered IP interface is configured for IP operation but lacks an IP address and mask
- Usefull on Point-to-Point links:
-- Conserving network address space
-- IP interface config IP & mask:
-- Unnumbered Assoc Address = existing IP on the router. The primarily source address of all packets that originate from the unnumbered interface
-- Cannot ping/manage an Unnumbered Interface
-- ARP is not used with an Unnumbered Interface
-- Unnumbered IP link does not appear in the routing table (has no network number)
-- Unnumbered IP link next hop = circuit instance number

OSPF : Open Shortest Path First
(based on cost)

+ used in large networks
+ supports numbered and unnumbered links
+ use of different subnets in the same network
+ supports equal cost paths
+ converges quickly
- CPU & Memory Intensive
- Requires carefull planning
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