Hour 28: IGP Review RIP

Consult the symbols legend at the end of the post for information on symbols.

RIP Review

Metric:

  • AD of 120
  • Hop-count. Max 15 hops.
  • Router adds 1 hop to each route sent to peers (locally connected routes have a metric of 0) this metric is installed in peer’s routing table. Remote peer does not add a hop to his updates, unless offset-list is used.
  • During redistribution from other protocols metric is set manually. This metric is announced to peers as it is. No additional metric is added when sending route to peers, unless offset-list is used.
  • [(RTR) offset-list <acl> {in | out} <offset> <IF>] Adds artificial metric to received or sent updates. If ACL is zero then no ACL is used. Can be used to filter updates by adding infinite offset (16). Route is not even added to database, it is dropped. Offset is added to ALL advertised routes, regardless if they are redistributed or originated by RIP.

Neighbors:

  • No Neighbor relationship and No Hello
  • [(IF) ip rip send version 1] Default is to send only RIPv2 messages but listens to v1 and v2. If v1 or v2 is manually defined in (RTR), only this version is sent and received on all interfaces, regardless of per-interface configuration.
  •  [(RTR)neighbor <ip>] sends unicasts updates to specified peer. Used in conjunction with [(RTR)passive-interface] on broadcast interface as the command does not suppress sending mcast/bcast updates, and peer will receive double updates
  • [(IF)ip rip v2-broadcast] suppresses mcast messages and sends bcast
  • V1: UDP/520 sent to broadcast (255.255.255.255)
  • V2: UDP/520 sent to 224.0.0.9 Continue reading

Hour 15: Distance Vector vs Link State analogy

One of the questions people new to networking often ask me is the difference between Distance Vector routing and Link State routing. There are many differences between them and you can research the details if you’d like to but I like to use two analogies to explain them. Here they are:

Distance Vector Routing: You are wandering in the Banff mountains of northern Alberta in Canada, a wonderful place to wander if you aren’t lost. But you are lost. You come upon a fork in the trail and you see a sign pointing west, reading “Edmonton 140km”. You have no choice but to trust the sign. You have no clue how the terrain is for the next 140km and you don’t know whether there is a better route or even if the sign is correct. In similar way, distance vector protocols provide road signs to networks. They provide the direction and the distance, but no details about what lies along the route.

Link-state Routing: You are in the biggest mall in Canada; the West Edmonton mall. You need to get to a computer store. You find the map of the mall and there’s a “you are here” dot on it, indicating where you are. From there you plan your way to the nearest computer store.

Just like every map in the mall is the same, the link-state database is the same in all routers within an area of a link-state routing protocol. The one difference between all the maps in the shopping mall is the “you are here” dot and so, the best path to a specific store will be different from each location in the mall. Link-state routing functions the same way, it calculates the best way to every network within the area, from their own perspective, using its own map; the Link-State Database.

I hope you liked these analogies as they’ve been really helpful to me.

On a side note: today I received my Cisco 2511 Access Server. I need to make a couple of roll-over cables to connect them to my home lab. I’m still waiting on 2x NM-1FE-2W WIC modules to be delivered. I need these for my 3600’s series Frame Relay switching router because the NM-1E-2W version does not support WIC-2T interface cards. So be careful if you plan on purchasing a 3600’s series router as only the FE modules support the WIC-2T interface cards.