Thursday 4 June 2015

OSPF Broadcast & Route Redistribution Basics


I have tomorrow off work so I've kind of spent a bit of time tonight working out what I can do with my baby lab (it definitely needs to grow more... but there's still a bit you can do with 4 routers and a layer 2 switch). I'm pretty tired now and ready to collapse into bed, but I thought I'd do a quick update.

Here's what my lab looks like at the moment (no rack yet... its definitely a work in progress and the goal is to add to it each payday. I think the rack will be my final purchase)

So what can I play around with this setup. Well, I did some OSPF broadcast (makes sense) and then did a little bit of route redistribution.

OSPF Broadcast

So when OSPF is run on an ethernet interface it is automatically assumed to be in a broadcast network. Here is the output of my R1 in a OSPF broadcast network situation:

Some points I learnt about OSPF broadcast networks tonight are:
  • as mentioned ethernet defaults to broadcast
  • the timers set to 10s for Hellos, 40s for Dead Timer
  • the broadcast address is
  • and although the DR router sends the updates, the next hop stays the originating router

Route Redistribution

I was getting pretty tired by this stage, but here is my R1 with RIP, Inter-Area OSPF, External EIGRP and External OSPF routes.
I guess the main takeaway I got from this is that every routing protocol uses its own metric:
  • RIP: hop count
  • EIGRP: a formula based on bandwidth, delay, reliability, load & mtu
  • OSPF: cost based on bandwidth
When redistributing routes into RIP you have to set a metric for the hop count. 3 is good, anything higher than 16 is ignored.

When redistributing routes into EIGRP you have to set values for all 5 values used in the formula. Apparently mtu isn't used but you still have to set it. It seems pretty standard to just use 1, 1, 1, 1, 1

When redistributing routes into OSPF you don't have to set a value for the metric, OSPF defaults to a metric of 20 with a metric-type of 2.

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