OSPF and EIGRP

Until recently, I didn’t delve much into the EIGRP protocol. Despite Cisco opening it up to IETF in 2013, I have yet to run into EIGRP on another vendor’s platform, so in my opinion it was still relegated to Cisco only networks.

However, I decided to take the plunge into Cisco’s certification track as a side project and completed the first step this week – the CCNA! Along the way I had to pick up some rudimentary information around the EIGRP protocol and how it compares to OSPF.¬†Contrary to some marketing experts, there are some key differences between EIGRP and OSPF that are useful to keep in the back of your head when designing networks.

First, the similarities:

  • They are both internal L3 routing protocols
  • They both converge quickly
  • They both scale very well
  • They both have summarization, filtering, and metric tuning capabilities.

Next, the differences of note for EIGRP:

  • EIGRP keeps the successor route in the routing table, which is the best possible route to the destination. It also holds feasible routes in the topology table, which are alternate routes to the destination that can be utilized if the main route crashes and burns. This provides pretty fast failure recovery. OSPF developed a similar feature to this called Fast Reroute.
  • EIGRP supports unequal cost multipathing. OSPF requires equal costs paths before it will load balance, something that can be achieved with some manual tweaks.
  • EIGRP’s routing metric can be tweaked. By default it will only consider bandwidth and delay, but there are five possible metrics – Bandwidth, Load, Delay, Reliability, and MTU (kind of, only used as a tiebreaker). By contrast, OSPF looks at link speed alone.
  • EIGRP offers more flexible summarization – each EIGRP router can offer summarization, while OSPF is limited to summarization at the ABR or ASBR. This is due to differences between link state and distance vector mechanics.
  • On the same token, EIGRP offers more flexible route filtering – each EIGRP router supports filtering, while OSPF must filter at the ABR, ASBR, or directly on each router individually. Filtering type 1 or type 2 LSAs within the same area can cause routing loops.

This is a basic level summary, but my takeaway is that EIGRP seems to support more “nerd knobs” than OSPF.

Final verdict? While the network geek within me likes the fancy details, I’m not sure that those nerd knobs make a big enough difference for me personally to recommend EIGRP over OSPF in a new network. The business owner doesn’t care much about the geeky details – they just want to know that traffic will reach what it’s supposed to reach in a timely fashion, and both EIGRP and OSPF support that admirably. I see more and more multivendor systems being deployed… for example, Palo Alto has a very well built feature set, but they only support OSPF. HP has some very solid access layer switches, but they only support OSPF. If you chose EIGRP, you have a painful path ahead of you in the future if you want to take advantage of those products. By choosing an open routing protocol, you ensure that you can choose the best product fit for your company in the future.

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