OSPF Routers on a Shared Ethernet Segment

[ksantillan06]

Hi.

I am currently studying multi area OSPF neighbor relationship topics.

Question is, why are some networks designed to have routers on a shared ethernet segment?

I am only familiar with routers representing branch offices connected to the HQ site. So this could mean the HQ would have 2 routers for redundancy and maybe another separate router to serve as a voice gateway.

Upon seeing some training videos, some examples illustrated 4 or 5 routers connected in a shared ethernet segment. What are some scenarios of having a network like this? Does this pertain to Metro-E ?

Thanks in advance.

Kev

[lmediavilla]

Most training have this design to show you how DR, BDR adn DROTHERS are elected, also about LSA 1, 2 and 3 is easier using the same ethernet segment.

Kind regards

[Jon Marshall · Accepted solution]

Kev

A lot of it depends on the size of the network. But you do get the situation more often than you would think. Bear in mind that not all devices that run OSPF are routers eg. L3 switches, firewalls etc. can run OSPF. So in the last place i worked we ran a pair of L3 switches for redundancy connected to a pair of firewalls. They all shared the same ethernet segment and ran OSPF to exchange routes. In this scenario a DR/BDR was needed and it was the pair of L3 switches.

Before L3 switches came on the scene you could only connect routers to L2 switches and so you used a shared vlan more often. With the advent of L3 switching you could then have routed ports so it is probably more common now to connect to routers using routed ports and each switch/router connection is a dedicated segment so you don't see that many OSPF routers on the same segment.

Jon

[ksantillan06]

Hi Jon.

Thanks for the reply. Just a follow up, is the DR/BDR election in a MetroE/VPLS network the same as the election in a LAN or shared ethernet segment?

[Jon Marshall]

Kevv

The short answer is i haven't used VPLS before so i can't say for sure. But if the underlying network supports shared access to the same segement and broadcasts then yes it should be ie. OSPF itself does not really care and the DR/BDR router election should be no different.

Not sure if i have answered your question but i have very limited knowledge of MetroE/VPLS setups.

Jon

[ksantillan06]

I see. Thanks Jon.

[Joseph W. Doherty]

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Posting

As Jon has already described, whether to have OSPF peers share a common segment have much to do with hardware capacity especially then vs. now.

Suppose you had 10 OSPF routers (not L3 switches), in a distribution role.  How might you interconnect them?

Well you could have an 11th additional "core" router, to which all the distribution routers connect.  But that 11th core router will likely need to be more powerful than any of your distribution routers, so you incur the additional cost of a more expensive (per unit) 11th router.  Also, you have an additional L3 hop, adding (a tiny bit) of latency.

Instead of a L3 core (the 11th router), you could use a L2 core.  Each distribution router now is on a shared segment with all the other distribution routers.  The L2 core might be provided by a hub or a switch.  Either likely much less expensive than a core router and a switch probably forwards faster than a router (again pre L3 switches).

So, basically, a shared segment, such as a L2 core, decreases cost and often (used to) increase performance.  (If you search Cisco's main site, you'll probably find some design documents for L2 cores.)

Could we still use shared segments today?  Yes, but with L3 switches, performance wasn't the issue it often was with "real" routers, and L3 cores, using p2p OSPF links, will generally detect OSPF neighbor loss faster than they will on shared segments (improving convergence time).  So, L3 cores are the more common design approach today.

[ksantillan06]

Noted Joseph. Thanks for the inputs.