OSPF – Understanding Multicast Use in OSPF

igmpmulticastospfrouterrouting

When it comes to OSPF and its use of multicast, am I correct in believing that IGMP forwarding and an IGMP querier is still required within the local segment?

My understanding would be without this, the frames would be treated as a broadcast and sent to everyone. Therefore removing the benefit of using multicast.

Based on this is there any mechanism for it to fall back to unicast, to prevent the above. Apologies if my understanding here is way off : )

Best Answer

In terms of what happens to OSPF hello/discovery multicast packets, you're right. The switches send them to everyone on the subnet/VLAN/broadcast domain, as IGMP snooping isn't happening for 224.0.0.0/24, just as Ron explained.

However - and I believe that's the part you didn't consider - a host can still discard these multicast earlier than if they were broadcast. If the host isn't interested in (any/this) multicast group(s), it can drop the ethernet frame (at L2) or the IPv4/IPv6 packet (at L3), without even looking at any inner payload. A broadcast packet however would have to be passed 'further up the stack' to see if it is of interest to any of the running applications/services/deamons.

To add to Ron's list of OSPF's network types/operating modes:

It's good practice to run edge networks connected to OSPF routers with passive interfaces, especially where the OSPF network type is "broadcast" by nature. For example, this is the case in the proverbial "25 client VLANs" of a large campus, each with a few dozen of hosts each at least:

passive-interface

keeps the chatter in the campus LAN down
reduces exposure of routers to end systems (script-happy CompSci students, anyone?) - no Hellos nor LSAs are exchanged, neither in nor out.
saves some ressources on the routers (what with not having to perform the same DR/BDR election with the same peer in dozens of VLANs)
still allows the routing information about these networks to be propagated as internal routes to the rest of the OSPF AS. Having them as external routes (by virtue of "redistribute connected") has some implications (desirable or unwanted), e.g. when aggregating, redistributing or filtering routes in other corners of the AS.

Related Solutions

OSPF over vPC on Nexus7k

As the firewalls aren't part of a vPC they won't be part of the normal vPC loop prevention.

The loop prevention only states that a packet cannot ingress on the peer link if it is destined to go out another vPC enabled port.

Not too sure on the multicast front as we don't use it in our environment and I haven't really looked into its behaviour on the 7K's.

Usually if you are running a routing protocol down on the switch stack the recommended design would be to not have it as a member of a vPC and just use OSPF to give you the same advantages that vPC gives you at L2.

Quagga OSPF – DR/BDR Mismatch and Losing Routes

To answer your question directly, this is expected behavior by OSPF and not a bug in Quagga.

So first, let's take a look at the DR/BDR section of the RFC.

Designated Router
    The Designated Router selected for the attached network.  The
    Designated Router is selected on all broadcast and NBMA networks
    by the Hello Protocol.  Two pieces of identification are kept
    for the Designated Router: its Router ID and its IP interface
    address on the network.  The Designated Router advertises link
    state for the network; this network-LSA is labelled with the
    Designated Router's IP address.  The Designated Router is
    initialized to 0.0.0.0, which indicates the lack of a Designated
    Router.

Backup Designated Router
    The Backup Designated Router is also selected on all broadcast
    and NBMA networks by the Hello Protocol.  All routers on the
    attached network become adjacent to both the Designated Router
    and the Backup Designated Router.  The Backup Designated Router
    becomes Designated Router when the current Designated Router
    fails.  The Backup Designated Router is initialized to 0.0.0.0,
    indicating the lack of a Backup Designated Router.

If the BDR field in the Hello packet header is set to 0.0.0.0, it means you do not have a BDR elected.

In your case this is because you have your other router set to a priority of 0, this makes the router ineligible to become a BDR (this is why you see "DROther" and not "BDR"). You just need to set the priority on your other router to something that isn't 0.

Here is the other piece from the RFC for some more context.

Router Priority
    An 8-bit unsigned integer.  When two routers attached to a
    network both attempt to become Designated Router, the one with
    the highest Router Priority takes precedence.  A router whose
    Router Priority is set to 0 is ineligible to become Designated
    Router on the attached network.  Advertised in Hello packets
    sent out this interface.

https://www.ietf.org/rfc/rfc2328.txt

Best Answer

Related Solutions

OSPF over vPC on Nexus7k

Quagga OSPF – DR/BDR Mismatch and Losing Routes

Related Topic