Multicast and IGPs

[tidjes]

Hi,

Most IGP (OSPF, EIGRP,..) we know rely on the hello packets to establish neiborships. Knowing that the hello packets are multicast packets, how do those routers establish neiborship when multicast routing protocols are disabled? Most routers have multicast routing disable by default but when we enable  an IGP on two routers, we still get neiborship even though multicast routing is disabled.

[M02@rt37 · Accepted solution]

Hello @tidjes 

IGP protocols like OSPF and EIGRP use multicast addresses to send hello packets (and other control messages), but this does not require multicast routing to be enabled. This is because these protocols rely on link-local multicast, which operates within the same L2 broadcast domain and does not require the router to forward multicast packets between interfaces. Multicast routing, such as PIM, is only necessary for routing multicast traffic across subnets, not for processing or sending multicast traffic on a local interface. 

[Joseph W. Doherty]

Although M02@rt37 correctly describes why multicast routing isn't needed, I also wanted to mention multicast, operating within a L2:broadcast domain is much like broadcasts operating within the same L2 broadcast domain, which aren't routed either. However, hosts that "see" a multicast frame/packet can by examination of just the multicast address, can exclude multicast of no interest.

Also, keep in mind, again like broadcast, hosts can be in different unicast subnets, but as you can have multiple unicast subnets in the same L2 broadcast domain, routers might physically accept routing frames/packets from undesired unicast subnets which would be logically ignored.