Which multicast addresses does OSPF use, and who listens to each?
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Say this first
224.0.0.5 is AllSPFRouters: every OSPF router listens and hellos go there. 224.0.0.6 is AllDRouters:
only the DR and BDR listen. On a broadcast segment a DROTHER sends its updates to 224.0.0.6, and the DR
floods them back to everyone on 224.0.0.5. In IPv6, OSPFv3 uses FF02::5 and FF02::6.
Go deeper
Flow on a broadcast segment
- R3, a DROTHER, sees a link change and sends an LSU to
224.0.0.6. - The DR and BDR receive it. The BDR listens so it can take over without resynchronizing.
- The DR floods the LSU to
224.0.0.5, so every router on the segment gets it. - Routers acknowledge to the DR.
Other details
- Both are link-local scope (
224.0.0.0/24), sent with TTL 1 and never routed. - The MAC addresses are
0100.5e00.0005and0100.5e00.0006. - On point-to-point links everything goes to
224.0.0.5, since there is no DR. - Unicast is used for retransmissions, and for all packets on NBMA networks where neighbors are configured statically.
They will ask next
Why does it matter for a firewall or a switch in the path?
A firewall in transparent mode or an ACL on the interface must allow IP protocol 89 to these groups. Some switches with IGMP snooping misbehave with link-local groups, which shows up as neighbors stuck in INIT.
Traps
- Swapping the two addresses.
.5is everyone,.6is the DR and BDR.
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