TP-Link JetStream T2700G-28TQ User Manual page 190

28-port gigabit stackable l2+ managed switch
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In the automatic election, the router would in the first place select
as the router ID. If the router doesn't pre-define the loopback interfaces, it would select the
address
highest physical interface IP address
3. OSPF Network Types
OSPF, a dynamic routing protocol running in the network layer, would apply different working
mechanism according to the features of different data link layers. There are four sorts of
relationships between the working mechanism of OSPF routing protocol and network type.
1)
Broadcast: When the network type is Ethernet or FDDI, OSPF protocol would broadcast the
Hello, LSU and LSAck packets. For instance, the Hello packet is multicast to the other OSPF
routers in the LAN and the destination address is the reserved 224.0.0.5, while the other
routers forward the link state update and acknowledgement data to OSPF DR with the
reserved multicast address as 224.0.0.6. In such broadcast type of network the DD and LSR
packets are unicast.
2)
NBMA (Non-Broadcast Multi-Access): In such type of network as frame relay, ATM or X.25,
where the routers need extra configuration to find neighbors, the OSPF protocol packets are
unicast.
P2MP (Point-to-MultiPoint): In general, P2MP type of network is converted from NBMA,
3)
where the Hello packet is multicast (224.0.0.5), LSU and LSAck packets are multicast
(224.0.0.5) or unicast, DD and LSR packets are unicast.
4)
P2P (Point-to-Point): When the link layer protocol is PPP or HDLC, the link always connects
a pair of routers, who could generally establish an adjacency relationship after becoming valid
neighbors. In this type of network, the protocol packets are multicast (224.0.0.5).
Our switches are all Ethernet ones. The network type of all the interfaces defaults to Broadcast,
and it also supports to be configured as P2P type that can automatically find neighbors. To ensure
the communication of multi-point networking, it's not recommended to manually configure the
network type of interfaces. In the following guide, we will mainly take the broadcast type of
interface for example to introduce the working principle of OSPF protocol.
4. Designated Router and Backup Designated Router
On broadcast networks or NBMA networks, usually there are multiple routers running OSPF
protocol at the same time. If the neighbor relationship between any two routers is adjacency, the
change of one router could result in the repeated forwarding of route updates and a waste of
network resources.
DR (Designated Router) and BDR (Backup Designated Router) defined by OSPF protocol would
maintain the entire network, while the other routers only need to establish adjacency relationships
with DR and BDR. DR is responsible to flood the routing information in the network to all the
neighbors. When DR fails, BDR will become the new DR, which avoids network block during the
DR re-election. Then a new BDR needs to be re-elected for sure, but the process would not affect
the communication even though it still requires quite a long time. Once DR and BDR are
determined in a network, unless they become invalid, any new routers joining or exiting would not
cause re-election.
As shown below, on a network of five routers, ten adjacency relations need to be established if one
between every two routers, but only seven adjacencies are required if DR and BDR are introduced.
To conclude, on a network of N routers, N*(N-1)/2 adjacencies are required in general, but the
adjacencies required will be (N-2)*2+1 if DR and BDR are introduced. Therefore, the more routers
on the network, the more significant the advantages will be.
as the router ID.
178
the highest loopback interface IP

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