address as the router ID which is thus always invariant outward. To ensure the uniqueness of
the router ID, it is recommended to manually configure the router ID or the loopback interface.
In the automatic election, the router would in the first place select
the highest loopback interface
IP address
as the router ID. If the router doesn’t pre-define the loopback interfaces, it would
select the
highest physical interface IP address
as the router ID.
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.
3)
P2MP (Point-to-MultiPoint):
In general, P2MP type of network is converted from NBMA,
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
204
Содержание T3700G-28TQ
Страница 1: ...User Guide T3700G 28TQ T3700G 52TQ 1910012358 REV3 0 0 November 2018...
Страница 11: ...XI...
