Ospf question
pakdub992
Registered Users Posts: 4 ■□□□□□□□□□
in CCNA & CCENT
Using Designated Routers on Ethernet Links
OSPF behaves differently on some types of interfaces, particularly comparing point-topoint
and Ethernet links. In particular, on Ethernet links, OSPF elects one of the routers on
the same subnet to act as the designated router (DR). The DR plays a key role in how the
database exchange process works, with different rules than with point-to-point links. To see
how, consider the example that begins with Figure 7-9. The figure shows five OSPFv2 routers
on the same Ethernet VLAN. These five OSPF routers elect one router to act as the DR,
and one router to be backup DR (BDR). The figure shows A and B as DR and BDR, for no
other reason than the Ethernet must have one of each.
The above paragraph has been taken from ICND2 Wendell Odom book. My question is regarding the bold sentence where it is mentioned that five OSPF routers on the same VLAN. However, my understanding was that VLANs can't span across Routers then how come routers can be on same Ethernet VLAN.
I will be greatly thankful to those who can clear my confusion.
OSPF behaves differently on some types of interfaces, particularly comparing point-topoint
and Ethernet links. In particular, on Ethernet links, OSPF elects one of the routers on
the same subnet to act as the designated router (DR). The DR plays a key role in how the
database exchange process works, with different rules than with point-to-point links. To see
how, consider the example that begins with Figure 7-9. The figure shows five OSPFv2 routers
on the same Ethernet VLAN. These five OSPF routers elect one router to act as the DR,
and one router to be backup DR (BDR). The figure shows A and B as DR and BDR, for no
other reason than the Ethernet must have one of each.
The above paragraph has been taken from ICND2 Wendell Odom book. My question is regarding the bold sentence where it is mentioned that five OSPF routers on the same VLAN. However, my understanding was that VLANs can't span across Routers then how come routers can be on same Ethernet VLAN.
I will be greatly thankful to those who can clear my confusion.
Comments
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ccie14023
Member Posts: 183
VLANs actually can span across routers, but let me not confuse you too much there since you're working on the basics, and the basic theory would state that they cannot.
The key word there is "across". If five routers are connected to a VLAN, each router just has one leg into that VLAN. In other words, the VLAN is not crossing any of the routers. If you take any one of the five routers, the VLAN terminates on it. If we have, say, VLAN 100, there will be an interface on each router in VLAN 100. However, VLAN 100 won't be on any other interface of that router.
Just because a VLAN cannot (theoretically) cross a router does not mean there can only be one router per VLAN. There can be as many as you want.
