- Recall
the Ethernet frame format:
- Fact:
- A bridge only
uses:
- SrcID = the Ethernet Address (48 bits) of the source
- DestID = the Ethernet Address (48 bits) of the destination
- A bridge only
uses:
- Therefore,
I will represent an
Ethernet frame as:
- Structure of a bridge:
- A bridge
has 2 ports:
Each port is connected to a different Ethernet segment
- Each port
of the bridge has an
associated
forwarding table :
Forwarding table:
- Forwarding table contains the Ethernet Addresses of node attached to the Ethernet segment
Note:
- Recall that:
- Ethernet is a broadcast network
- Therefere:
- A bridge can hear (= receive) messages transmitted on both Ethernet segment
- A bridge
has 2 ports:
- Operation of an
Ethernet node:
- When an Ethernet node receives
an Ethernet frame:
if ( destination address == my Ethernet address ) { Deliver the data portion in the Ethernet frame } else { Discard frame }
- When an Ethernet node receives
an Ethernet frame:
- Operation of a bridge
- When a bridge receives an
(Ethernet) frame on a
port P:
if ( Destination address ∈ Forwarding Table(P) ) { Discard frame
}
else
{
Forward the frame on the other port
}
- When a bridge receives an
(Ethernet) frame on a
port P:
- Note:
- The bridge
does not change
any part of
an Ethernet frame !!!
- Later, you will learn about routers do need to change some part of the Ethernet frame
- The bridge
does not change
any part of
an Ethernet frame !!!
- Fact:
- Multiple Ethernet segments
connected by
bridges:
is considered as one Ethernet network
- Multiple Ethernet segments
connected by
bridges:
- Administrative problem
in the
operation of
a bridge:
- A bridge needs to know:
- The Ethernet Addresses of the all nodes in every Ethernet segment
(This information needed to correctly forward a Ethernet frame)
- A bridge needs to know:
- Early days:
- Once upon a time:
- network administrators had to enter the forwarding tables of a bridge manually !!!
- Once upon a time:
- Transparent bridging:
- Nowadays,
a bridge will
learn
the Ethernet address
- This technique is known as transparent bridging
- Nowadays,
a bridge will
learn
the Ethernet address