Information Technology Reference
In-Depth Information
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Default behavior
—Prior to IRB and the CRB features (see later), you could bridge or route a
protocol only on a platform basis. That is, if the
ip route
command was used, for example, then IP
routing was done on all interfaces. In this situation, IP could not be bridged on any of the router's
interfaces.
•
Concurrent routing and bridging (CRB)
—With CRB, you can determine whether to bridge or
route a protocol on an interface basis. That is, you can route a given protocol on some interfaces and
bridge the same protocol on bridge group interfaces within the same router. The router can then be
both a router and a bridge for a given protocol, but there cannot be any kind of communication
between routing-defined interfaces and bridge group interfaces. For a given protocol, the router can
be logically considered as different independent devices: one router and one or more bridges, as
shown in Figure 20-5.
Figure20-5 The Router Can Be Logically Considered as Different Independent Devices
Bridge CRB
Interface E0
IP address X
Interface 1
IP address Y
Interface E2
Bridge-group 1
interface E3
Bridge-group 1
Bridge 1 protocol IEEE
Bridge-
group1
E0
E1
E2
E3
In this configuration for the IP protocol, the Cisco device is acting like a router for interfaces E0 and
E1 and is acting like a bridge for interfaces E2 and E3. Note that there is no communication possible
between the two functions (a host connected on E0 would never be able to reach a host connected
on E2 through the router with this configuration).
•
Integrated routing and bridging (IRB)
—IRB provides the capability to route between a bridge
group and a routed interface using a concept called Bridge-Group Virtual Interface (BVI). Because
bridging is in the data link layer and routing is in the network layer, they have different protocol
configuration models. With IP, for example, bridge-group interfaces belong to the same network and
have a collective IP network address, while each routed interface represents a distinct network and
has its own IP network address. The concept of Bridge-Group Virtual Interface was created to enable
these interfaces to exchange packets for a given protocol. Conceptually, the Cisco router looks like
a router connected to one or more bridge groups, as shown in Figure 20-6.
