Local-Area Networks and LAN Switching - CCIE Routing and Switching Exam Certification Guide - page 135

forwarding ports. When VLANs are mapped into a MISTP instance, it reduces the number of
BPDUs because only MISTP BPDUs are sent between the switches configured for MISTP,
which allows STP to scale to larger networks such as MANs.

CRB

CRB, introduced in IOS 11.0, enables the administrator to both route and bridge the same
protocol on separate interfaces. With CRB, the routed traffic is confined to the routed interfaces,
and bridged traffic is confined to the interfaces configured with bridge groups. Prior to CRB,
routers could only bridge or route the same protocol—but not both.

As shown in Figure 4-18, IP traffic is routed on Ethernet interfaces 0 and 1 and concurrently
bridged between Ethernet interfaces 2 and 3.

CRB Figure 4-18

E3: bridge group 1

BRIDGE IP

E0: 192.168.1.1/24

E2: bridge group 1

ROUTE IP E1: 192.168.2.1/24

The router configuration is displayed in Example 4-5.

Configuration Example of CRB

Example 4-5

bridge 1 protocol ieee
bridge crb
!
interface ethernet 0
ip address 192.168.1.1 255.255.255.0
!
interface ethernet 1
ip address 192.168.2.1 255.255.255.0
!
interface ethernet 2
bridge-group 1
no ip address
!
interface ethernet 3
bridge-group 1
no ip address