Specifications

Configuring Routing Between VLANs
How to Configure Routing Between VLANS
20
Configuring the Hot Standby Router Protocol over ISL
The Hot Standby Router Protocol (HSRP) provides fault tolerance and enhanced routing performance
for IP networks. HSRP allows Cisco IOS routers to monitor each other’s operational status and very
quickly assume packet forwarding responsibility in the event the current forwarding device in the HSRP
group fails or is taken down for maintenance. The standby mechanism remains transparent to the
attached hosts and can be deployed on any LAN type. With multiple Hot Standby groups, routers can
simultaneously provide redundant backup and perform loadsharing across different IP subnets.
Figure 79 illustrates HSRP in use with ISL providing routing between several VLANs.
Figure 79 Hot Standby Router Protocol in VLAN Configurations
A separate HSRP group is configured for each VLAN subnet so that Cisco IOS router A can be the
primary and forwarding router for VLANs 10 and 20. At the same time, it acts as backup for VLANs 30
and 40. Conversely, Router B acts as the primary and forwarding router for ISL VLANs 30 and 40, as
well as the secondary and backup router for distributed VLAN subnets 10 and 20.
Running HSRP over ISL allows users to configure redundancy between multiple routers that are
configured as front ends for VLAN IP subnets. By configuring HSRP over ISLs, users can eliminate
situations in which a single point of failure causes traffic interruptions. This feature inherently provides
some improvement in overall networking resilience by providing load balancing and redundancy
capabilities between subnets and VLANs.
To configure HSRP over ISLs between VLANs, you need to create the environment in which it will be
used. Perform the tasks described in the following sections in the order in which they appear.
SUMMARY STEPS
1. enable
2. configure terminal
3. interface type slot/port.subinterface-number
Cisco IOS
router
Cisco VLAN
switch
Cisco VLAN
switch
ISL ISL
ISL
HSRP
VLAN 20 VLAN 10 VLAN 40
VLAN 10 VLAN 30 VLAN 20
Cisco IOS
router
S6620