IP hosts can use several methods of deciding which default router or default gateway to use—DHCP, BOOTP, ICMP Router Discovery Protocol (IRDP), manual configuration, or even by running a routing protocol (although having hosts run a routing protocol is not common today).
The most typical methods—using DHCP or manual configuration—result in the host knowing a single IP address of its default gateway. Hot Standby Router Protocol (HSRP), Virtual Router Redundancy Protocol (VRRP), and Gateway Load Balancing Protocol (GLBP) represent a chronological list of some of the best tools for overcoming the issues related to a host knowing a single IP address as its path to get outside the subnet.
HSRP allows multiple routers to share a virtual IP and MAC address so that the end-user hosts do not realize when a failure occurs. Some of the key HSRP features are as follows:
■ Virtual IP address and virtual MAC active on the Master router
■ Standby routers listen for Hellos from the Active router, defaulting to a 3-second hello interval and 10-second dead interval
■ Highest priority (IOS default 100, range 1–255) determines the Active router, with preemption disabled by default
■ Supports tracking, whereby a router’s priority is decreased when a tracked object (interface or route) fails
■ Up to 255 HSRP groups per interface, enabling an administrative form of load balancing
■ Virtual MAC of 0000.0C07.ACxx, where xx is the hex HSRP group
■ Virtual IP address must be in the same subnet as the routers’ interfaces on the same LAN
■ Virtual IP address must be different from any of routers’ individual interface IP addresses
■ Supports clear-text and MD5 authentication (through a key chain)
Because HSRP uses only one Active router at a time, any other HSRP routers are idle. To provide load sharing in an HSRP configuration, the concept of Multiple HSRP, or MHSRP, was developed.
In MHSRP, two or more HSRP groups are configured on each HSRP LAN interface, where the configured priority determines which router will be active for each HSRP group.
MHSRP requires that each DHCP client and statically configured host is issued a default gateway corresponding to one of the HSRP groups and requires that they’re distributed appropriately. Thus, in an MHSRP configuration with two routers and two groups, all other things being equal, half of the hosts should have one HSRP group address as its default gateway, and the other half of the hosts should use the other HSRP group address.
HSRP is Cisco proprietary, has been out a long time, and is widely popular. VRRP (RFC 3768) provides a standardized protocol to perform almost the exact same function. The Cisco VRRP implementation has the same goals in mind as HSRP but with these differences:
■ VRRP uses a multicast virtual MAC address (0000.5E00.01xx, where xx is the hex VRRP group number).
■ VRRP uses the IOS object tracking feature, rather than its own internal tracking mechanism, to track interface states for failover purposes.
■ VRRP defaults to use pre-emption, but HSRP defaults to not use pre-emption. Both can be configured to either use pre-emption or not.
■ The VRRP term Master means the same thing as the HSRP term Active.
■ In VRRP, the VRRP group IP address is the interface IP address of one of the VRRP routers.
GLBP is a newer Cisco-proprietary tool that adds load-balancing features in addition to gatewayredundancy features. Hosts still point to a default gateway IP address, but GLBP causes different hosts to send their traffic to one of up to four routers in a GLBP group. To do so, the GLBP Active Virtual Gateway (AVG) assigns each router in the group a unique virtual MAC address, following the format 0007.B400.xxyy, where xx is the GLBP group number, and yyis a different number for each router (01, 02, 03, or 04). When a client ARPs for the (virtual) IP address of its default gateway, the GLBP AVG replies with one of the four possible virtual MACs. By replying to ARP requests with different virtual MACs, the hosts in that subnet will in effect balance the traffic across the routers, rather than send all traffic to the one active router.
Cisco IOS devices with GLBP support permit configuring up to 1024 GLBP groups per physical
interface and up to four hosts per GLBP group.