Medianet Reference Guide - Cisco
Medianet Reference Guide - Cisco
Medianet Reference Guide - Cisco
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Chapter 3<br />
<strong>Medianet</strong> Availability Design Considerations<br />
Network Availability Technologies<br />
Members of a GLBP group elect one gateway to be the active virtual gateway (AVG) for that group.<br />
Other group members provide backup for the AVG in the event that the AVG becomes unavailable. The<br />
function of the AVG is that it assigns a virtual MAC address to each member of the GLBP group. Each<br />
gateway assumes responsibility for forwarding packets sent to the virtual MAC address assigned to it by<br />
the AVG. These gateways are known as active virtual forwarders (AVFs) for their virtual MAC address.<br />
The AVG is also responsible for answering Address Resolution Protocol (ARP) requests for the virtual<br />
IP address. Load sharing is achieved by the AVG replying to the ARP requests with different virtual<br />
MAC addresses (corresponding to each gateway router).<br />
In Figure 3-22, Router A is the AVG for a GLBP group, and is primarily responsible for the virtual IP<br />
address 172.16.128.3; however, Router A is also an AVF for the virtual MAC address 0007.b400.0101.<br />
Router B is a member of the same GLBP group and is designated as the AVF for the virtual MAC address<br />
0007.b400.0102. All hosts have their default gateway IP addresses set to the virtual IP address of<br />
172.16.128.3. However, when these use ARP to determine the MAC of this virtual IP address, Host A<br />
and Host C receive a gateway MAC address of 0007.b400.0101 (directing these hosts to use Router A as<br />
their default gateway), but Host B and Host D receive a gateway MAC address 0007.b400.0102<br />
(directing these hosts to use Router B as their default gateway). In this way, the gateway routers<br />
automatically load share.<br />
Figure 3-22<br />
GLBP Topology<br />
WAN/VPN<br />
Both WAN links are active<br />
Both WAN links are active<br />
Router A<br />
Router B<br />
GLBP Group<br />
172.16.128.1<br />
Virtual Mac: 0007.b400.0101<br />
Virtual Router<br />
Master<br />
Virtual<br />
Router<br />
172.16.128.3<br />
Virtual Router<br />
Backup<br />
172.16.128.2<br />
Virtual Mac: 0007.b400.0102<br />
Host A Host B Host C<br />
Host D<br />
Virtual Mac: Virtual Mac: Virtual Mac:<br />
0007.b400.0102 0007.b400.0101 0007.b400.0102<br />
All Hosts are configured with a default gateway IP address of 172.16.128.3<br />
228680<br />
If Router A becomes unavailable, Hosts A and C do not lose access to the WAN because Router B<br />
assumes responsibility for forwarding packets sent to the virtual MAC address of Router A, and for<br />
responding to packets sent to its own virtual MAC address. Router B also assumes the role of the AVG<br />
for the entire GLBP group. Communication for the GLBP members continues despite the failure of a<br />
router in the GLBP group.<br />
Additionally, like HSRP and VRRP, GLBP supports object tracking, preemption, and SSO awareness.<br />
OL-22201-01<br />
<strong>Medianet</strong> <strong>Reference</strong> <strong>Guide</strong><br />
3-27
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