http://www.cisco.com/en/US/docs/ios/12_2s/feature/guide/fs_glbp2.html
Load Sharing You can configure GLBP in such a way that traffic from LAN clients can be shared by multiple routers, thereby sharing the traffic load more equitably among available routers. Multiple Virtual Routers GLBP supports up to 1024 virtual routers (GLBP groups) on each physical interface of a router, and up to four virtual forwarders per group.
Preemption The redundancy scheme of GLBP enables you to preempt an active virtual gateway with a higher priority backup virtual gateway that has become available. Forwarder preemption works in a similar way, except that forwarder preemption uses weighting instead of priority and is enabled by default.
Authentication You can use a simple text password authentication scheme between GLBP group members to detect configuration errors. A router within a GLBP group with a different authentication string than other routers will be ignored by other group members.
http://www.cisco.com/en/US/docs/switches/datacenter/sw/5_x/nxs/unicast/configuration/guide/l3_glbp.html
GLBP Authentication GLBP has three authentication types: MD5 authentication Plain text authentication No authentication MD5 authentication provides greater security than plain text authentication. MD5 authentication allows each GLBP group member to use a secret key to generate a keyed MD5 hash that is part of the outgoing packet. At the receiving end, a keyed hash of an incoming packet is generated. If the hash within the incoming packet does not match the generated hash, the packet is ignored. The key for the MD5 hash can either be given directly in the configuration using a key string or supplied indirectly through a key chain. You can also choose to use a simple password in plain text to authenticate GLBP packets, or choose no authentication for GLBP.

CCNA Self-Study (ICND Exam): Extending Switched Networks with Virtual LANs http://www.ciscopress.com/articles/article.asp?p=102157&seqNum=2
Topic 2, IP Routing Technologies

CCNA - EIGRP Common Question http://www.orbitco-ccna-pastquestions.com/CCNA---EIGRP-Common-Question.php
Looking at the output above, there is no IP route for 10.1.1.4 address on AcmeB routing table. If the router can no find a specific path in its routing table to a particular route,( In this case no path is found so AcmeB) the router will inform the source host with an ICMP message that the destination is unreachable and this will be through the same interface it has received the packet (interface Fa0/0 network 192.168.3.0/28 from the exhibit).

Understanding VLAN Trunk Protocol (VTP) http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094c52.shtml
Introduction VLAN Trunk Protocol (VTP) reduces administration in a switched network. When you configure a new VLAN on one VTP server, the VLAN is distributed through all switches in the domain. This reduces the need to configure the same VLAN everywhere. VTP is a Cisco-proprietary protocol that is available on most of the Cisco Catalyst series products.