Layer 3 availability mechanisms

Layer 3 availability mechanisms

Routing protocols

Routing protocols allow routers to dynamically learn the topology of the network. Should the topology of the network change, routing protocols update their internal topology table, which allows them to route around failure.

There are two types of routing protocol, distance vector and link state. Distance vector protocols, including RIP and IGRP, exchange their entire routing table periodically. To each route, they add their metric (for RIP, this is “hop count”) and insert it in the routing table. If updates fail to arrive before the router’s timer expires, it purges the route and looks for another path. These protocols are usually slow to converge. See Table 62: Sample convergence times (single link failure) on page 343.

Link-state protocols, such as OSPF, take a more holistic view of the network. They compute the entire topology of the network and insert the best path to a destination in the routing table. Link state protocols exchange their routing tables only once, when routers first establish a relationship. After that, they only send updates. They also send hello messages periodically to ensure that the other routers are still present. Link state protocols converge much more quickly than distance vector protocols, and thus are generally better suited to networks that require high availability.

VRRP and HSRP

Virtual Router Redundancy Protocol (VRRP) and the related Cisco proprietary Hot Standby Router Protocol (HSRP) provide a mechanism to deal with router failure without disrupting endpoints on the network. In essence, these protocols work by assigning a virtual IP address and MAC address for the routers. This address is given to endpoints as their default gateway. The two routers send periodic hello messages marked with a priority value between each other. The high-priority router assumes the virtual address, and traffic flows through it. If the primary router fails or its capabilities become degraded (such as if a WAN link fails), the secondary router takes over. This is a useful mechanism to protect endpoints from router failures, and works with IP Telephony endpoints.

Issue 6 January 2008 341

Page 341
Image 341
Avaya 555-245-600 manual Layer 3 availability mechanisms, Routing protocols, Vrrp and Hsrp