CBT Designated Routers

CBT uses HELLO messages to elect a designated router on multiaccess networks The rationale for using a CBT DR is the same as that for DVMRP-designated forwarders and MOSPF DRs Because CBT does not use an RPF check when forwarding packets, a DR is especially important for preventing loops when there are multiple upstream paths to the core, as in Figure 5-39

Each CBT interface is configured with a preference value between 0 and 255, and this value is carried in the HELLO message A value between 1 and 254 indicates that the router is eligible to become the DR, with the lower number indicating a higher preference—that is, a router with a preference of 10 is "more eligible" than a router with a preference of 20 A preference of 0 indicates that the router is the DR

When a CBT router first becomes active on a multiaccess link, it sends two HELLO messages in succession to advertise its presence and its preference value The router then listens for HELLOs, with one of the following three results

• A HELLO with a lower preference value is heard from another router on the network

• All HELLOs heard on the network have a higher preference value

• No other HELLOs are heard on the network

Figure 5-39 CBT Elects a Designated Router on Multiaccess Networks to Manage Multiple Upstream Paths to the Core

Core

Figure 5-39 CBT Elects a Designated Router on Multiaccess Networks to Manage Multiple Upstream Paths to the Core

Core

In the first case, the new router knows that the router with the lower preference value is elected as the DR In the other two cases, the new router assumes the role of DR and advertises that fact by setting the preference to 0 in its HELLOs If all HELLOs have equal preference values, the router with the lowest IP address is elected as the DR

In steady state, the DR sends a HELLO every 60 seconds both as an advertisement of its status and as a keepalive The DR also sends a HELLO in response to a HELLO from a new router Other routers do not send HELLOs or respond to HELLOs from new routers

In some cases, the elected DR may not be on the path to the core Suppose that RTA in Figure 5-39 is elected as the DR, but RTB is the best next-hop router to the core In this case, when RTC forwards a JOIN_REQUEST to RTA, RTA unicasts the JOIN_REQUEST back across the multiaccess link to RTB This redirection occurs only with JOIN_REQUESTs, when RTB sends a JOIN_ACK, the message is sent directly to RTC

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