Segregating the World into Autonomous Systems
Exterior routing protocols were created to control the expansion of routing tables and to provide a more structured view of the Internet by segregating routing domains into separate administrations, called autonomous systems (ASs), which each have their own independent routing policies and unique IGPs. During the early days of the Internet, an exterior gateway protocol called EGP[] (not to be confused with Exterior Gateway Protocols in general) was used. The NSFNET used EGP t..
The packet format of IGMP and PIM is useful in understanding the operation of PIM. Understanding the packet format also helps you in troubleshooting PIM problems, in case sniffer traces need to be looked at. This section covers the important packet format of IMGP and PIM. IGMP Packet Format IGMP messages are always sent with a TTL of 1 and are IP-encapsulated with a protocol number of 2. Figure 12-11 shows the IGMP version 2 packet format. The IGMP version 1 packet format i..
Multicast Open Shortest Path First
MOSPF is defined in RFC 1584, “Multicast Extensions to OSPF,” which is an extension to the OSPFv2 unicast routing protocol. The basic operation of MOSPF is described here: ■ MOSPF uses the group membership LSA, Type 6, which it floods throughout the originating router’s area. As with unicast OSPF, all MOSPF routers in an area must have identical linkstate databases so that every MOSPF router in an area can calculate the same SPT. ■ The SPT is calculated “on-dema..
IS-IS Multiple Area Configuration
This section uses the network in Figure 7-12 to investigate the properties of a multiple area IS-IS network. The loopback addresses, system IDs, and NSAP addresses for the routers in area 17 are listed in Table 7-3. Figure 7-12. Multiple Area IS-IS Network [View full size image] Table 7-3. Additional Loopback and System IDs for Figure 7-12 Router Loopback System ID NSAP F 172.16.17..
Protecting Against Sudden Loss of BPDUs
STP BPDUs are used as probes to learn about a network topology. When the switches participating in STP converge on a common and consistent loop-free topology, BPDUs still must be sent by the root bridge and must be relayed by every other switch in the STP domain. The STP topology’s integrity then depends on a continuous and regular flow of BPDUs from the root. What happens if a switch doesn’t receive BPDUs in a timely manner or when it doesn’t receive any? The sw..
