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A broadcast is a data packet that is destined for multiple hosts. Broadcasts can occur at the data link layer and the network layer. Data-link broadcasts are sent to all hosts attached to a particular physical network. Network layer broadcasts are sent to all hosts attached to a particular logical network. The Transmission Control Protocol/Internet Protocol (TCP/IP) supports the following types of broadcast packets:
Because broadcasts are recognized by all hosts, a significant goal of router configuration is to control unnecessary proliferation of broadcast packets. Cisco routers support two kinds of broadcasts: directed and flooded. A directed broadcast is a packet sent to a specific network or series of networks, whereas a flooded broadcast is a packet sent to every network. In IP internetworks, most broadcasts take the form of User Datagram Protocol (UDP) broadcasts.
Although current IP implementations use a broadcast address of all ones, the first IP implementations used a broadcast address of all zeros. Many of the early implementations do not recognize broadcast addresses of all ones and fail to respond to the broadcast correctly. Other early implementations forward broadcasts of all ones, which causes a serious network overload known as a broadcast storm. Implementations that exhibit these problems include systems based on versions of BSD UNIX prior to Version 4.3.
In the brokerage community, applications use UDP broadcasts to transport market data to the desktops of traders on the trading floor. This case study gives examples of how brokerages have implemented both directed and flooding broadcast schemes in an environment that consists of Cisco routers and Sun workstations. Figure 19-1 illustrates a typical topology. Note that the addresses in this network use a 10-bit netmask of 255.255.255.192.
In Figure 19-1, UDP broadcasts must be forwarded from a source segment (Feed network) to many destination segments that are connected redundantly. Financial market data, provided, for example, by Reuters, enters the network through the Sun workstations connected to the Feed network and is disseminated to the TIC servers. The TIC servers are Sun workstations running Teknekron Information Cluster software. The Sun workstations on the trader networks subscribe to the TIC servers for the delivery of certain market data, which the TIC servers deliver by means of UDP broadcasts. The two routers in this network provide redundancy so that if one router becomes unavailable, the other router can assume the load of the failed router without intervention from an operator. The connection between each router and the Feed network is for network administration purposes only and does not carry user traffic.
Two different approaches can be used to configure Cisco routers for forwarding UDP broadcast traffic: IP helper addressing and UDP flooding. This case study analyzes the advantages and disadvantages of each approach.
IP helper addressing is a form of static addressing that uses directed broadcasts to forward local and all-nets broadcasts to desired destinations within the internetwork.
To configure helper addressing, you must specify the ip helper-address command on every interface on every router that receives a broadcast that needs to be forwarded. On Router A and Router B, IP helper addresses can be configured to move data from the TIC server network to the trader networks. IP helper addressing in not the optimal solution for this type of topology because each router receives unnecessary broadcasts from the other router, as shown in Figure 19-2.
In this case, Router A receives each broadcast sent by Router B three times, one for each segment, and Router B receives each broadcast sent by Router A three times, one for each segment. When each broadcast is received, the router must analyze it and determine that the broadcast does not need to be forwarded. As more segments are added to the network, the routers become overloaded with unnecessary traffic, which must be analyzed and discarded.
When IP helper addressing is used in this type of topology, no more than one router can be configured to forward UDP broadcasts (unless the receiving applications can handle duplicate broadcasts). This is because duplicate packets arrive on the trader network. This restriction limits redundancy in the design and can be undesirable in some implementations.
Although IP helper addressing is well-suited to nonredundant, nonparallel topologies that do not require a mechanism for controlling broadcast loops, in view of these drawbacks, IP helper addressing does not work well in this topology. To improve performance, network designers considered several other alternatives:
With alternatives eliminated, the network designers turned to a simpler implementation that supports redundancy without duplicating packets and that ensures fast convergence and minimal loss of data when a router fails: UDP flooding.
UDP flooding uses the spanning tree algorithm to forward packets in a controlled manner. Bridging is enabled on each router interface for the sole purpose of building the spanning tree. The spanning tree prevents loops by stopping a broadcast from being forwarded out an interface on which the broadcast was received. The spanning tree also prevents packet duplication by placing certain interfaces in the blocked state (so that no packets are forwarded) and other interfaces in the forwarding state (so that packets that need to be forwarded are forwarded).
To enable UDP flooding, the router must be running software that supports transparent bridging and bridging must be configured on each interface that is to participate in the flooding. If bridging is not configured for an interface, the interface will receive broadcasts, but the router will not forward those broadcasts and will not use that interface as a destination for sending broadcasts received on a different interface.
When configured for UPD flooding, the router uses the destination address specified by the ip broadcast-address command on the output interface to assign a destination address to a flooded UDP datagram. Thus, the destination address might change as the datagram propagates through the network. The source address, however, does not change.
With UDP flooding, both routers shown in Figure 19-1 use a spanning tree to control the network topology for the purpose of forwarding broadcasts. The key commands for enabling UDP flooding are as follows:
bridge group protocol protocol ip forward-protocol spanning tree bridge-group group input-type-list access-list-number
The bridge protocol command can specify either the dec keyword (for the DEC spanning-tree protocol) or the ieee keyword (for the IEEE Ethernet protocol). All routers in the network must enable the same spanning tree protocol. The ip forward-protocol spanning tree command uses the database created by the bridge protocol command. Only one broadcast packet arrives at each segment, and UDP broadcasts can traverse the network in both directions.
To determine which interface forwards or blocks packets, the router configuration specifies a path cost for each interface. The default path cost for Ethernet is 100. Setting the path cost for each interface on Router B to 50 causes the spanning tree algorithm to place the interfaces in Router B in forwarding state. Given the higher path cost (100) for the interfaces in Router A, the interfaces in Router A are in the blocked state and do not forward the broadcasts. With these interface states, broadcast traffic flows through Router B. If Router B fails, the spanning tree algorithm will place the interfaces in Router A in the forwarding state, and Router A will forward broadcast traffic.
With one router forwarding broadcast traffic from the TIC server network to the trader networks, it is desirable to have the other forward unicast traffic. For that reason, each router enables the ICMP Router Discovery Protocol (IRDP), and each workstation on the trader networks runs the irdp daemon. On Router A, the preference keyword sets a higher IRDP preference than does the configuration for Router B, which causes each irdp daemon to use Router A as its preferred default gateway for unicast traffic forwarding. Users of those workstations can use netstat -rn to see how the routers are being used.
On the routers, the holdtime, maxadvertinterval, and minadvertinterval keywords reduce the advertising interval from the default so that the irdp daemons running on the hosts expect to see advertisements more frequently. With the advertising interval reduced, the workstations will adopt Router B more quickly if Router A becomes unavailable. With this configuration, when a router becomes unavailable, IRDP offers a convergence time of less than one minute.
IRDP is preferred over the Routing Information Protocol (RIP) and default gateways for the following reasons:
Figure 19-3 shows how data flows when the network is configured for UDP flooding.
If the hosts on the trader networks do not support IRDP, the Hot Standby Routing Protocol (HSRP) can be used to select which router will handle unicast traffic. HSRP allows the standby router to take over quickly if the primary router becomes unavailable. For information about configuring HSRP, see "Using HSRP for Fault-Tolerant IP Routing."
By default, the router performs UDP flooding by process switching the UDP packets. To increase performance on AGS+ and Cisco 7000 series routers, enable fast switching of UDP packets by using the following command:
ip forward-protocol turbo-flood
The following commands configure UDP flooding on Router A. Because this configuration does not specify a lower path cost than the default and because the configuration of Router B specifies a lower cost than the default with regard to UDP flooding, Router A acts as a backup to Router B. Because this configuration specifies an IRDP preference of 100 and because Router B specifies a IRDP preference of 90 (ip irdp preference 90), Router A forwards unicast traffic from the trader networks, and Router B is the backup for unicast traffic forwarding.
!Router A: ip forward-protocol spanning-tree ip forward-protocol udp 111 ip forward-protocol udp 3001 ip forward-protocol udp 3002 ip forward-protocol udp 3003 ip forward-protocol udp 3004 ip forward-protocol udp 3005 ip forward-protocol udp 3006 ip forward-protocol udp 5020 ip forward-protocol udp 5021 ip forward-protocol udp 5030 ip forward-protocol udp 5002 ip forward-protocol udp 1027 ip forward-protocol udp 657 ! interface ethernet 0 ip address 200.200.200.61 255.255.255.0 ip broadcast-address 200.200.200.255 no mop enabled ! interface ethernet 1 ip address 164.53.7.61 255.255.255.192 ip broadcast-address 164.53.7.63 ip irdp ip irdp maxadvertinterval 60 ip irdp minadvertinterval 45 ip irdp holdtime 60 ip irdp preference 100 bridge-group 1 bridge-group 1 input-type-list 201 no mop enabled ! interface ethernet 2 ip address 164.53.8.61 255.255.255.192 ip broadcast-address 164.53.8.63 ip irdp ip irdp maxadvertinterval 60 ip irdp minadvertinterval 45 ip irdp holdtime 60 ip irdp preference 100 bridge-group 1 bridge-group 1 input-type-list 201 no mop enabled ! interface ethernet 3 ip address 164.53.9.61 255.255.255.192 ip broadcast-address 164.53.9.63 ip irdp ip irdp maxadvertinterval 60 ip irdp minadvertinterval 45 ip irdp holdtime 60 ip irdp preference 100 bridge-group 1 bridge-group 1 input-type-list 201 no mop enabled ! interface ethernet 4 ip address 164.53.10.61 255.255.255.192 ip broadcast-address 164.53.10.63 ip irdp ip irdp maxadvertinterval 60 ip irdp minadvertinterval 45 ip irdp holdtime 60 ip irdp preference 100 bridge-group 1 bridge-group 1 input-type-list 201 no mop enabled ! router igrp 1 network 164.53.0.0 ! ip name-server 255.255.255.255 snmp-server community public RW snmp-server host 164.53.7.15 public bridge 1 protocol dec bridge 1 priority 255 access-list 201 deny 0xFFFF 0x0000
The following commands configure UDP flooding on Router B. Because this configuration specifies a lower path cost than the default (bridge-group 1 path-cost 50) and because the configuration of Router A accepts the default, Router B forwards UDP packets. Because this configuration specifies an IRDP preference of 90 (ip irdp preference 90) and because Router A specifies a IRDP preference of 100, Router B acts as the backup for Router A for forwarding unicast traffic from the trader networks.
!Router B ip forward-protocol spanning-tree ip forward-protocol udp 111 ip forward-protocol udp 3001 ip forward-protocol udp 3002 ip forward-protocol udp 3003 ip forward-protocol udp 3004 ip forward-protocol udp 3005 ip forward-protocol udp 3006 ip forward-protocol udp 5020 ip forward-protocol udp 5021 ip forward-protocol udp 5030 ip forward-protocol udp 5002 ip forward-protocol udp 1027 ip forward-protocol udp 657 ! interface ethernet 0 ip address 200.200.200.62 255.255.255.0 ip broadcast-address 200.200.200.255 no mop enabled ! interface ethernet 1 ip address 164.53.7.62 255.255.255.192 ip broadcast-address 164.53.7.63 ip irdp ip irdp maxadvertinterval 60 ip irdp minadvertinterval 45 ip irdp holdtime 60 ip irdp preference 90 bridge-group 1 bridge-group 1 path-cost 50 bridge-group 1 input-type-list 201 no mop enabled ! interface ethernet 2 ip address 164.53.8.62 255.255.255.192 ip broadcast-address 164.53.8.63 ip irdp ip irdp maxadvertinterval 60 ip irdp minadvertinterval 45 ip irdp holdtime 60 ip irdp preference 90 bridge-group 1 bridge-group 1 path-cost 50 bridge-group 1 input-type-list 201 no mop enabled ! interface ethernet 3 ip address 164.53.9.62 255.255.255.192 ip broadcast-address 164.53.9.63 ip irdp ip irdp maxadvertinterval 60 ip irdp minadvertinterval 45 ip irdp holdtime 60 ip irdp preference 90 bridge-group 1 bridge-group 1 path-cost 50 bridge-group 1 input-type-list 201 no mop enabled ! interface ethernet 4 ip address 164.53.10.62 255.255.255.192 ip broadcast-address 164.53.10.63 ip irdp ip irdp maxadvertinterval 60 ip irdp minadvertinterval 45 ip irdp holdtime 60 ip irdp preference 90 bridge-group 1 bridge-group 1 path-cost 50 bridge-group 1 input-type-list 201 no mop enabled ! router igrp 1 network 164.53.0.0 ! ip name-server 255.255.255.255 snmp-server community public RW snmp-server host 164.53.7.15 public bridge 1 protocol dec bridge 1 priority 255 access-list 201 deny 0xFFFF 0x0000
Although IP helper addressing is useful in networks that do not require redundancy, when configured in networks that feature redundancy, IP helper addressing results in packet duplication that severely reduces router and network performance.
By configuring UDP flooding, one router forwards UDP traffic without burdening the second router with duplicate packets. By dedicating one router to the task of forwarding UDP traffic, the second router becomes available for forwarding unicast traffic. At the same time, because each router is configured as the backup for the other router, redundancy is maintained; if either router fails, the other router can assume the work of the failed router without intervention from an operator. When compared with IP helper addressing, UDP flooding makes the most efficient use of router resources.
Posted: Wed Apr 10 10:47:30 PDT 2002
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