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CX11x, CX31x, CX710 (Earlier Than V6.03), and CX91x Series Switch Modules V100R001C10 Configuration Guide 12

The documents describe the configuration of various services supported by the CX11x&CX31x&CX91x series switch modules The description covers configuration examples and function configurations.
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This section describes the implementation of RIP.

RIP Principles

RIP is based on the Distance-Vector (DV) algorithm. RIP uses hop count (HC) to measure the distance to the destination. The distance is called the metric value. In RIP, the default HC from a router to its directly connected network is 0, and the HC from a router to a reachable network through another router is 1, and so on. That is to say, the HC equals the number of routers passed from the local network to the destination network. To speed up network convergence, RIP defines the HC as an integer that ranges from 0 to 15. An HC 16 or greater is defined as infinity, that is, the destination network or the host is unreachable. For this reason, RIP is not applied to large-scale networks.

RIP Routing Table

When RIP starts on a router, the RIP routing table contains only the routes to the directly connected interfaces. After neighboring routers on different network segments learn the routing entries from each other, they can communicate with each other.

Figure 7-18 RIP routing table generation

Figure 7-18 shows the process of RIP routing table generation.
  • RIP starts, and then RouterA broadcasts Request packets to neighboring routers.
  • When receiving the Request packet, RouterB encapsulates its own RIP routing table into the Response packet and broadcasts the Response packet to the network segment connected to the interface receiving the Request packet.
  • RouterA generates a routing table based on the Response packet sent from RouterB.
RIP Update and Maintenance

RIP uses four timers to update and maintain routing information:

  • Update timer: When this timer expires, a router immediately sends an Update packet.
  • Age timer: If a RIP device does not receive an Update packet from a neighbor within the aging time, the RIP device considers the route unreachable.
  • Garbage-collect timer: If a RIP device does not receive an Update packet of an unreachable route within the timeout interval, the device deletes the routing entry from the RIP routing table.
  • Suppress timer: When a RIP device receives an Update packet with the Cost field being 16 from a neighbor, the route is suppressed and the suppress timer starts. To avoid route flapping, the RIP device does not accept any Update packet before the suppress timer expires even if the Cost field in an Update packet is smaller than 16. After the suppress timer expires, the RIP device accepts new Update packets.

Relationships between RIP routes and timers:

  • The interval for sending Update packets is determined by the Update timer, which is 30 seconds by default.
  • Each routing entry has two timers: age timer and Garbage-collect timer. When a RIP device adds a learned route to the local RIP routing table, the age timer starts for the routing entry. If the RIP device does not receive an Update packet from the neighbor within the age time, the RIP device sets the Cost value of the route to 16 (unreachable) and starts the Garbage-collect timer. If the RIP device still does not receive an Update packet within the Garbage-collect timer, the RIP device deletes the routing entry from the RIP routing table.
Triggered Update

When routing information changes, a device immediately sends an Update packet to its neighbors, without waiting for Update timer expiration. This function avoids loops.

Figure 7-19 Triggered update

As shown in Figure 7-19, RouterC first learns that network is unreachable.
  • If RouterC does not support triggered update when detecting a link fault, it has to wait until the Update timer expires. If RouterC receives an Update packet from RouterB before its Update timer expires, RouterC learns a wrong route to network In this case, the next hops of the routes from RouterB or RouterC to network are RouterC and RouterB respectively. A routing loop is generated.
  • If RouterC supports triggered update when detecting a link fault, RouterC immediately sends an Update packet to RouterB so that a routing loop is prevented.

RIP-2 Enhanced Features

Two versions are available for RIP: RIP-1 and RIP-2. RIP-2 is an extension to RIP-1.

Comparison Between RIP-1 and RIP-2

RIP version 1 (RIP-1) is a classful (as opposed to classless) routing protocol. It supports the advertisement of protocol packets only in broadcast mode. Figure 7-20 shows the packet format. The RIP-1 protocol packet does not carry any mask, so it can identify only the routes of the natural network segment such as Class A, Class B, and Class C, and does not support route aggregation or discontinuous subnet.

RIP version 2 (RIP-2), is a classless routing protocol. Figure 7-21 shows the packet format.

Figure 7-20 RIP-1 packet format

Figure 7-21 RIP-2 packet format

Compared with RIP-1, RIP-2 has the following advantages:

  • Supports route tag and can flexibly control routes on the basis of the tag in the routing policy.

  • Has packets that contain mask information and support route summarization and Classless Inter-Domain Routing (CIDR).

  • Supports the next hop address and can select the optimal next hop address in the broadcast network.

  • Supports sending update packets in multicast mode. Only RIP-2 routers can receive protocol packets. This reduces resource consumption.

  • Provides packets authentication to enhance security.

RIP-2 Route Summarization

When different subnet routes in the same natural network segment are transmitted to other network segments, these routes are summarized into one route of the same segment. This process is called route summarization.

RIP-1 packets do not carry mask information, so RIP-1 can advertise only the routes with natural masks. Because RIP-2 packets carry mask information, RIP-2 supports subnetting. RIP-2 route summarization improves extensibility and efficiency and minimizes the routing table size of a large-sized network.

Route summarization is classified into two types:

  • RIP process-based classful summarization

    Summarized routes are advertised using nature masks. For example, route (metric=2) and route (metric=3) are summarized as a route (metric=2) in the natural network segment. RIP-2 supports classful summarization to obtain the optimal metric.

  • Interface-based summarization

    A user can specify a summarized address. For example, a route (metric=2) can be configured on the interface as a summarized route of route (metric=2) and route (metric=3).

Split Horizon and Poison Reverse

Split Horizon

Split horizon ensures that a route learned by RIP on an interface is not sent to neighbors from the interface. This feature reduces bandwidth consumption and avoids routing loops.

Split horizon provides two models for different networks: interface-based split horizon and neighbor-based split horizon. Broadcast, P2P, and P2MP networks use interface-based split horizon, as shown in Figure 7-22.

Figure 7-22 Interface-based split horizon

RouterA sends routing information destined for to RouterB. If split horizon is not configured, RouterB sends the route learned from RouterA back to RouterA. Thus RouterA can learn two routes destined for a direct route with hop count 0 and a route with the next hop RouterB and hop count 2.

However, only the direct route in the RIP routing table on RouterA is active. When the route from RouterA to network is unreachable, RouterB does not receive the unreachable message immediately and still notifies RouterA that network is reachable. Therefore, RouterA receives incorrect routing information that network is reachable through RouterB, and RouterB considers that network is reachable through RouterA. A routing loop is thus generated. With the split horizon feature, RouterB does not send the route destined for back to RouterA. Routing loops are avoided.

On a Non-Broadcast Multiple Access (NBMA) network, an interface connects to multiple neighbors; therefore, split horizon is performed based on neighbors. Routes are advertised in unicast mode. The routes received by an interface are differentiated by neighbors. The route learned from a neighbor is not sent back through the same interface.

Figure 7-23 Neighbor-based split horizon

As shown in Figure 7-23, after split horizon is configured on an NBMA network, RouterA sends route learned from RouterB to RouterC, but does not send it to RouterB.

Poison Reverse

Poison reverse ensures that RIP sets the cost of the route learned from an interface of a neighbor to 16 (unreachable) and then sends the route from the same interface back to the neighbor. This feature deletes useless routes from the routing table and avoids routing loops.

Figure 7-24 Poison reverse

As shown in Figure 7-24, after receiving a route from RouterA, RouterB sends an unreachable message (with the route Cost being 16) to RouterA. RouterA then does not learn the route from RouterB. A routing loop is avoided.

Multi-process and Multi-instance

The multi-process feature associates a RIP process with multiple interfaces, ensuring that the specific process performs all the protocol-related operations only on these interfaces. With the multi-process feature, multiple RIP processes can run on a device independently. Route exchange between RIP processes is similar to route exchange between routing protocols.

RIP multi-instance associates a VPN instance with a RIP process so that the VPN instance can be associated with all interfaces on this process.


A link fault or topology change causes routers to recalculate routes. Therefore, route convergence must be quick enough to ensure network performance. A solution to speed up route convergence is to quickly detect faults and notify routing protocols of the faults.

Bidirectional Forwarding Detection (BFD) detects faults on links between neighboring routers. Associated with a routing protocol, BFD can rapidly detect link faults and report the faults to the protocol so that the protocol quickly triggers route convergence. Traffic loss caused by topology changes is minimized. After RIP is associated with BFD, BFD rapidly detects link faults and reports the faults to RIP so that RIP quickly responds to network topology changes.

Table 7-7 lists the link fault detection mechanisms and convergence speed before and after BFD is associated with RIP.

Table 7-7 BFD speeds up convergence

RIP and BFD Association Feature

Link Fault Detection Mechanism

Convergence Speed


The RIP age timer expires. By default, the timeout interval is 180 seconds.

Second-level (> 180 seconds)


The BFD session goes Down.

Second-level (< 30 seconds)


BFD is classified into static BFD and dynamic BFD:

  • Static BFD

    In static BFD, BFD session parameters (including local and remote discriminators) are set manually using commands, and BFD session setup requests are manually delivered.

  • Dynamic BFD

    In dynamic BFD, BFD session setup is triggered by routing protocols. The local discriminator is dynamically allocated and remote discriminator is obtained from the peer. A routing protocol notifies BFD of the neighbor parameters (including destination and source addresses), and then BFD sets up a session based on the received parameters. When a link fault occurs, the protocol associated with BFD quickly detects that the BFD session is Down, and switches traffic to the backup link. This feature minimizes data loss.

A device can implement static BFD even if the peer device does not support BFD. Dynamic BFD is more flexible than static BFD.


After RIP is associated with BFD, BFD reports link faults to RIP within several milliseconds. The RIP router then deletes the faulty links from the local routing table and starts the backup link. This feature increases route convergence speed.

Figure 7-25 RIP and BFD association network

Implementation of RIP and BFD association:

  • As shown in Figure 7-25, RouterA, RouterB, RouterC, and RouterD set up RIP neighbor relationships. RouterB is the next hop on the route from RouterA to RouterD. RIP and BFD association is configured on RouterA and RouterB.
  • When the link between RouterA and RouterB is faulty, BFD quickly detects the fault and notify RouterA of the fault. RouterA deletes the route with RouterB as the next hop, and then recalculates a route. The new route passes RouterC and RouterB and reaches RouterD.
  • When the link between RouterA and RouterB recovers, a session is set up again. RouterA receives routing information from RouterB and selects the optimal route.


The device with a distributed architecture supports RIP Non-stop Routing (NSR). RIP backs up all route data from the Active Main Board (AMB) to the Standby Main Board (SMB). Whenever the AMB fails, the SMB becomes active and takes over the AMB. RIP, therefore, can keep the normal operation of services. RIP NSR ensures that real-time data is highly synchronized between the AMB and SMB. Therefore, during the AMB/SMB switchover, the neighbor will not detect the fault on the local device.

Updated: 2019-08-09

Document ID: EDOC1000041694

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