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

On the Internet, network connecting devices control traffic and ensure data transmission quality. Common network connecting devices include hubs, bridges, switches, and routers. These network devices have similar basic principles. The following uses a router as an example to describe basic principles.

As a typical network connecting device, a router selects routes and forwards packets. Upon receiving a packet, a router selects a proper path, which has one or multiple hops, to send the packet to the next router according to the destination address in the packet. The last router is responsible for sending the packet to the destination host.

A route is a path along which packets are sent from the source to the destination. When multiple routes are available to send packets from a router to the destination, the router can select the optimal route from an IP routing table to forward the packets. Optimal route selection depends on the routing protocol preferences and metrics of routes. When multiple routes have the same routing protocol preference and metric, load balancing can be implemented among these routes to relieve network pressure. When multiple routes have different routing protocol preferences and metrics, route backup can be implemented among these routes to improve network reliability.

Routes support static routes and dynamic routes including Routing Information Protocol (RIP) routes, Open Shortest Path First (OSPF) routes, Intermediate System-to-Intermediate System (IS-IS) routes, and Border Gateway Protocol (BGP) routes.

Differences Between Static Routes and Dynamic Routes

Static routes are easy to configure, have low requirements on the system, and apply to simple, stable, and small networks. The disadvantage of static routes is that they cannot automatically adapt to network topology changes. Therefore, static routes require subsequent maintenance.

Dynamic routing protocols have their routing algorithms. Therefore, dynamic routes can automatically adapt to network topology changes and apply to the networks on which Layer 3 devices are deployed. The configurations of dynamic routes are complex. Dynamic routes have higher requirements on the system than static ones and consume network resources and system resources.

Classification of Dynamic Routing Protocols

Based on the application range, dynamic routing protocols are classified into the following types:

• Interior Gateway Protocol (IGP): runs inside an AS, such as RIP, OSPF, and IS-IS.

• Exterior Gateway Protocol (EGP): runs between different ASs, such as BGP.

Based on the type of algorithm they use, dynamic routing protocols are classified into the following types:

• Distance-vector routing protocol: includes RIP and BGP. BGP is also called path-vector protocol.

• Link-state routing protocol: includes OSPF and IS-IS.

The preceding algorithms differ mainly in route discovery and calculation methods.

Routing Table

Each router maintains a local core routing table, and each routing protocol maintains its routing table.

• Local core routing table

  A router uses the local core routing table to store preferred routes. The router then sends the preferred routes to the FIB table to guide packet forwarding. The router selects routes according to the priorities of protocols and costs stored in the routing table.

  NOTE:

  A router that supports Layer 3 Virtual Private Network (L3VPN) maintains a local core routing table for each VPN instance.

• Protocol routing table

  A protocol routing table stores the routing information discovered by the protocol.

  A routing protocol can import and advertise the routes that are discovered by other routing protocols. For example, if a router that runs the Open Shortest Path First (OSPF) protocol needs to use OSPF to advertise direct routes, static routes, or Intermediate System-Intermediate System (IS-IS) routes, the router must import the routes into the OSPF routing table.

Routing Table Contents

You can run the display ip routing-table command on a router to view brief information about the routing table of the router. The command output is as follows:

<HUAWEI> display ip routing-table
Proto: Protocol        Pre: Preference
Route Flags: R - relay, D - download to fib, T - to vpn-instance
------------------------------------------------------------------------------
Routing Table: _public_
         Destinations : 14       Routes : 14                                    
                                                                                
Destination/Mask    Proto   Pre  Cost      Flags NextHop         Interface      
                                                                                
        0.0.0.0/0   Static  60   0          RD   10.137.216.1    Vlanif20
     10.10.10.0/24  Direct  0    0           D   10.10.10.10     Vlanif20
    10.10.10.10/32  Direct  0    0           D   127.0.0.1       InLoopBack0    
   10.10.10.255/32  Direct  0    0           D   127.0.0.1       InLoopBack0    
     10.10.11.0/24  Direct  0    0           D   10.10.11.1      LoopBack0      
     10.10.11.1/32  Direct  0    0           D   127.0.0.1       InLoopBack0    
   10.10.11.255/32  Direct  0    0           D   127.0.0.1       InLoopBack0    
   10.137.216.0/23  Direct  0    0           D   10.137.217.208  Vlanif20      
 10.137.217.208/32  Direct  0    0           D   127.0.0.1       InLoopBack0    
 10.137.217.255/32  Direct  0    0           D   127.0.0.1       InLoopBack0    
      127.0.0.0/8   Direct  0    0           D   127.0.0.1       InLoopBack0    
      127.0.0.1/32  Direct  0    0           D   127.0.0.1       InLoopBack0    
127.255.255.255/32  Direct  0    0           D   127.0.0.1       InLoopBack0    
255.255.255.255/32  Direct  0    0           D   127.0.0.1       InLoopBack0 

A routing table contains the following key data for each IP packet:

• Destination: identifies the destination IP address or the destination network address of an IP packet.

• Mask: works with the destination address to identify the address of the network segment where the destination host or router resides.

  The network segment address of the destination host or router is obtained through the "AND" operation on the destination address and network mask. For example, if the destination address is 1.1.1.1 and the mask is 255.255.255.0, the address of the network segment where the host or router resides is 1.1.1.0.

  The network mask is composed of several consecutive 1s. These 1s can be expressed in either the dotted decimal notation or the number of consecutive 1s in the mask. For example, the network mask can be expressed either as 255.255.255.0 or 24.

• Proto: indicates the protocol through which routes are learned.

• Pre: indicates the routing protocol preference of a route. There may multiple routes to the same destination, which have different next hops and outbound interfaces. These routes may be discovered by different routing protocols or manually configured. A router selects the route with the highest preference (the smallest value) as the optimal route. For the routing protocol preference, see Routing Protocol Preference.

• Cost: indicates the route cost. When multiple routes to the same destination have the same preference, the route with the lowest cost is selected as the optimal route.

  NOTE:

  The Preference value is used to compare the preferences of different routing protocols, while the Cost value is used to compare the preferences of different routes of the same routing protocol.

• NextHop: indicates the IP address of the next device that an IP packet passes through.

• Interface: indicates the outbound interface through which an IP packet is forwarded.

As shown in Figure 7-1, RouterA connects to three networks, so it has three IP addresses and three outbound interfaces. Figure 7-1 shows the routing table of RouterA.

Figure 7-1 Routing table

Automatic Restoration After the Number of Routes Exceeds the Upper Limit

If a protocol fails to add routes to the local core routing table due to a specific route limitation, the system records the failure with the protocol name and routing table ID.

After routes of protocols are deleted from the local core routing table, and the number of routes falls below the upper limit, the system prompts all the protocols that failed to add routes to the local core routing table to re-add the routes to the local core routing table. This process restores most of the routes in the local core routing table. The size of released table space determines whether all routes in the local core routing table can be restored.

Matching with FIB Table

After route selection, routers send active routes in the routing table to the FIB table. When a router receives a packet, the router searches the FIB table for the optimal route to forward the packet.

Each entry in the FIB table contains the physical or logical interface through which a packet is sent to a network segment or host to reach the next router. An entry also indicates whether the packet can be sent to a destination host in a directly connected network.

The router performs the "AND" operation on the destination address in the packet and the network mask of each entry in the FIB table. The router then compares the result of the "AND" operation with the entries in the FIB table to find a match and chooses the optimal route to forward packets according to the longest match.

Assume that a router has the following routing table:

Routing Tables:

Destination/Mask    Proto  Pre  Cost     Flags NextHop         Interface

 0.0.0.0/0    Static   60   0       D   120.0.0.2      GigabitEthernet1/0/0

 8.0.0.0/8    Static   60   3       D   120.0.0.2      GigabitEthernet1/0/0

 9.0.0.0/8    Static   60   50      D   20.0.0.2       GigabitEthernet3/0/0

 9.1.0.0/16   Static   60   4       D   120.0.0.2      GigabitEthernet2/0/0

 20.0.0.0/8   Direct   0    0       D   20.0.0.1       GigabitEthernet4/0/0

After receiving a packet that carries the destination address 9.1.2.1, the router searches the following FIB table:

 FIB Table:

 Total number of Routes : 5

Destination/Mask   Nexthop         Flag TimeStamp     Interface              TunnelID

0.0.0.0/0          120.0.0.2       SU   t[37]         GigabitEthernet1/0/0  0x0

8.0.0.0/8          120.0.0.2       DU   t[37]         GigabitEthernet1/0/0  0x0

9.0.0.0/8          20.0.0.2        DU   t[9992]       GigabitEthernet3/0/0  0x0

9.1.0.0/16         120.0.0.2       DU   t[9992]       GigabitEthernet2/0/0  0x0

20.0.0.0/8         20.0.0.1        U    t[9992]       GigabitEthernet4/0/0  0x0

The router performs the "AND" operation on the destination address 9.1.2.1 and the masks 0, 8, and 16 to obtain the network segment addresses: 0.0.0.0/0, 9.0.0.0/8, and 9.1.0.0/16. The three addresses match three entries in the FIB table. The router chooses the entry 9.1.0.0/16 according to the longest match, and forwards the packet through GigabitEthernet2/0/0.

Load Balancing

Routers support the multi-route mode, allowing you to configure multiple routes with the same destination and preference. If the destinations and costs of multiple routes discovered by the same routing protocol are the same, load balancing can be performed among the routes.

During load balancing, a router forwards packets based on the 5-tuple (source IP address, destination IP address, source port, destination port, and transport protocol) in the packets. When the 5-tuple information is the same, the router always chooses the next-hop address that is the same as the last one to send packets. When the 5-tuple information is different, the router forwards packets over idle paths.

Figure 7-2 Networking diagram of load balancing

As shown in Figure 7-2, RouterA forwards the first packet P1 to 10.1.1.0/24 through GE1/17/0 and needs to forward subsequent packets to 10.1.1.0/24 and 10.2.1.0/24 respectively. The forwarding process is as follows:

• When forwarding the second packet P2 to 10.1.1.0/24, RouterA forwards P2 and subsequent packets destined for 10.1.1.0/24 through GE1/17/0 if it finds that the 5-tuple information of P2 is the same as that of P1 destined for 10.1.1.0/24.

• When forwarding the first packet P1 to 10.2.1.0/24, RouterA forwards this packet and subsequent packets destined for 10.2.1.0/24 through GE2/17/0 if it finds that the 5-tuple information of P1 destined for 10.2.1.0/24 is different from that of P1 destined for 10.1.1.0/24.

Route Backup

Route backup can improve network reliability. You can configure multiple routes to the same destination as required. The route with the highest preference functions as the primary route, and the other routes with lower preferences function as backup routes.

A router generally uses the primary route to forward data. When the primary link fails, the primary route becomes inactive. The router selects a backup route with the highest preference to forward data. In this manner, data is switched from the primary route to a backup route. When the primary link recovers, the router selects the primary route to forward data again because the primary route has the highest preference. Data is then switched back from the backup route to the primary route.

Definition

When a router detects a fault at the physical or data link layer, IP fast reroute (FRR) enables the router to report the fault to the upper-layer routing system and to immediately use a backup link to forward packets. IP FRR is a method that implements fast route backup.

Purpose

On traditional IP networks, when a fault occurs at the lower layer of the forwarding link, the physical interface on the router becomes Down. After the router detects the fault, it informs the upper-layer routing system to recalculate routes and then update routing information. Usually, it takes the routing system several seconds to re-select an available route.

Second-level convergence is intolerable to the services that are quite sensitive to delay and packet loss because it may lead to service interruption. For example, Voice over Internet Protocol (VoIP) services are only tolerant of millisecond-level interruption.

IP FRR ensures that the forwarding system rapidly detects a link fault and then uses a backup route to restore services as soon as possible.

IP FRR Classification and Implementation

IP FRR, which is designed for routes on IP networks, is classified into IP FRR on the public network and IP FRR on the private network.

• IP FRR on the public network: protects routers of the public network.

• IP FRR on the private network: protects Customer Edges (CEs).

IP FRR is implemented as follows:

1. If the primary link is available, you can configure an IP FRR policy to provide the forwarding information of the backup route to the forwarding engine.

2. If the forwarding engine detects a link fault, the engine uses the backup link to forward traffic before the routes on the control plane converge.

IP FRR Typical Applications

As shown in Figure 7-3, IP FRR is configured to improve network reliability. CE1 is dual-homed to PE1 and PE2 and has two outbound interfaces and two next hops configured. That is, link B functions as the backup of link A. When link A fails, traffic can be rapidly switched to link B.

Figure 7-3 IP FRR

Definition

Route convergence is the action of recalculating routes to replace existing routes in the case of network topology changes. The integration of network services urgently requires differentiated services. Routes for key services, such as Voice over IP (VoIP), video conferences, and multicast services, need to be converged rapidly, while routes for common services can be converged relatively slowly. In this case, the system needs to converge routes based on their convergence priorities to improve network reliability.

Priority-based convergence is a mechanism that allows the system to converge routes based on the convergence priority. You can set different convergence priorities for routes: critical, high, medium, and low, which are in descending order of priority. The system then converges routes according to the scheduling weight to guide service forwarding.

Principles

Routing protocols first compute and deliver routes of high convergence priorities to the system. You can reconfigure the scheduling weight values as required. Table 7-3 lists the default convergence priorities of public routes.

NOTE:

For private routes, only the convergence priority of 32-bit host routes of OSPF and IS-IS is identified as medium and the convergence priorities of the other routes are identified as low.

Priority-based Route Convergence

Figure 7-4 shows the networking for multicast services. OSPF and IS-IS run on the network; the receiver connects to RouterA; the multicast source server 10.10.10.10/32 connects to RouterB. The route to the multicast source server must be converged faster than other routes, such as 12.10.10.0/24. You can set the convergence priority of route 10.10.10.10/32 to be higher than that of route 12.10.10.0/24. When routes are converged on the network, the route to the multicast source server 10.10.10.10/32 is converged first. This ensures the transmission of multicast services.

Figure 7-4 Networking diagram of priority-based route convergence