Using IS-IS ATT-Bit Control Feature
Using the IS-IS Attach-Bit
Control Feature
Introduction
In Intermediate System-to-Intermediate System (IS-IS) networks, routing inter-area traffic from Layer 1 areas is accomplished by sending the traffic to the nearest Layer 1/Layer 2 router. A Layer 1/Layer 2 router identifies itself by setting an attach-bit (ATT-bit) in its Layer 1 link-state packet (LSP). In some situations, however, it might not be desirable to set the ATT-bit. For example, if there are multiple Layer 1/Layer 2 routers within a Layer 1 area and one of the Layer 1/Layer 2 routers loses its backbone connection, continuing to send inter-area traffic to this Layer 1/Layer 2 router can cause the traffic to be dropped. Cisco IOS® Software now introduces a new capability to allow network administrators to control when a Layer 1/Layer 2 router should set the ATT bit and avert dropped traffic.
Overview
In networks running hierarchical routing protocols—IS-IS or Open Shortest Path First (OSPF) Protocol, for example—it is beneficial, for redundancy purposes, to have multiple paths reach the backbone area from a local area. If one of the paths is lost to the backbone area, the other path can continue to be used for forwarding inter-area traffic. With IS-IS, routing the inter-area traffic is accomplished by sending the traffic to the closest Layer 1/Layer 2 router. Layer 1/Layer 2 routers identify themselves by setting the ATT-bit in their Layer 1 LSPs. Upon receiving an LSP with the ATT-bit set, a Layer 1 router knows that the LSP originator is a Layer 1/Layer 2 router that can be used to route inter-area traffic. When there are multiple Layer 1/Layer 2 routers in one local area, the Layer 1 routers within that local area forward inter-area traffic to the nearest Layer 1/Layer 2 router (Figure 1).
In Figure 1, the network element (NE) devices in Area 1 are acting as Layer 1 routers. They use either Rtr1 or Rtr2 Layer 1/Layer 2 routers to forward the traffic destined to areas outside of their local area. Assume all the links have equal cost. NE1 would use Rtr1 because it is closer than Rtr2. On the other hand, NE3 would use Rtr2. NE2 would perform load balancing to Rtr1 and Rtr2 because they are equidistant to NE2.
Figure 1
Sample Connectionless Network Service (CLNS) Network Topology
Issue
With the introduction of the multi-area support feature, Layer 1/Layer 2 routers can connect to multiple Layer 1 areas. This has effectively reduced the number of Layer 1/Layer 2 routers needed because multiple Layer 1 areas can share one Layer 1/Layer 2 router. On the other hand, it can complicate networks. In earlier Cisco IOS Software implementations, a Layer 1/Layer 2 router would set the ATT-bit in its Layer 1 LSP if it connects to multiple Layer 1 areas. Thus, if the backbone connection is lost, the Layer 1/Layer 2 router would still set the ATT-bit in the Layer 1 LSP. Consequently, the Layer 1 devices associated with that Layer 1/Layer 2 router would continue sending inter-area traffic to the Layer 1/Layer 2 router and cause the traffic to be dropped. For example, in Figure 1, Rtr1 has connections to two Layer 1 areas in addition to the backbone area. If the connection between Rtr1 and its upstream router were lost, Rtr1 would still set the ATT-bit in its LSP. Consequently, NE1 would still send inter-area traffic to Rtr1. However, because Rtr1 has lost its connection to the L2 area, it uses Rtr2 to route inter-area traffic. This causes the traffic to be sent back to NE1. Thus, a routing loop—an undesirable situation—is formed.
To address this problem, Cisco IOS Software implements a new capability to allow users to have greater control of setting the ATT-bit. Instead of setting the ATT-bit whenever seeing other areas, a Cisco router can now set the ATT-bit based on the criteria specified in a route map. Users can use the "match" command associated with a route map to match a Connectionless Network Service (CLNS) area address. When the specified area address is not found in the CLNS routing table, the "match" condition fails, the route map is said to "not be satisfied," and the ATT-bit will not be set. A complete configuration example will be discussed in the "Feature Usage Examples" section.
Command Syntax
This new command is configured under "router isis". It enables the ATT-bit control capability.
router(config-router)#set-attach-bit route-map
Control Feature
Introduction
In Intermediate System-to-Intermediate System (IS-IS) networks, routing inter-area traffic from Layer 1 areas is accomplished by sending the traffic to the nearest Layer 1/Layer 2 router. A Layer 1/Layer 2 router identifies itself by setting an attach-bit (ATT-bit) in its Layer 1 link-state packet (LSP). In some situations, however, it might not be desirable to set the ATT-bit. For example, if there are multiple Layer 1/Layer 2 routers within a Layer 1 area and one of the Layer 1/Layer 2 routers loses its backbone connection, continuing to send inter-area traffic to this Layer 1/Layer 2 router can cause the traffic to be dropped. Cisco IOS® Software now introduces a new capability to allow network administrators to control when a Layer 1/Layer 2 router should set the ATT bit and avert dropped traffic.
Overview
In networks running hierarchical routing protocols—IS-IS or Open Shortest Path First (OSPF) Protocol, for example—it is beneficial, for redundancy purposes, to have multiple paths reach the backbone area from a local area. If one of the paths is lost to the backbone area, the other path can continue to be used for forwarding inter-area traffic. With IS-IS, routing the inter-area traffic is accomplished by sending the traffic to the closest Layer 1/Layer 2 router. Layer 1/Layer 2 routers identify themselves by setting the ATT-bit in their Layer 1 LSPs. Upon receiving an LSP with the ATT-bit set, a Layer 1 router knows that the LSP originator is a Layer 1/Layer 2 router that can be used to route inter-area traffic. When there are multiple Layer 1/Layer 2 routers in one local area, the Layer 1 routers within that local area forward inter-area traffic to the nearest Layer 1/Layer 2 router (Figure 1).
In Figure 1, the network element (NE) devices in Area 1 are acting as Layer 1 routers. They use either Rtr1 or Rtr2 Layer 1/Layer 2 routers to forward the traffic destined to areas outside of their local area. Assume all the links have equal cost. NE1 would use Rtr1 because it is closer than Rtr2. On the other hand, NE3 would use Rtr2. NE2 would perform load balancing to Rtr1 and Rtr2 because they are equidistant to NE2.
Figure 1
Sample Connectionless Network Service (CLNS) Network Topology
Issue
With the introduction of the multi-area support feature, Layer 1/Layer 2 routers can connect to multiple Layer 1 areas. This has effectively reduced the number of Layer 1/Layer 2 routers needed because multiple Layer 1 areas can share one Layer 1/Layer 2 router. On the other hand, it can complicate networks. In earlier Cisco IOS Software implementations, a Layer 1/Layer 2 router would set the ATT-bit in its Layer 1 LSP if it connects to multiple Layer 1 areas. Thus, if the backbone connection is lost, the Layer 1/Layer 2 router would still set the ATT-bit in the Layer 1 LSP. Consequently, the Layer 1 devices associated with that Layer 1/Layer 2 router would continue sending inter-area traffic to the Layer 1/Layer 2 router and cause the traffic to be dropped. For example, in Figure 1, Rtr1 has connections to two Layer 1 areas in addition to the backbone area. If the connection between Rtr1 and its upstream router were lost, Rtr1 would still set the ATT-bit in its LSP. Consequently, NE1 would still send inter-area traffic to Rtr1. However, because Rtr1 has lost its connection to the L2 area, it uses Rtr2 to route inter-area traffic. This causes the traffic to be sent back to NE1. Thus, a routing loop—an undesirable situation—is formed.
To address this problem, Cisco IOS Software implements a new capability to allow users to have greater control of setting the ATT-bit. Instead of setting the ATT-bit whenever seeing other areas, a Cisco router can now set the ATT-bit based on the criteria specified in a route map. Users can use the "match" command associated with a route map to match a Connectionless Network Service (CLNS) area address. When the specified area address is not found in the CLNS routing table, the "match" condition fails, the route map is said to "not be satisfied," and the ATT-bit will not be set. A complete configuration example will be discussed in the "Feature Usage Examples" section.
Command Syntax
This new command is configured under "router isis
router(config-router)#set-attach-bit route-map
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