Dual-Homed Remote Sites

Articles ยป Dual-Homed Remote Sites

In most mission-critical networks, the remote sites have at least two upstream links to two core routers (see the next diagram). In these designs, the amount of DUAL traffic generated by an outage is significantly higher than in the previous scenario, as both central routers start querying the remote offices.

Dual-homed hub-and-spoke WAN network

Dual-homed hub-and-spoke WAN network

Furthermore, if the core routers lose their direct connectivity (for example, the LAN interface of one of them fails), the transit traffic might start flowing across remote sites, congesting their slow-speed links:

Link failure turns spoke sites into transit sites

Link failure turns spoke sites into transit sites

Both undesired effects are solved very effectively by the EIGRP stub routers. The stub routers are not queried in the DUAL process, thus the WAN link congestion is avoided. Furthermore, as these routers never propagate EIGRP routes to other EIGRP neighbors, they cannot become transit routers.

For example, the next printout displays the EIGRP topology table on router A1 when the connectivity between A1 and A2 was lost and the remote office routers were not configured as stub routers. The routes beyond A2 are reachable through all remote office routers (B1 and B2).

Core routes reachable through remote office routers
a1#show ip eigrp topology 10.1.0.0 255.255.255.0
IP-EIGRP (AS 1): Topology entry for 10.1.0.0 255.255.255.0
  State is Passive, Query origin flag is 1, 2 Successor(s), FD is 2684416
  Routing Descriptor Blocks:
  172.16.1.33 (Serial0/0/0.401), from 172.16.1.33, Send flag is 0x0
      Composite metric is (2684416/2172416), Route is Internal
      Vector metric:
        Minimum bandwidth is 1544 Kbit
        Total delay is 40100 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 2
  172.16.1.2 (Serial0/0/0.100), from 172.16.1.2, Send flag is 0x0
      Composite metric is (2684416/2172416), Route is Internal
      Vector metric:
        Minimum bandwidth is 1544 Kbit
        Total delay is 40100 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 2

When the remote office routers are configured as stub routers, A1 loses its connectivity to 10.1.0.0/24 when its connection to A2 is lost:

Core route no longer available via remote offices
%DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 10.0.0.6 (FastEthernet0/0) is down: holding time expired
a1#show ip eigrp topology 10.1.0.0 255.255.255.0
% IP-EIGRP (AS 1): Route not in topology table

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