IsIS-IS Applicable to the Network Where One CE Is Dual-Homed to Two PEs?
IS-IScan be used in the network where one CE is dual-homed to two PEs. In IS-IS andOSPF, the configurations of the networking where one CE is dual-homed to twoPEs are different.
Figure1 and Figure 2 show two applications of CE-PE dual-homing.
Figure 1 Networkingdiagram of CE-PE dual-homing 1 
Figure 2 Networkingdiagram of CE-PE dual-homing 2 
Asshown in the networking, a PE imports BGP routes to IS-IS and thus IS-IS on thePE can advertise the routes to the peer PE through a CE. IS-IS on the peer PElearns the routes as IS-IS routes. By default, IS-IS routes are prior to BGProutes. Thus, the RM module prefers the IS-IS routes and then BGP routes becomeinactive. In this case, the PE cannot import the BGP routes, causing routeflapping.
IS-ISis not as good as OSPF in preventing route loops. OSPF marks imported BGProutes with DN bits to prevent route loops. IS-IS does not provide mechanismsto prevent route loops. Thus, IS-IS needs to tag the BGP routes it imports.After learning the routes, the peer PE filters out the routes through policies.
