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Configuration Guide - IP Unicast Routing

S7700 and S9700 V200R011C10

This document describes IP Unicast Routing configurations supported by the switch, including the principle and configuration procedures of IP Routing Overview, Static Route, RIP, RIPng, OSPF, OSPFv3, IS-IS(IPv4), IS-IS(IPv6), BGP, Routing Policy ,and PBR, and provides configuration examples.
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IS-IS Route Leaking

IS-IS Route Leaking

Normally, Level-1 routers manage routes in Level-1 areas. All Level-2 and Level-1-2 routers form a contiguous backbone area. Level-1 areas can only connect to the backbone area and cannot connect to each other.

A Level-1-2 router encapsulates learned Level-1 routing information into a Level-2 LSP and floods the Level-2 LSP to other Level-2 and Level-1-2 routers. Then Level-1-2 and Level-2 routers know routing information about the entire IS-IS routing domain. To reduce the size of routing tables, a Level-1-2 router, by default, does not advertise the learned routing information of other Level-1 areas and the backbone area to its Level-1 area. In this case, Level-1 routers do not know routing information outside the local area. As a result, Level-1 routers cannot select the optimal route to a destination outside the local area.

IS-IS route leaking can solve this problem. You can configure access control lists (ACLs) and routing policies and mark routes with tags on Level-1-2 routers to select eligible routes. Then a Level-1-2 router can advertise routing information of other Level-1 areas and backbone area to its Level-1 area.

Figure 7-9  IS-IS route leaking

In Figure 7-9, RouterA sends a packet to RouterF. The selected optimal route should be RouterA->RouterB->RouterD->RouterE->RouterF. This is because the cost of this route is 40, which is smaller than the cost (70) of the other route (RouterA->RouterC->RouterE->RouterF). However, when you check the route on RouterA to view the path of the packets sent to RouterF, the selected route is RouterA->RouterC->RouterE->RouterF but not the optimal route from RouterA to RouterF.

RouterA (Level-1 router) does not know routes outside its area, so it sends packets outside its area through the default route generated by the nearest Level-1-2 router. Therefore, the optimal route is not used to forward the packets.

If route leaking is enabled on Level-1-2 routers (RouterC and RouterD), Level-1 routers in Area 10 can know routes outside Area 10 and passing through the two Level-1-2 routers. After route calculation, the forwarding path becomes RouterA->RouterB->RouterD->RouterE->RouterF, which is the optimal route from RouterA to RouterF.

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Updated: 2019-04-01

Document ID: EDOC1000178324

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