Exporting Interface routes

The current ipsrd treats the directly attached networks differently than the learned networks. Ipsrd advertise specific host address of its directly attached(point-to-point interface), but does not advertise any attached network address to the dynamic routing domain by default. In order for routing domain, RIP/OSPF, to know about the directly attached networks, one needs to export interface routes.

Example: network is as shown in Figure-1

On GW-1

  1. Config:route export:interface route to RIP
  2. select on Enable export routes from interface routes into RIP, Apply
  3. select on Export all interface routes into RIP, Apply

Now, other routers, like SW-1 and SW-2 know the route to network of 205.226.22.0 through RIP.

Exporting Aggregate Routes

There are two situation one wants to do Exporting Aggregate Routes.

I. The current Ipsrd does not like to do any auto (implicit) aggregation since in the global state with routing this can lead to *many* situations where the wrong thing is happening. For RIP version 1, since it does not have subnet information, ipsrd will not export subnetted network to another networks. For OSPF and RIP version 2, there is no need to do this.

Example 1: Export natural network in RIP 1 using aggregate exporting.

In Figure-1, RIP version 1 is enabled on Interface E1 which has network 205.226.26.0, Interface E2 has network 205.226.24.0 but it is subnetted with 30 bits to be network 205.226.24.100(the type of routing protocol enabled on E2 is irrelevant). Interface E1 will not advertise 205.226.24.100 to RIP 1 because it is not a valid network address in network 205.226.26.0. Interface E1 will not advertise 205.226.24.0 to RIP version 1 either because the current ipsrd can not do auto implicit aggregation. The future release will correct this behavior to compatible with RIP spec which requires a router to filter out subnet information by default when it is a border router of two natural networks. Therefore, a route aggregation export is required in the current release.

On GW-3,

  1. Config: route aggregation.
  2. Type 205.226.24.0 in add new aggregate Field, Type 24 in Mask length Field, Apply
  3. Select RIP for New Contributing Protocol, Apply
  4. Select on Contribute all routes from RIP.
  5. Config: route export: aggregate to RIP
  6. Select on Enable Export of routes from Aggregates into RIP. Apply
  7. Select on Export all Aggregates into RIP. Apply
  8. Type 2 in Metric from routes into RIP Field. Apply

Now, 205.226.24.0 network will be advertised on the interface of 205.226.26.0 as well as on the interface of 205.226.25.0. Assuming both interfaces are enabled for RIP version 1.

II When one router is on the border of several networks that can aggregate to one network number with a super netmask, one can do exporting aggregate routes to reduce routing table size. This applies to any protocols, including RIP version 1 and 2, OSPF and IGRP and it also applies to subnetted environment.

Example2: Export aggregate route in OSPF to reduce routing table size

In Figure-1, if IP networks of 205.226.24.0 to 205.226.27.0 are all located on the right hand of the network diagram. Since 205.226.24.0 to 205.226.27.0 can be grouped as 205.226.24.0 with mask bit of 22(a super net mask), interface can advertise an aggregate route of 205.226.24.0/22 for networks 205.226.24.0, 205.226.25.0. 225.226.26.0 and 225.226.27.0 that resides on the right side of the map.

  1. config:route aggregate
  2. Type 205.226.24.0 in add new aggregate Field, Type 22 in Mask length Field, Apply
  3. Select OSPF for New Contributing Protocol, Apply
  4. Select on Contribute all routes from OSPF.
  5. Config:route export:aggregate route to OSPF
  6. Select on Enable Export of routes from Aggregates into OSPF. Apply
  7. Select on Export all Aggregates into OSPF. Apply

Copyright © 1997 Ipsilon Networks, Inc.
Portions copyright © Digital Equipment Corporation 1998. All rights reserved.
Updated January 8, 1998
Send comments to Digital Equipment Corporation, doc-quality@lkg.mts.dec.com