Archive for the ‘Redistribution’ category

Strategy 1 for Redistribution | The Theory

June 29, 2006

The following is an extensive treatment of the redistribution tactic discussed in class on Tuesday, June 27, 2006.

The Problem

When we have different interior gateway protocols (IGPs) running with multiple points of redistribution, sub-optimal paths and routing loops can easily occur.

1. On autonomous system border routers (ASBRs), this occurs when the IGP originating a particular route is ignored and a router running an IGP with a lower administrative distance (AD) is selected as the next-hop address in the routing table.

Redistribution Strategy 1

In the above example, Router 2 has a 50% chance of selecting the OSPF-advertised route through R1 -> R4 -> R3 to get to the loopback address 172.16.103.1/24. Although this is a native RIP route, OSPF is preferred because the AD of OSPF (110) is lower than that of RIP (120), thus its routes are “more trusted.” (more…)

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RIP Routing with Tags!!!

June 27, 2006

Last week we noted that we could tag RIP routing updates and filter them at the redistribution points, but couldn’t view the tags using show ip route or show ip rip database.

In order to view the tags on particular RIP routes, issue debug ip rip in privileged-exec mode to get output like that below:

Router# debug ip rip
*Apr 30 01:09:31.711: RIP: build update entries
*Apr 30 01:09:31.711:   2.2.2.0/24 via 0.0.0.0, metric 2, tag 0
*Apr 30 01:09:31.711:   4.4.4.0/24 via 0.0.0.0, metric 1, tag 0

Redistributing iBGP routes

June 27, 2006

This tip applies if you are redistributing from BGP, but your routes aren’t redistributing into your IGP. If the routes that don’t redistribute are originated within the AS, you may need to use the bgp redistribute-internal command, which can be applied under the bgp router prompt or an address-family prompt. This command allows routes that are originated within the same BGP AS to be redistributed out into other routing protocols. This behavior is normally disabled by default to prevent routing problems.

For more information, here is the entry under univercd:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cr/hirp_r/rte_bgh1.htm#wp1072295