Routing Protocol Changes and Migration

Migrating from one routing protocol to another is always a disruptive change to the network.It requires careful planning to minimize the outages, and even then, they are inevitable,although their duration can be kept very low. Therefore, a routing protocol migration always requires a maintenance window.

Routing protocol migration is usually accomplished with the following steps:

Step 1. Plan the migration strategy.
Step 2. Activate the new routing protocol on all routers in the topology, raising its
administrative distance (AD) above the ADs of the current IGP. If the new IGP
is Routing Information Protocol (RIP) or Enhanced Interior Gateway Routing
Protocol (EIGRP), redistribution from the current into the new IGP has to be
configured on each router as well. The current IGP is left intact.
Step 3. Verify the new IGP’s adjacencies and optionally the working database
contents.
Step 4. Deactivate the current IGP in a gradual fashion.
Step 5. Remove the temporary settings from the new IGP.

EIGRP

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EIGRP topology table and the FD value

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Per Cisco doc:

"The feasible distance is the  best metric to reach the destination or the best metric that was known  when the route went active. This value is used in the feasibility  condition check. If the reported distance of the router (the metric  after the slash) is less than the feasible distance, the feasibility  condition is met and that path is a feasible successor."

The blue and the red number normally match, and that is the FD.

R1#sh ip eigrp topology  10.6.6.0/24

EIGRP-IPv4 Topology Entry for AS(1)/ID(10.1.1.1) for 10.6.6.0/24

  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 161280

  Descriptor Blocks:

  10.0.13.3 (FastEthernet0/0), from 10.0.13.3, Send flag is 0x0

      Composite metric is (161280/158720), route is Internal

      Vector metric:

        Minimum bandwidth is 100000 Kbit

        Total delay is 5300 microseconds

        Reliability is 255/255

        Load is 1/255

        Minimum MTU is 1500

        Hop count is 3

        Originating router is 10.6.6.6

R1#sh ip eigrp topology

EIGRP-IPv4 Topology Table for AS(1)/ID(10.1.1.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,

       r - reply Status, s - sia Status

P 10.6.6.0/24, 1 successors, FD is 161280

        via 10.0.13.3 (161280/158720), FastEthernet0/0

        via 172.16.15.5 (670720/158720), Serial1/0

R1#sh ip route 10.6.6.6

D        10.6.6.0/24 [90/161280] via 10.0.13.3, 00:21:14, FastEthernet0/0

      172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks

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There are several reasons why the EIGRP neighbor router(s) might not respond to the Query. Common reasons for this include the following:

1.       The neighbor router’s CPU is overloaded and the router either cannot respond in time or is even unable to process all incoming packets including the EIGRP packets.

2.       Quality issues on the link are causing packets to be lost.

3.       Low-bandwidth links are congested and packets are being delayed or dropped.

4.       The network topology is excessively large or complex, either requiring the Query to propagate to a significant depth or causing an inordinate number of prefixes to be impacted by a single link or node failure.

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