Category Archives: Routing

OSPF Network Type – Multipoint

Recently I was working through one of the CBT nuggets OSPF NBMA labs by memory to try and reinforce the configuration and while I was practising I found that I was having issues with the multipoint OSPF area.

R3 is the hub router and R5 and R6 are the spoke routers.

area 100

R5 and R6 interfaces are left as the default OSPF network type which is point to point.

For this scenario the hello-interval is changed on R3’s interface to match the point to point hello interval, which is 10 seconds by default and this allows the neighbourship to form.

without-multipoint

What I noticed is that the ip ospf network point-to-multipoint command had not been enabled on R3’s interface. The neighbourship formed but R3 was not receiving any routes from R5 or R6, as you can see from the above picture the 5.5.5.0/24 and 6.6.6.0/24 networks are not in the routing table.

with-multipoint

Once the ip ospf network point-to-multipoint command is added, R3 is then able to correctly learn routes from R5 and R6 (the  5.5.5.0/24 and 6.6.6.0/24 networks).

I spent some time troubleshooting this problem, including running debugs but none of the debug commands gave me any feedback which lead me towards finding the issue. It was not until I went back and watched the video that I found the interface type had not been correctly configured and this was the reason R3 was not receiving these routes.

I wrote this post as a reminder to myself when configring OSPF over NBMA, even if the neighbours have formed and everything looks fine with the configuration if you still see issues with routes being learned then check the network type as this can cause problems, like the one mentioned here.

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OSPF Summary route and redistribution

Inter Area summarisation

area range [area number][prefix] [mask]

A router configuration command used to summarise subnets internally to the OSPF domain between different areas. The area number is the area where the networks are currently received from.

External Summarisation and Redistribution

summary address [prefix][mask]

A router configuration command used to create a summary address for routes learned externally from the OSPF domain.

redistribute [routing protocol or prefix][metric] [metric-type][subnets]

A router configuration command used to redistribute routes learned from another routing protocol, you can assign a metric or metric type depending on the internal protocol (metric-type for OSPF/ISIS or just metric for all internal protocols). The subnets keyword tells OSPF to advertise the network as it is received and not to just advertise the classful network as this is the default.

Summary and Redistribution

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CCNP Route v1.0 EIGRP Notes

  • Fast Convergence
  • Partial triggered udpates
  • Multiple protocol support (Ipv4/6, appletalk, IPX)
  • Multicast and Unicast (communications between routers)
  • VLSM support (classless)
  • Datalink layer protocol and topologies – no extra config it just works (eg: FR, broadcast, non broadcast, LAN, WAN)

Four Key Technologies

  • Neighbour discovery/re-discovery – hello packets
  • RTP – reliable transport protocol
  • Dual finite state machine – Decision process for ALL route communications
  • PDMs – Protocol dependant modules, IPv4/6,appletalk, ipx all operate independantly of each other

EIGRP Operation

For IP routing the process is:

Routing Information -> EIGRP Neighbour Table -> Topology Table -> Routing Table

  • Neighbour table, all adjacent neighbours
  • Topology Table, all learned routes to all destinations
  • Routing Table, best route to each destination

Successor = Best route (least cost path)
Feasible Successor = 2nd best route (backup route)
Feasible Distance = The metric of the successor route

The routing table is essentially a subset of the topology table

The topology table stores the RIB of each neighbour, the AD for each distination and the FD> to reah each destination.

Eigrp Packets

Hello – multicast discovery
Update – route changes, sent only to affected routers
Query – RTP, multicast then unicast, look for a successor route
Reply – Response to query, unicast, RTP
Ack – Acknowledge update/query/reply, unicast

All EIGRP overhead traffic is generated through the primary interface address

All packets carrying routing information are sent reliably

Retransmission of a reliable packet occurs up to 16 times or until the hold timer expires. After 16 tries the router resets the neighbourship.

DUAL Feasibility Condition

If your metric is lower than mine, you are loop free

Advertised Distance (AD) = Metric from the neighbour routers point of view
Local Distance = Metric to reach the neighbour

Dual Metric Calculation

Bandwidth (lowest link in Kbps) + Delay (in 10s of microseconds)

K Values

K1 & K3 = 1
K2 & K4 & K5 = 0
Identical K values are required to for an adjacency

Planning EIGRP Implemenations

  • IP Addressing Plan
  • Network TOpology
  • EIGRP Traffic Engineering

Verifying EIGRP Operation

show ip route eigrp
show ip eigrp neighbors
show ip eigrp interfaces
show ip eigrp topology [all-links]
show ip eigrp traffic
show ip protocols

debug ip eigrp
debug eigrp packets
debug ip eigrp summary
debug ip eigrp neighbors

EIGRP Commands

ip default-network – candidate default route
ip summary-address – sumarise on an interface
ip bandwidth-percent eigrp [as] [%] – percentage of interface bandwidth used for eigrp operations (updates/queries/replies)
no ip split-horizon eigrp [as] – turn off split horizon for multi-point FR
ip authentication mode eigrp [as] md5 – enable md5 authentication on the interface
ip authentication key chain eigrp [as] [key chain name] – specify the key chain to use for eigrp authentication

General Notes

Key chain authentication is configured globally and then added per interface
The summary route will stay in the RIB as long as one of the summarised prefixes is still reachable
Summary route have a default AD = 5

All IP routing protocols on Cisco routers can perform equal cost load balancing.

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