ROUTE Notes – OSPF – Neighbours

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Or ‘neighbors’ if your an American or a router…

OSPF, like EIGRP uses the concept of Hello packets to build adjacencies.

OSPF Hellos packets are sent to the multicast address 224.0.0.05

Included in the Hello packet is:

  • Router ID
  • Stub Area Flag

The following are interface specific:

  • Hello Interval
  • Dead Interval
  • Subnet
  • List of neighbours on interface
  • Area ID
  • Router Priority
  • Designated Router IP (DR)
  • Backup Designated Router IP (BDR)
  • Authentication

Before 2 routers become neighbours the following requirements must be met:

  • Same subnet
  • Same Area
  • Hello/Dead Interval Timers
  • Unique RIDs
  • MTU match
  • Authentication Pass (if configured)

Hello Intervals

** Key Point = timers are interface specific **

Hello Default is 10 seconds

Dead Timer is 40 seconds

When a router receives a Hello it resets it’s down counting timer back to 40 again.

Convergence can improved by tuning Hello timers although, if an interface fails, OSPF will immeadiately realise and all neighbours reached through that interface will be marked as dead.

Prudence is required when modifying timers as any changes will probably cause the neighbours to drop as the timers will no longer match.

Router ID

Router ID is assigned to an OSPF router in the following order

  1. ‘router-id’ OSPF sub-command
  2. Highest IP of any UP/UP Loopback Interface
  3. Highest IP of any UP/UP Interface

Routers with matching RIDs will not form an adjacency. However routers in the same VLAN will form a relationship with both (the duplicate RIDs will be shown in the output of ‘sh ip ospf neighbors’) but routes will not be learnt from the newest of the duplicate neighbours.

MTU

Mismatched MTU values will not prevent 2 routers from becoming neighbours. However it does mean that they will not be able to exchange topology information.

The command ‘ip mtu’ will set the maximum IP packet allowed where ‘mtu’ sets the MTU for all L3 protocols.

Authentication

The are 3 types of authentication in OSPF:

  • 0 – None
  • 1 – Clear Text
  • 2 – MD5

Type 0 is the default

Authentication is either enabled per interface or area wide under OSPF settings.

Authentication (if enabled) requires that the authentication keys must be configured per interface.

You can configure the same key to be used area wide to save typing, however if an interface key exists as well this will take precedence.