ENTERPRISE, JNCIA, JNCIE, JNCIP, JNCIS, SERVICE PROVIDER

OSPF SUMMARY

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OSPF PACKET TYPES

HELLO PACKETS

Adjacencies formation:

An adjacency could fail if there is a mismatch of any of the following parameters:

  • MTU (stuck in exstart)
  • Network address
  • Subnet mask
  • Hello interval and/or dead interval
  • Area type
  • Authentication
  • Area id
  • Interface type (broadcast, ptp, ptmp…)

OR if there is a Duplicate RID or IP address

DR ELECTION

  • Highest priority wins
  • Default priority is 128
  • Priority = 0 means ineligible
  • Highest RID if same priority
  • Election NOT deterministic
    • Election occurs within the first 40 sec of OSPF coming up
  • No preemption
  • point-to-point link (set protocols ospf area <area> interface <interface> interface-type p2p) => no DR

LSA TYPES:

  • ONLY LSA with domain scope = LSA type 5!!!

LSA TYPES AND AREA TYPE:

LSAs HEADER:

LINK STATE TYPE AND LINK STATE ID:

Meaning of LINK STATE ID field in the LSA HEADER depends on the LSA type:

LSA TYPE 1

Meaning of LINK ID and LINK DATA fields, within the ROUTER LSA (TYPE 1), depends on the LINK TYPE:

How to remember? For Link Types 1, 2, and 4 Link ID = neighbors info, Link Data = Local info.

NOTE: A point to point link is advertised with TWO LSAs Type 1 (Link type 1 and link type 3):

LSA TYPE 2

Network LSA does NOT contain any prefix information, though it advertises the subnet mask for the network.

LSA TYPE 3

For LSAs type 3, the advertised prefix is in the LINK STATE ID (in the LSA HEADER).

LSA TYPE 4

For LSAs type 4, the advertised ASBR RID is in the LINK STATE ID (in the LSA HEADER).

LSA TYPE 5

External LSAs header E-bit:

LSA TYPE 7

Same format as LSA Type 5

Translated into an AS external LSA (Type 5) by the ABR at the NSSA border. This CANNOT be disabled!

If more than one ABR exists the one with the highest RID does the translation.

Other LSAs supported by Junos:

  • Type 9: used for graceful restart capability
  • Type 10: used for MPLS traffic engineering

ADVERTISEMENT OF DEFAULT ROUTE INTO AREAS

Default route not advertised into NSSA area or stub area by default. Use default-metric command.

Default-route advertised as an LSA type 3 for a STUB area; as an LSA type 7 or type 3 on NSSA depending on configuration.

ROUTE SUMMARIZATION

Only an ABR can summarize prefixes.

You CANNOT summarize LSAs type 1 and type 2, but an ABR can summarize prefixes learned from LSAs type 1 and type 2 and place the summary into LSAs type 3, instead of the specific prefixes.

This is NOT possible!
Default behavior.

Also, just like LSAs type 1 and type 2 cannot be summarized, LSAs type 5 cannot be summarize. However, an ABR that is translating LSAs type 7 into LSAs type 5 can summarize prefixes, within the LSA type 5.

Regular area:

set area <area#> area-range <prefix_summary> [restrict]

  • Configured on the ABR only!!!
  • Summarizes prefixes injected by the ABR, into an area (within LSAs type 3.
  • ABR learns about these prefixes from LSAs type 1 and type 2.
  • Specific prefixes are suppressed automatically
  • Restrict option can be used to filter prefixes.

EXAMPLE:

set area 1 area-range 10.1.0/22

Summarizes all prefixes within the 10.1.0/22 range.

set area 1 area-range 10.1.0/22 [restrict]

Because all specific prefixes are suppressed automatically, and the restrict suppresses the summary, this effectively filters LSAs type 3.

The example summarizes all prefixes within the 10.1.0/22 range, but the restrict action suppresses the update.

NSSA area:

set area <area#> nssa default-lsa area-range <prefix_summary> [restrict]

  • Configured on the ABR only!!!
  • Summarizes prefixes injected by the ABR, into an area (within LSAs type 5) when translating from LSAs type 7 into LSAs type 5..
  • ABR learns about these prefixes from LSAs type 7
  • Specific prefixes are suppressed automatically
  • Restrict option can be used to filter prefixes.

EXAMPLE:

set area 1 nssa default-lsa area-range 10.1.0/22

Summarizes all prefixes within the 10.1.0/22 range.

set area 1 area-range 10.1.0/22 [restrict]

Because all specific prefixes are suppressed automatically, and the restrict suppresses the summary, this effectively filters LSAs type 5 (translated from type 7) within the range.

The example summarizes all prefixes within the 10.1.0/22 range, but the restrict action suppresses the update.

OSPF ROUTE FILTERING / ROUTING POLICIES and REDISTRIBUTION

LSAs filtering is NOT possible. The database of ALL routers within an area must be identical. You can limit propagation of some LSAs by converting the area into a stub, nssa, or stub/nssa no-summaries.

Routing policies can be used to control creation and propagation of LSAs type 3 and LSAs type 5.

LSAs 1 and 2 cannot controlled with any routing policies policies.

JUNOS <=> IOS

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