- If you change the network type, you will also change the hello and dead timers accordingly.
- OSPF defines 6 network types:
1. Broadcast networks
- Default network on ethernet and FDDI.
- Will elect a DR and a BDR. (highest priority wins, default priority is 1, 0=ineligible DR/BDR election)
- Uses the multicast MAC 224.0.0.5 (0100.5E00.0005) for All SPF Routers and 224.0.0.6 (0100.5E00.0006) for All DRouters.
- There is NO next-hop modification. The next-hop IP remains that of the originating router.
- Layer3 to layer2 resolution is required.
- Broadcast networks can’t have unicast neighbors configured.
- 10 hello / 40 dead-interval.
Note: @ multipoint FR DR must be explicitly define.
2. Non-Broadcast networks
- Can connect more than two routers but have no native broadcast capability.
- Non-Broadcast is the default network type on multipoint frame-relay interface, eg a main interface.
- OSPF routers on NBMA networks elect a DR and BDR, but all OSPF packets are unicast between each manually specified neighbor with the "neighbor" command. (must have to operate)
- The next-hop IP is not changed and remains the IP address of the originating router.
- The default priority is 1, and should be disabled (=0) on ALL SPOKES, to prevent a spoke from becoming a blackhole DR/BDR.
- 30 hello / 120 dead-interval.
3. Point-to-point networks
- Default on T1, DS-3, or SONET links, point-to-point sub-interface on frame-relay and ATM networks.
- Uses the multicast destination to AllSPFRouters (224.0.0.5), except for retransmitted LSAs which are unicast.
- NO DR/BDR election, OSPF configured as per normal.
- The next-hop IP is that of the advertising router.
- OSPF ignores subnet mask mismatch on point-to-point links.
- 10 hello / 40 dead-interval.
4. Point-to-multipoint networks
- Cisco proprietary, and not a default option, but best choice for NBMA networks.
- Are special configurations of NBMA networks in which the networks are treated as a collection of point-to-point links.
- Does not elect a DR and BDR, and the OSPF packets are multicast (224.0.0.5) to each known neighbor.
- The next-hop IP is that of the advertising neighbor.
- Layer3 to layer2 resolution is ONLY needed for the directly connected neighbors.
- Non-direct neighbors use recursive layer3 IP routing to reach each other.
- In addition the endpoints of point-to-multipoint networks are advertised as host routes instead
of the actual networks. ie /32 in the routing table.
- 30 hello / 120 dead-interval.
5. Point-to-multipoint non-broadcast networks
- Cisco proprietary, the same as point-to-multipoint, but configured with the additional 'non-broadcast' keyword.
- No DR/BDR election, uses unicast appose to multicast, to each manually specified neighbor.
- As a result the directly connected neighbor must be manually defined with the 'neighbor' command. This command is only required on the one side.
- The next-hop IP is that of the advertising neighbor.
- IP routing will be used to establish reachability between devices that are non-adjacent at layer2.
- Was created to allow for the assignment of the cost per neighbor appose to using the interface's cost.
- Remember that the cost is based on the 'incoming' interface's bandwidth and not the bandwidth of the neighbor's interface.
- 30 hello / 120 dead-interval.
6. Virtual links
- Are used to link an area to the backbone through a non-backbone area. (Also known as a transit area)
- Can also be used to connect two parts of a partitioned backbone through a non-backbone area.
- Must be configured between two ABRs of which one must be connected to area 0.
- The transit area cannot be a stub area, and must have full routing information.
- The virtual link will transition to the fully functional point-to-point interface state when a route to the neighbouring ABR is found in the route table.
- OSPF ignores subnet mask mismatch on point-to-point links.
- A virtual link is seen as an interface in area 0.
- All area 0 attributes are inherited by routers attached to the virtual link, including summarization and authentication.
- To see the cost of using the transit area use "sh ip ospf virtual-link" and refer to 'cost of using'.
- The cost of the virtual link is the cost of the route to the neighbors interface via the transit area.
- Configures one end of the virtual link. {ABR-RID} = Area
area {transit-area} virtual-link {ABR-ID} Border Router-ID
note: must be configured on both sides. Shows the state of a virtual link, the cost of transit area, transit interface
sh ip ospf virtual-link
- used to connect discontiguous Area 0 (bad design)
authentication - if authentication is configured on Area 0, it must be configured on the far end virtual link router " area 0 authentication" on remote-end.
Nested virtual-link (not recommended design)
- you can extend to more than one area away
- one hop at a time
- as you install virtual link, you bring area 0 outwards to the remote area .
area0 area1 area2 area3 area0
-------ABR------------ABR-------------ABR------------ABR------------
|<------VL----->|<-----VL----->|<-----VL------>|
--- cannot build virtual link without area 0.
OSPF w/0 Area0
- in a single area system, area 0 is not needed.
- once you have more than 1 area area 0 is needed.
- virtual-link itself is part of area 0.
i.e area 100 area 200
-----------------------RTR1---------------------------
area0 must exists in rtr1, you can add lo0(work perfectly fine) or interface in area 0.
==================
OSPF over GRE
- OSPF virtual links cannot transit stub areas.
- If a virtual link over a stub area is required, the only solution is to use a GRE tunnel.
- The tunnel interface must have a IP address with a network statement in area0.
loopback interface
- Default for loopback interfaces.
- will show as stub host
- Assumes only a single attached router. OSPF advertises stub networks as host routes(/32).
- Don’t confuse this with stub areas!
- if you want the mask to be entered in the routing 3 ways
1. use "ip ospf network point-to-point"
2. use area range command (@ABR)
3. redistributed connected.
LAB SCENARIO: Need to change the default network type of the interface loopback, but you are not allowed to change using "ip ospf network type" command;
2 method:
1. area range - configured on ABR
i.e. loopback @ area 24
area 24 range ip mask
- it will create as network summary route
- at other routers it will shows as int type 0- route
- between areas O IA (inter Area)
2. redistribute connected - it will take the full mask
note: watch the problem, it might violate the CCIE lab not to do. putting lo0 @ area 24 example, as soon as u redistribute connected it will show as E2 type 5 routes which dont have any reference to any area it just appears as external routes.
@ other routing table. will show 2 different routes
- O for the stub /32
- E2 route w/c is the /24 for the network.
note: Reach ability wise this is not necessarily achieving what we wanted.
Neighbor command
- use unicast packets
- used to signify when a pair cannot be reached via multicast.
- only necessary on one side of the connection (hub)
- typically the hub side is configured though can configure both also.
show commands:
Displays OSPF-related interface information, DR, BDR, etc
sh ip ospf interface
Interface command:
- Same as OSPF network command. Places the interface in a specified area
ip ospf {pid} area {area-id}
- Change the network type to broadcast. Timers: 10/40
ip ospf network broadcast
- Change the network type to NBMA. Timers: 30/120. Require manual neighbors
ip ospf network non-broadcast
- Change the network type to point-to-point. Timers: 10/40
ip ospf network point-to-point
- Change the network type to point-to-multipoint. Timers: 30/120
ip ospf network point-to-multipoint
- Change to network type to point-to-multipoint Timers: 30/120
ip ospf network point-to-multi [non-broadcast]non-broadcast
- Highest priority wins, (Default = 1, Ineligible = 0)
ip ospf priority {number}
OSPF process command:
- enabling OSPF with process ID of 1. (locally significant)
router ospf 1
- Defines an interface on which OSPF runs and its area ID.
network {ip} {mask} area {area-id}
- Manually specifies a neighbor
- Optionally define priority or cost for the neighbor.
neighbor {ip} [priority {pri}] [cost {cost}]
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