Od presenečenja, da se oprema nekega proizvajalca obnaša drugače kot oprema drugega, je do brskanja po RFC-jih le še korak.

V testnem okolju sem na sosednjem usmerjevalniku (router2) nastavil pot do nekega ciljnega IP-naslova tako, da ja ta pot kazala nazaj proti mojemu usmerjevalniku (router1). Presenetilo me je, da moj usmerjevalnik te poti ni prepoznal kot veljavne in je ni vpisal v svojo usmerjevalno tabelo. Naslov OSPF Forward Address za to pot, ki ga je izvedel od soseda, je bil namreč on sam. Zato je razumljivo, da je bila zanj ta pot neveljavna.

Ker se mi zdelo nelogično, da nek usmerjevalnik nastavi Forward Address na nekaj “povsem neuporabnega”, sem najprej preveril, ali enako velja tudi za kakšno drugo vrsto usmerjevalnikov. Na usmerjevalnik router1 sem zato na enak način povezal še usmerjevalnik drugega proizvajalca (router71).


(podrobnejše nastavitve najdete tule)

Sledilo je presenečenje – usmerjevalnik router2 je v Forward Address vpisal IPv4-naslov vmesnika na router1 (10.0.12.1), usmerjevalnik router71 pa prazno polje 0.0.0.0.

router1#show ip ospf database external 10.0.2.255
            OSPF Router with ID (10.1.1.1) (Process ID 4)
                Type-5 AS External Link States
  LS age: 128
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.0.2.255 (External Network Number )
  Advertising Router: 10.2.2.2
  LS Seq Number: 8000002E
  Checksum: 0x89B6
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 10.0.12.1
        External Route Tag: 0

router1#show ip ospf database external 10.0.71.255
            OSPF Router with ID (10.1.1.1) (Process ID 4)
                Type-5 AS External Link States
  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 385
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.0.71.255 (External Network Number )
  Advertising Router: 10.71.71.71
  LS Seq Number: 8000004B
  Checksum: 0xA690
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 0
        Forward Address: 0.0.0.0
        External Route Tag: 0

Je oboje pravilno in skladno s specifikacijami usmerjevalnega protokola OSPF za IPv4?

Odprem RFC – OSPF v2 in iz dokumenta razberem namen forwarding address:

If the forwarding address is set to 0.0.0.0, packets should be sent to the ASBR (Autonomous System Boundary Router) itself.
If the forwarding address is non-zero, look up the forwarding address in the routing table. The matching routing table entry must specify an intra-area or inter-area path; if no such path exists, do nothing with the LSA and consider the next in the list.
When the forwarding address field is non-zero, it should point to a router belonging to another Autonomous System.

Videti je, da sta proizvajalca usmerjevalnikov router2 in router71 razmišljala različno, vendar oba skladno z RFC 2328, saj v slednjem nastavitev Forward Address ni strogo zapovedana. Tule Cisco pojasni, kdaj bo usmerjevalnik nastavil Forward Address različen od 0.0.0.0.

Očitno ima Forward Address različen od 0.0.0.0 smisel le na broadcast povezavi, ne pa na point-to-point povezavi. To preverim tako, da povezavo med router1 in router2 definiram kot point-to-point. Forward Address se spremeni v 0.0.0.0:

router1>show ip ospf int fa0/0
FastEthernet0/0 is up, line protocol is up
  Internet Address 10.0.12.1/24, Area 0
  Process ID 4, Router ID 10.1.1.1, Network Type POINT_TO_POINT, Cost: 1
  Topology-MTID Cost Disabled Shutdown Topology Name
        0 1 no no Base
  Transmit Delay is 1 sec, State POINT_TO_POINT
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:02
  Supports Link-local Signaling (LLS)
  Cisco NSF helper support enabled
  IETF NSF helper support enabled
  Index 1/1, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 2
  Last flood scan time is 0 msec, maximum is 4 msec
  Neighbor Count is 1, Adjacent neighbor count is 1
    Adjacent with neighbor 10.2.2.2
  Suppress hello for 0 neighbor(s)

router1>show ip ospf database external 10.0.2.255
            OSPF Router with ID (10.1.1.1) (Process ID 4)
                Type-5 AS External Link States
  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 708
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.0.2.255 (External Network Number )
  Advertising Router: 10.2.2.2
  LS Seq Number: 80000031
  Checksum: 0x6AE9
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

router1>show ip route 10.0.2.255
Routing entry for 10.0.2.255/32
  Known via "ospf 4", distance 110, metric 20, type extern 2, forward metric 1
  Last update from 10.0.12.2 on FastEthernet0/0, 00:45:40 ago
  Routing Descriptor Blocks:
  * 10.0.12.2, from 10.2.2.2, 00:45:40 ago, via FastEthernet0/0
      Route metric is 20, traffic share count is 1


Danes je večina povezav v sodobnih hrbteničnih omrežjih realizirana kot point-to-point. Zmede z OSPF forwarding address v teh omrežjih ni.

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