Crossposting, original publication
OSFP, being a link-state protocol, eliminates topology loops by building a shortest path tree within one zone using Dijkstra's algorithm. However, the behavior of OSPF between zones is more like the behavior of distance-vector protocols, which exchange only prefixes and corresponding metrics without any information about the actual topology; for this reason, some authors may refer to OSPF as a hybrid routing protocol. The mechanism for protecting against routing loops between zones, however, is quite simple: all zones must exchange routing information through the backbone zone, zone 0, direct exchange of routes between zones is not possible.
, OSPF – OSPF Virtual Link (VL). , VL – ; , VL . , VL , - . , VL.
, google.com. GNS3 Cisco 7200:
(loopback0) OSPF RID ; loopback ABR 0. : 192.168.xy.x|y/24 Rx Ry (, 192.168.12.1 f0/1 R1). OSPF, R1 R3 VL.
, ; R1 R5:
R1#ping 5.5.5.5 so lo 0 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 5.5.5.5, timeout is 2 seconds: Packet sent with a source address of 1.1.1.1 !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 20/30/40 ms R1#traceroute 5.5.5.5 so lo 0 numeric Type escape sequence to abort. Tracing the route to 5.5.5.5 VRF info: (vrf in name/id, vrf out name/id) 1 192.168.14.4 16 msec 24 msec 20 msec 2 192.168.45.5 48 msec 16 msec 24 msec
, . :
R1(config)#router os 1 R1(config-router)#no capability transit
Transit area capability – OSPFv2 , VL. : , VL, LSA3; LSA3 , . OSPFv1 , , VL. , . :
R1#traceroute 5.5.5.5 so lo 0 n Type escape sequence to abort. Tracing the route to 5.5.5.5 VRF info: (vrf in name/id, vrf out name/id) 1 192.168.12.2 44 msec 16 msec 20 msec 2 192.168.23.3 20 msec 40 msec 40 msec 3 192.168.35.5 76 msec 44 msec 44 msec
, : R1 VL OSPFv1 transit capability. ? :
R2(config)#int f1/0 R2(config-if)#ip os cost 100
R3(config)#int f1/0 R3(config-if)#ip os cost 100
, ?
R1#traceroute 5.5.5.5 so lo 0 n
Type escape sequence to abort.
Tracing the route to 5.5.5.5
VRF info: (vrf in name/id, vrf out name/id)
1 192.168.12.2 20 msec 16 msec 16 msec
2 192.168.12.1 24 msec 16 msec 16 msec
3 192.168.12.2 36 msec 32 msec 44 msec
4 192.168.12.1 28 msec 36 msec 40 msec
5 192.168.12.2 44 msec 48 msec 64 msec
6 192.168.12.1 60 msec 60 msec 60 msec
7 192.168.12.2 80 msec 80 msec 80 msec
8 192.168.12.1 84 msec 76 msec 76 msec
, – . :
VL R1-R3;
Transit capability ;
R2-R3 .
: R1 next-hop 5.5.5.5/32 R2:
R2#sho ip ro 5.5.5.5 255.255.255.255 longer-prefixes 5.0.0.0/32 is subnetted, 1 subnets O IA 5.5.5.5 [110/4] via 192.168.12.1, 00:06:21, FastEthernet0/1
R2 ; R1 :
R1#sho ip ro 5.5.5.5 255.255.255.255 longer-prefixes 5.0.0.0/32 is subnetted, 1 subnets O 5.5.5.5 [110/103] via 192.168.12.2, 00:08:07, FastEthernet0/1
OSPFv1:
5.5.5.5/32 VL 0;
, VL.
R2 VL, transit area; , LSA3.
, – transit capability, VL. , : OSPFv1 OSPFv2 transit area. , , , . , : , - VL OSPFv1 .
OSPFv2 RFC OSPFv1:
“When summarizing information into a virtual link's transit area, version 2 of the OSPF specification prohibits the collapsing of multiple backbone IP networks/subnets into a single summary link.”
Section F.2.3, RFC 1247
, RFC , . 0 ABR, LSA3 VL. :
VL 0; - LSA3 VL;
LSA3.
, R3 5.5.5.0/24, R2 5.5.5.0/25 R4, . ? 0, , .. . , , 0, ; ABR - LSA1/2 LSA3. , 0, . ; LSA1-LSA3 , .
Conclusion: it is better not to touch some little-known default settings.
PS There is an equally interesting way to shoot yourself in the foot .
Assisted in editing the article: Anastasia Kuralyova, Maxim Klimanov.