OSPF — virtual link across a transit area
- Routing
- Advanced
- 14 min
- 4 steps
- Cisco IOS-style
Three areas (0, 1, 2) chained: RA0 (area 0) ↔ RA1 (area 1, ABR to area 2) ↔ RA2 (area 2). Without a virtual link, RA0 can't reach RA2's prefixes — RA1 is technically an ABR for area 1+2 but lacks an area-0 interface. Configure a virtual link RA0↔RA1 through area 1 to fix it.
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Free · no install · no account
Scenario
An OSPF virtual link is a *logical* point-to-point adjacency between two ABRs that traverses a non-backbone "transit area". Two use cases: 1. Discontiguous backbone — a remote area-0 fragment connects back to the main backbone through a non-zero transit. 2. Indirect ABR-to-area-0 connection — an ABR with no direct area-0 interface joins area 0 logically through the transit. This lab exercises case 2: RA1 has interfaces in areas 1 + 2 but not in 0; without a v-link to RA0 (which is in area 0 + area 1), RA1 can't generate Type-3 summaries for area-2 prefixes into area 0. The v-link makes RA1 a *logical* area-0 member. Constraints (RFC 2328 §15.1): - Both endpoints must be ABRs. - The transit area must NOT be a stub family. - Each end names the *other ABR's* router-id.
Topology
3 × router · Cisco IOS-style
RA0— routerRA1— routerRA2— router
What you will do
On
RA0, runshow ip route. Confirm3.3.3.3/32is missing — RA0 has no path to area-2 prefixes yet.On
RA0, configure the v-link end:area 1 virtual-link 2.2.2.2(RA1's router-id).On
RA1, configure the matching reverse end:area 1 virtual-link 1.1.1.1(RA0's router-id).Back on
RA0, runshow ip routeagain.3.3.3.3/32should now appear asO IAvia the v-link path.
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This lab in other languages
OSPF — Virtual Link über eine Transit-AreaOSPF — virtual link a través de un área de tránsitoOSPF — virtual link através de uma área de trânsitoOSPF — virtual link à travers une zone de transitOSPF — एक transit area के पार virtual linkOSPF — 跨过境区域的 virtual linkOSPF — virtual link attraverso un'area di transitoOSPF — トランジットエリアを越える virtual linkOSPF — 통과 에어리어를 가로지르는 virtual linkOSPF — virtual link через транзитную областьOSPF — transit alan üzerinden virtual linkOSPF — virtual link qua transit area