On Demand Routing (ODR)

ODR requires cdp to be enabled on connected interface between hub and spokes. the hub need to have "router odr" command that's all we require to configure odr. Hub router will send a default route to all spokes while spokes will send their connected prefixes to Hub.

We need to enable cdp on virtual-template if PPPoFR is running, in this case it does matter whether cdp is enabled on physical interface or not. But if cdp is enable on both physical interface and virtual template, the hub will have two routes installed for same destination one via virtual -access and other via physical interface. Though this does not make any difference whether two routes are install or just one because ultimately even virtual-access is also using physical interface for packet forwarding. And obviously spokes will have a default route advertise by hub.

Redistribution example

Lets take up a scenario based question on redistribution

R1 running both EIGRP and OSPF.  EIGRP to R2 and OSPF to R3
R1 s0/0 to R2 s0/0 and R1 S0/1 to R3 S0/1

R1 has 2 LAN interface F0/0 and F0/1

R1 Redistributes these LAN interfaces into EIGRP.  

R1 then mutually redistributes EIGRP and OSPF

Question: Why isn't R3 seeing the LAN interfaces of R1? Since they were redistributed on EIGRP and then EIGRP was redistributed into OSPF....wouldn't R3 see them?

Answer:  When you redistribute 2 LAN connected interfaces into EIGRP on R1, these two interfaces will land up in the eigrp topology database i.e show ip eigrp topology" but not on the eigrp routing table i.e "show ip route eigrp" because R1 does a comparision of administrative distance(AD) between same routes(2 LAN interfaces) learned via connected (AD = 0) and EIGRP (AD = 170) clearly connected wins and hence appear in the routing table. Now since Eigrp has these routes in its database so it passes them to R2, R2 is learning these routes only via EIGRP so R2 need not to perform any comparision and hence install in its routing table.

Remember even after the above redistribution: 

"Show ip route eigrp will have nothing while "show ip eigrp int" will have only connected link i.e s0/0

When mutual redistribution takes place between EIGRP and OSPF on R1 (Redistribution always takes place from the routing table)

When EIGRP redistribute into OSPF it does this

1. R1's OSPF databese "show ip ospf database" will have "Show ip route eigrp" and "show ip eigrp int" route so the only route land up in R1's database is prefix of S0/0 "show ip ospf database.

2. R1 passes this route (prefix of R1's S0/0) to R3 and R3 learned these routes only via ospf so R3 installs them in its routing table

When OSPF redistribute into EIGRP it does this

1. R1's EIGRP databese "show ip eigrp topology" will have "Show ip route eigrp" and "show ip eigrp int" routes so the only route land up in R1's EIGRP database is prefix of S0/1 "show ip eigrp topology".

2. R1 passes this route (prefix of R1's S0/1) to R2 and R2 learned these routes only via eigrp so R3 installs them in its routing table

Therefore ultimately R3 can not have connected routes in its routing table, in order to do so we have to redistribute these connected interfaces into eigrp.