08-21-2019 05:00 AM
we have two ISP with running Open shortest path (ospf) ( Layer 3 Switch - ISP Customer Edge router ) and BGP ( ISP Router) at HUB location.
From HUB location multiple Spoke location connected with single ISP. so spoke location can connect respective ISP.
newly one Branch office implement with two link ( PRI and sec ) ISP over (MPLS).
whenever user can reach new Branch office Subnet by asymmetric route manner.
one more issue while one ISP link down at Branch office that Subnet user not able reach via other ISP .
Exam: in HUB location user can reach in Branch office ISP users - forward path via ISP 1 and return path via ISP 2.
how we can resolve this routing issue
08-21-2019 05:09 AM
is this exam question? do you have the diagram and can you share the configuration so we can add?
08-21-2019 05:12 AM
no exam question. please the solution with routing protocol.
08-21-2019 05:25 AM - edited 08-21-2019 05:28 AM
Hello cciesubbu,
generally speaking eBGP routes are preferred over OSPF routes for their lower AD.
So we could expect that the ISP using eBGP as PE-CE both on the HUB site and the remote site is preferred.
However, eBGP routes need to redistributed into OSPF and they become external OSPF routes from the point of view of devices not directly connected to the ISP1 PE node.
However, there are other possible options to explore that include the fact that the other ISP may be using the OSPF super-area backbone emulation that can change the way OSPF routes are propagated over MP BGP.
The important concept here is that of OSPF domain id, that when it is not set manually it is equal to the OSPF process-id (as configured on PE nodes it is not important what CE nodes use as OSPF process)
The OSPF emulation allows a PE to create a MP BGP prefix with additional attributes including an extcommunity representing the OSPF domain.
When the OSPF domain is the same at both VRF sites the remote PE re-builds an LSA type 3 to represent the imported OSPF routes for OSPF routes that were internal or inter area on the other VRF site O, O IA -> O IA LSA type 3. External routes are seen as external routes.
If this is the case the O IA route coming from OSPF via MP BGP is preferred over the O E1 or O E2 route generated by redistribution og BGP into OSPF coming from the other MPLS provider.
If the OSPF domain-id does not match remote routes are seen as external routes OE1 or O E2 regardless of the type of route on the other site O, O IA , O E 1/2 ----> O E 1/2 routes.
I think this network scenario is a good opportunity to make you play with the OSPF domain-id concept and OSPF emulation over MP BGP.
However, if you provide more details you can get better help.
Hope to help
Giuseppe
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