• What feature we can use to prevent routing loop for dual home CE?
PE set “down bit” for LSA type 3 and reject LSA type 3 from CE with “down bit” set
See OSPF Divers.
• What feature we can use to keep external route as LSA type 5 or type 7 for network with dual OSPF processes when using MPLS L3VPN as inter-site connection?
Configure domain-ID on PE. PE with same domain-ID advertises LSA 1/2/3 as LSA3 to CE. PE with difference domain-ID advertises all LSA as LSA type 5
• Summarization: Ingress PE
– What if you want Site1 (area 1) to send a summary route to all other
– Summarization is not possible since ABR does not exist in Site1.
– PE-1 can summarize via BGP and advertise a aggregate block to all other sites
• Summarization: Egress PE
– What if you need to send a summary route to Site3 (area3)?
– Can’t summarize from each individual site; no ABR exists within the sites (area1 or area2)
– Summarize all the other site routes, on PE-3
– Type-5 metric will be selected from best to BGP MED (multi-exit discriminator)
But be-careful with Loop:
– Summary is originated in OSPF
– Summary route will be propagated to all sites as a result of redistribution from OSPF into BGP
– Summary route should be filtered while redistributing OSPF into BGP on PE3 unless it is desirable to send summary to some select PEs
• What feature we can use to prefer MPLS Backbone link over intra area backdoor link?
Run “OSPF Sham-link” between PEs and adjust cost on backdoor Link.
Source ccdewiki
Source BRKRST-2337 OSPF Deployment in Modern Networks
Daniel LARDEUX 